[PATCH 0/2] fuse: remove extra page copies in writeback

[PATCH 0/2] fuse: remove extra page copies in writeback

[Joanne Koong]

Currently when we handle writeback in fuse, we allocate and copy data
to a temporary folio in order to mitigate the following deadlock scenario
that may arise if reclaim waits on writeback to complete:
* single-threaded FUSE server is in the middle of handling a request
  that needs a memory allocation
* memory allocation triggers direct reclaim
* direct reclaim waits on a folio under writeback
* the FUSE server can't write back the folio since it's stuck in
  direct reclaim
(more details can be found here [1])

The first patch adds an ASOP_NO_RECLAIM_IN_WRITEBACK flag to the struct
address_space_operations to indicate that reclaim when in writeback should
be skipped for filesystems with this flag set. More details can be found in
the commit message of the first patch, but this only actually affects the
case of legacy cgroupv1 when it encounters a folio that already has the
reclaim flag set (in the other cases, we already skip reclaim when in
writeback). Doing so allows us to get rid of needing to allocate and copy over
pages to a temporary folio when handling writeback in fuse (2nd patch).
Benchmarks can be found in commit message of the 2nd patch.

Thanks,
Joanne

[1] https://lore.kernel.org/linux-kernel/495d2400-1d96-4924-99d3-8b2952e05fc3@linux.alibaba.com/

Joanne Koong (2):
  mm: skip reclaiming folios in writeback contexts that may trigger
    deadlock
  fuse: remove tmp folio for writebacks and internal rb tree

 fs/fuse/file.c     | 321 ++++-----------------------------------------
 include/linux/fs.h |  14 ++
 mm/vmscan.c        |   6 +-
 3 files changed, 45 insertions(+), 296 deletions(-)

-- 
2.43.5

[PATCH 1/2] mm: skip reclaiming folios in writeback contexts that may trigger deadlock

[Joanne Koong]

Currently in shrink_folio_list(), reclaim for folios under writeback
falls into 3 different cases:
1) Reclaim is encountering an excessive number of folios under
   writeback and this folio has both the writeback and reclaim flags
   set
2) Dirty throttling is enabled (this happens if reclaim through cgroup
   is not enabled, if reclaim through cgroupv2 memcg is enabled, or
   if reclaim is on the root cgroup), or if the folio is not marked for
   immediate reclaim, or if the caller does not have __GFP_FS (or
   __GFP_IO if it's going to swap) set
3) Legacy cgroupv1 encounters a folio that already has the reclaim flag
   set and the caller did not have __GFP_FS (or __GFP_IO if swap) set

In cases 1) and 2), we activate the folio and skip reclaiming it while
in case 3), we wait for writeback to finish on the folio and then try
to reclaim the folio again. In case 3, we wait on writeback because
cgroupv1 does not have dirty folio throttling, as such this is a
mitigation against the case where there are too many folios in writeback
with nothing else to reclaim.

The issue is that for filesystems where writeback may block, sub-optimal
workarounds need to be put in place to avoid potential deadlocks that may
arise from the case where reclaim waits on writeback. (Even though case
3 above is rare given that legacy cgroupv1 is on its way to being
deprecated, this case still needs to be accounted for)

For example, for FUSE filesystems, when a writeback is triggered on a
folio, a temporary folio is allocated and the pages are copied over to
this temporary folio so that writeback can be immediately cleared on the
original folio. This additionally requires an internal rb tree to keep
track of writeback state on the temporary folios. Benchmarks show
roughly a ~20% decrease in throughput from the overhead incurred with 4k
block size writes. The temporary folio is needed here in order to avoid
the following deadlock if reclaim waits on writeback:
* single-threaded FUSE server is in the middle of handling a request that
  needs a memory allocation
* memory allocation triggers direct reclaim
* direct reclaim waits on a folio under writeback (eg falls into case 3
  above) that needs to be written back to the fuse server
* the FUSE server can't write back the folio since it's stuck in direct
  reclaim

This commit allows filesystems to set a ASOP_NO_RECLAIM_IN_WRITEBACK
flag in the address_space_operations struct to signify that reclaim
should not happen when the folio is already in writeback. This only has
effects on the case where cgroupv1 memcg encounters a folio under
writeback that already has the reclaim flag set (eg case 3 above), and
allows for the suboptimal workarounds added to address the "reclaim wait
on writeback" deadlock scenario to be removed.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 include/linux/fs.h | 14 ++++++++++++++
 mm/vmscan.c        |  6 ++++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index e3c603d01337..808164e3dd84 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -394,7 +394,10 @@ static inline bool is_sync_kiocb(struct kiocb *kiocb)
 	return kiocb->ki_complete == NULL;
 }
 
+typedef unsigned int __bitwise asop_flags_t;
+
 struct address_space_operations {
+	asop_flags_t asop_flags;
 	int (*writepage)(struct page *page, struct writeback_control *wbc);
 	int (*read_folio)(struct file *, struct folio *);
 
@@ -438,6 +441,12 @@ struct address_space_operations {
 	int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
 };
 
+/**
+ * This flag is only to be used by filesystems whose folios cannot be
+ * reclaimed when in writeback (eg fuse)
+ */
+#define ASOP_NO_RECLAIM_IN_WRITEBACK	((__force asop_flags_t)(1 << 0))
+
 extern const struct address_space_operations empty_aops;
 
 /**
@@ -586,6 +595,11 @@ static inline void mapping_allow_writable(struct address_space *mapping)
 	atomic_inc(&mapping->i_mmap_writable);
 }
 
+static inline bool mapping_no_reclaim_in_writeback(struct address_space *mapping)
+{
+	return mapping->a_ops->asop_flags & ASOP_NO_RECLAIM_IN_WRITEBACK;
+}
+
 /*
  * Use sequence counter to get consistent i_size on 32-bit processors.
  */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 749cdc110c74..2beffbdae572 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1110,6 +1110,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 		if (writeback && folio_test_reclaim(folio))
 			stat->nr_congested += nr_pages;
 
+		mapping = folio_mapping(folio);
+
 		/*
 		 * If a folio at the tail of the LRU is under writeback, there
 		 * are three cases to consider.
@@ -1165,7 +1167,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 			/* Case 2 above */
 			} else if (writeback_throttling_sane(sc) ||
 			    !folio_test_reclaim(folio) ||
-			    !may_enter_fs(folio, sc->gfp_mask)) {
+			    !may_enter_fs(folio, sc->gfp_mask) ||
+			    (mapping && mapping_no_reclaim_in_writeback(mapping))) {
 				/*
 				 * This is slightly racy -
 				 * folio_end_writeback() might have
@@ -1320,7 +1323,6 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 		if (folio_maybe_dma_pinned(folio))
 			goto activate_locked;
 
-		mapping = folio_mapping(folio);
 		if (folio_test_dirty(folio)) {
 			/*
 			 * Only kswapd can writeback filesystem folios
-- 
2.43.5

Re: [PATCH 1/2] mm: skip reclaiming folios in writeback contexts that may trigger deadlock

[Shakeel Butt]

On Fri, Oct 11, 2024 at 03:34:33PM GMT, Joanne Koong wrote:
> Currently in shrink_folio_list(), reclaim for folios under writeback
> falls into 3 different cases:
> 1) Reclaim is encountering an excessive number of folios under
>    writeback and this folio has both the writeback and reclaim flags
>    set
> 2) Dirty throttling is enabled (this happens if reclaim through cgroup
>    is not enabled, if reclaim through cgroupv2 memcg is enabled, or
>    if reclaim is on the root cgroup), or if the folio is not marked for
>    immediate reclaim, or if the caller does not have __GFP_FS (or
>    __GFP_IO if it's going to swap) set
> 3) Legacy cgroupv1 encounters a folio that already has the reclaim flag
>    set and the caller did not have __GFP_FS (or __GFP_IO if swap) set
> 
> In cases 1) and 2), we activate the folio and skip reclaiming it while
> in case 3), we wait for writeback to finish on the folio and then try
> to reclaim the folio again. In case 3, we wait on writeback because
> cgroupv1 does not have dirty folio throttling, as such this is a
> mitigation against the case where there are too many folios in writeback
> with nothing else to reclaim.
> 
> The issue is that for filesystems where writeback may block, sub-optimal
> workarounds need to be put in place to avoid potential deadlocks that may
> arise from the case where reclaim waits on writeback. (Even though case
> 3 above is rare given that legacy cgroupv1 is on its way to being
> deprecated, this case still needs to be accounted for)
> 
> For example, for FUSE filesystems, when a writeback is triggered on a
> folio, a temporary folio is allocated and the pages are copied over to
> this temporary folio so that writeback can be immediately cleared on the
> original folio. This additionally requires an internal rb tree to keep
> track of writeback state on the temporary folios. Benchmarks show
> roughly a ~20% decrease in throughput from the overhead incurred with 4k
> block size writes. The temporary folio is needed here in order to avoid
> the following deadlock if reclaim waits on writeback:
> * single-threaded FUSE server is in the middle of handling a request that
>   needs a memory allocation
> * memory allocation triggers direct reclaim
> * direct reclaim waits on a folio under writeback (eg falls into case 3
>   above) that needs to be written back to the fuse server
> * the FUSE server can't write back the folio since it's stuck in direct
>   reclaim
> 
> This commit allows filesystems to set a ASOP_NO_RECLAIM_IN_WRITEBACK
> flag in the address_space_operations struct to signify that reclaim
> should not happen when the folio is already in writeback. This only has
> effects on the case where cgroupv1 memcg encounters a folio under
> writeback that already has the reclaim flag set (eg case 3 above), and
> allows for the suboptimal workarounds added to address the "reclaim wait
> on writeback" deadlock scenario to be removed.
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  include/linux/fs.h | 14 ++++++++++++++
>  mm/vmscan.c        |  6 ++++--
>  2 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index e3c603d01337..808164e3dd84 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -394,7 +394,10 @@ static inline bool is_sync_kiocb(struct kiocb *kiocb)
>  	return kiocb->ki_complete == NULL;
>  }
>  
> +typedef unsigned int __bitwise asop_flags_t;
> +
>  struct address_space_operations {
> +	asop_flags_t asop_flags;
>  	int (*writepage)(struct page *page, struct writeback_control *wbc);
>  	int (*read_folio)(struct file *, struct folio *);
>  
> @@ -438,6 +441,12 @@ struct address_space_operations {
>  	int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
>  };
>  
> +/**
> + * This flag is only to be used by filesystems whose folios cannot be
> + * reclaimed when in writeback (eg fuse)
> + */
> +#define ASOP_NO_RECLAIM_IN_WRITEBACK	((__force asop_flags_t)(1 << 0))
> +
>  extern const struct address_space_operations empty_aops;
>  
>  /**
> @@ -586,6 +595,11 @@ static inline void mapping_allow_writable(struct address_space *mapping)
>  	atomic_inc(&mapping->i_mmap_writable);
>  }
>  
> +static inline bool mapping_no_reclaim_in_writeback(struct address_space *mapping)
> +{
> +	return mapping->a_ops->asop_flags & ASOP_NO_RECLAIM_IN_WRITEBACK;

Any reason not to add this flag in enum mapping_flags and use
mapping->flags field instead of adding a field in struct
address_space_operations?

> +}
> +
>  /*
>   * Use sequence counter to get consistent i_size on 32-bit processors.
>   */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 749cdc110c74..2beffbdae572 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1110,6 +1110,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  		if (writeback && folio_test_reclaim(folio))
>  			stat->nr_congested += nr_pages;
>  
> +		mapping = folio_mapping(folio);
> +
>  		/*
>  		 * If a folio at the tail of the LRU is under writeback, there
>  		 * are three cases to consider.
> @@ -1165,7 +1167,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  			/* Case 2 above */
>  			} else if (writeback_throttling_sane(sc) ||
>  			    !folio_test_reclaim(folio) ||
> -			    !may_enter_fs(folio, sc->gfp_mask)) {
> +			    !may_enter_fs(folio, sc->gfp_mask) ||
> +			    (mapping && mapping_no_reclaim_in_writeback(mapping))) {
>  				/*
>  				 * This is slightly racy -
>  				 * folio_end_writeback() might have
> @@ -1320,7 +1323,6 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  		if (folio_maybe_dma_pinned(folio))
>  			goto activate_locked;
>  
> -		mapping = folio_mapping(folio);
>  		if (folio_test_dirty(folio)) {
>  			/*
>  			 * Only kswapd can writeback filesystem folios
> -- 
> 2.43.5
> 

Re: [PATCH 1/2] mm: skip reclaiming folios in writeback contexts that may trigger deadlock

[Joanne Koong]

On Sat, Oct 12, 2024 at 9:55 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Fri, Oct 11, 2024 at 03:34:33PM GMT, Joanne Koong wrote:
> > Currently in shrink_folio_list(), reclaim for folios under writeback
> > falls into 3 different cases:
> > 1) Reclaim is encountering an excessive number of folios under
> >    writeback and this folio has both the writeback and reclaim flags
> >    set
> > 2) Dirty throttling is enabled (this happens if reclaim through cgroup
> >    is not enabled, if reclaim through cgroupv2 memcg is enabled, or
> >    if reclaim is on the root cgroup), or if the folio is not marked for
> >    immediate reclaim, or if the caller does not have __GFP_FS (or
> >    __GFP_IO if it's going to swap) set
> > 3) Legacy cgroupv1 encounters a folio that already has the reclaim flag
> >    set and the caller did not have __GFP_FS (or __GFP_IO if swap) set
> >
> > In cases 1) and 2), we activate the folio and skip reclaiming it while
> > in case 3), we wait for writeback to finish on the folio and then try
> > to reclaim the folio again. In case 3, we wait on writeback because
> > cgroupv1 does not have dirty folio throttling, as such this is a
> > mitigation against the case where there are too many folios in writeback
> > with nothing else to reclaim.
> >
> > The issue is that for filesystems where writeback may block, sub-optimal
> > workarounds need to be put in place to avoid potential deadlocks that may
> > arise from the case where reclaim waits on writeback. (Even though case
> > 3 above is rare given that legacy cgroupv1 is on its way to being
> > deprecated, this case still needs to be accounted for)
> >
> > For example, for FUSE filesystems, when a writeback is triggered on a
> > folio, a temporary folio is allocated and the pages are copied over to
> > this temporary folio so that writeback can be immediately cleared on the
> > original folio. This additionally requires an internal rb tree to keep
> > track of writeback state on the temporary folios. Benchmarks show
> > roughly a ~20% decrease in throughput from the overhead incurred with 4k
> > block size writes. The temporary folio is needed here in order to avoid
> > the following deadlock if reclaim waits on writeback:
> > * single-threaded FUSE server is in the middle of handling a request that
> >   needs a memory allocation
> > * memory allocation triggers direct reclaim
> > * direct reclaim waits on a folio under writeback (eg falls into case 3
> >   above) that needs to be written back to the fuse server
> > * the FUSE server can't write back the folio since it's stuck in direct
> >   reclaim
> >
> > This commit allows filesystems to set a ASOP_NO_RECLAIM_IN_WRITEBACK
> > flag in the address_space_operations struct to signify that reclaim
> > should not happen when the folio is already in writeback. This only has
> > effects on the case where cgroupv1 memcg encounters a folio under
> > writeback that already has the reclaim flag set (eg case 3 above), and
> > allows for the suboptimal workarounds added to address the "reclaim wait
> > on writeback" deadlock scenario to be removed.
> >
> > Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> > ---
> >  include/linux/fs.h | 14 ++++++++++++++
> >  mm/vmscan.c        |  6 ++++--
> >  2 files changed, 18 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index e3c603d01337..808164e3dd84 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -394,7 +394,10 @@ static inline bool is_sync_kiocb(struct kiocb *kiocb)
> >       return kiocb->ki_complete == NULL;
> >  }
> >
> > +typedef unsigned int __bitwise asop_flags_t;
> > +
> >  struct address_space_operations {
> > +     asop_flags_t asop_flags;
> >       int (*writepage)(struct page *page, struct writeback_control *wbc);
> >       int (*read_folio)(struct file *, struct folio *);
> >
> > @@ -438,6 +441,12 @@ struct address_space_operations {
> >       int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
> >  };
> >
> > +/**
> > + * This flag is only to be used by filesystems whose folios cannot be
> > + * reclaimed when in writeback (eg fuse)
> > + */
> > +#define ASOP_NO_RECLAIM_IN_WRITEBACK ((__force asop_flags_t)(1 << 0))
> > +
> >  extern const struct address_space_operations empty_aops;
> >
> >  /**
> > @@ -586,6 +595,11 @@ static inline void mapping_allow_writable(struct address_space *mapping)
> >       atomic_inc(&mapping->i_mmap_writable);
> >  }
> >
> > +static inline bool mapping_no_reclaim_in_writeback(struct address_space *mapping)
> > +{
> > +     return mapping->a_ops->asop_flags & ASOP_NO_RECLAIM_IN_WRITEBACK;
>
> Any reason not to add this flag in enum mapping_flags and use
> mapping->flags field instead of adding a field in struct
> address_space_operations?

No, thanks for the suggestion - I really like your idea of adding this
to enum mapping_flags instead as AS_NO_WRITEBACK_RECLAIM. I don't know
why I didn't see mapping_flags when I was looking at this. I'll make
this change for v2.


Thanks,
Joanne

>
> > +}
> > +
> >  /*
> >   * Use sequence counter to get consistent i_size on 32-bit processors.
> >   */
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 749cdc110c74..2beffbdae572 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1110,6 +1110,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >               if (writeback && folio_test_reclaim(folio))
> >                       stat->nr_congested += nr_pages;
> >
> > +             mapping = folio_mapping(folio);
> > +
> >               /*
> >                * If a folio at the tail of the LRU is under writeback, there
> >                * are three cases to consider.
> > @@ -1165,7 +1167,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >                       /* Case 2 above */
> >                       } else if (writeback_throttling_sane(sc) ||
> >                           !folio_test_reclaim(folio) ||
> > -                         !may_enter_fs(folio, sc->gfp_mask)) {
> > +                         !may_enter_fs(folio, sc->gfp_mask) ||
> > +                         (mapping && mapping_no_reclaim_in_writeback(mapping))) {
> >                               /*
> >                                * This is slightly racy -
> >                                * folio_end_writeback() might have
> > @@ -1320,7 +1323,6 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >               if (folio_maybe_dma_pinned(folio))
> >                       goto activate_locked;
> >
> > -             mapping = folio_mapping(folio);
> >               if (folio_test_dirty(folio)) {
> >                       /*
> >                        * Only kswapd can writeback filesystem folios
> > --
> > 2.43.5
> >

[PATCH 2/2] fuse: remove tmp folio for writebacks and internal rb tree

[Joanne Koong]

Currently, we allocate and copy data to a temporary folio when
handling writeback in order to mitigate the following deadlock scenario
that may arise if reclaim waits on writeback to complete:
* single-threaded FUSE server is in the middle of handling a request
  that needs a memory allocation
* memory allocation triggers direct reclaim
* direct reclaim waits on a folio under writeback
* the FUSE server can't write back the folio since it's stuck in
  direct reclaim

To work around this, we allocate a temporary folio and copy over the
original folio to the temporary folio so that writeback can be
immediately cleared on the original folio. This additionally requires us
to maintain an internal rb tree to keep track of writeback state on the
temporary folios.

This change sets the address space operations flag
ASOP_NO_RECLAIM_IN_WRITEBACK so that FUSE folios are not reclaimed and
waited on while in writeback, and removes the temporary folio +
extra copying and the internal rb tree.

fio benchmarks --
(using averages observed from 10 runs, throwing away outliers)

Setup:
sudo mount -t tmpfs -o size=30G tmpfs ~/tmp_mount
 ./libfuse/build/example/passthrough_ll -o writeback -o max_threads=4 -o source=~/tmp_mount ~/fuse_mount

fio --name=writeback --ioengine=sync --rw=write --bs={1k,4k,1M} --size=2G
--numjobs=2 --ramp_time=30 --group_reporting=1 --directory=/root/fuse_mount

        bs =  1k          4k            1M
Before  351 MiB/s     1818 MiB/s     1851 MiB/s
After   341 MiB/s     2246 MiB/s     2685 MiB/s
% diff        -3%          23%         45%

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 fs/fuse/file.c | 321 +++++--------------------------------------------
 1 file changed, 27 insertions(+), 294 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 4304e44f32e6..9fee9f3062db 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -415,74 +415,11 @@ u64 fuse_lock_owner_id(struct fuse_conn *fc, fl_owner_t id)
 
 struct fuse_writepage_args {
 	struct fuse_io_args ia;
-	struct rb_node writepages_entry;
 	struct list_head queue_entry;
-	struct fuse_writepage_args *next;
 	struct inode *inode;
 	struct fuse_sync_bucket *bucket;
 };
 
-static struct fuse_writepage_args *fuse_find_writeback(struct fuse_inode *fi,
-					    pgoff_t idx_from, pgoff_t idx_to)
-{
-	struct rb_node *n;
-
-	n = fi->writepages.rb_node;
-
-	while (n) {
-		struct fuse_writepage_args *wpa;
-		pgoff_t curr_index;
-
-		wpa = rb_entry(n, struct fuse_writepage_args, writepages_entry);
-		WARN_ON(get_fuse_inode(wpa->inode) != fi);
-		curr_index = wpa->ia.write.in.offset >> PAGE_SHIFT;
-		if (idx_from >= curr_index + wpa->ia.ap.num_pages)
-			n = n->rb_right;
-		else if (idx_to < curr_index)
-			n = n->rb_left;
-		else
-			return wpa;
-	}
-	return NULL;
-}
-
-/*
- * Check if any page in a range is under writeback
- */
-static bool fuse_range_is_writeback(struct inode *inode, pgoff_t idx_from,
-				   pgoff_t idx_to)
-{
-	struct fuse_inode *fi = get_fuse_inode(inode);
-	bool found;
-
-	if (RB_EMPTY_ROOT(&fi->writepages))
-		return false;
-
-	spin_lock(&fi->lock);
-	found = fuse_find_writeback(fi, idx_from, idx_to);
-	spin_unlock(&fi->lock);
-
-	return found;
-}
-
-static inline bool fuse_page_is_writeback(struct inode *inode, pgoff_t index)
-{
-	return fuse_range_is_writeback(inode, index, index);
-}
-
-/*
- * Wait for page writeback to be completed.
- *
- * Since fuse doesn't rely on the VM writeback tracking, this has to
- * use some other means.
- */
-static void fuse_wait_on_page_writeback(struct inode *inode, pgoff_t index)
-{
-	struct fuse_inode *fi = get_fuse_inode(inode);
-
-	wait_event(fi->page_waitq, !fuse_page_is_writeback(inode, index));
-}
-
 /*
  * Wait for all pending writepages on the inode to finish.
  *
@@ -876,7 +813,7 @@ static int fuse_do_readpage(struct file *file, struct page *page)
 	 * page-cache page, so make sure we read a properly synced
 	 * page.
 	 */
-	fuse_wait_on_page_writeback(inode, page->index);
+	folio_wait_writeback(page_folio(page));
 
 	attr_ver = fuse_get_attr_version(fm->fc);
 
@@ -1024,8 +961,7 @@ static void fuse_readahead(struct readahead_control *rac)
 		ap = &ia->ap;
 		nr_pages = __readahead_batch(rac, ap->pages, nr_pages);
 		for (i = 0; i < nr_pages; i++) {
-			fuse_wait_on_page_writeback(inode,
-						    readahead_index(rac) + i);
+			folio_wait_writeback(page_folio(ap->pages[i]));
 			ap->descs[i].length = PAGE_SIZE;
 		}
 		ap->num_pages = nr_pages;
@@ -1147,7 +1083,7 @@ static ssize_t fuse_send_write_pages(struct fuse_io_args *ia,
 	int err;
 
 	for (i = 0; i < ap->num_pages; i++)
-		fuse_wait_on_page_writeback(inode, ap->pages[i]->index);
+		folio_wait_writeback(page_folio(ap->pages[i]));
 
 	fuse_write_args_fill(ia, ff, pos, count);
 	ia->write.in.flags = fuse_write_flags(iocb);
@@ -1583,7 +1519,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
 			return res;
 		}
 	}
-	if (!cuse && fuse_range_is_writeback(inode, idx_from, idx_to)) {
+	if (!cuse && filemap_range_has_writeback(mapping, pos, (pos + count - 1))) {
 		if (!write)
 			inode_lock(inode);
 		fuse_sync_writes(inode);
@@ -1780,13 +1716,17 @@ static ssize_t fuse_splice_write(struct pipe_inode_info *pipe, struct file *out,
 static void fuse_writepage_free(struct fuse_writepage_args *wpa)
 {
 	struct fuse_args_pages *ap = &wpa->ia.ap;
+	struct folio *folio;
 	int i;
 
 	if (wpa->bucket)
 		fuse_sync_bucket_dec(wpa->bucket);
 
-	for (i = 0; i < ap->num_pages; i++)
-		__free_page(ap->pages[i]);
+	for (i = 0; i < ap->num_pages; i++) {
+		folio = page_folio(ap->pages[i]);
+		folio_end_writeback(folio);
+		folio_put(folio);
+	}
 
 	fuse_file_put(wpa->ia.ff, false);
 
@@ -1799,7 +1739,7 @@ static void fuse_writepage_finish_stat(struct inode *inode, struct page *page)
 	struct backing_dev_info *bdi = inode_to_bdi(inode);
 
 	dec_wb_stat(&bdi->wb, WB_WRITEBACK);
-	dec_node_page_state(page, NR_WRITEBACK_TEMP);
+	dec_node_page_state(page, NR_WRITEBACK);
 	wb_writeout_inc(&bdi->wb);
 }
 
@@ -1822,7 +1762,6 @@ static void fuse_send_writepage(struct fuse_mount *fm,
 __releases(fi->lock)
 __acquires(fi->lock)
 {
-	struct fuse_writepage_args *aux, *next;
 	struct fuse_inode *fi = get_fuse_inode(wpa->inode);
 	struct fuse_write_in *inarg = &wpa->ia.write.in;
 	struct fuse_args *args = &wpa->ia.ap.args;
@@ -1858,18 +1797,8 @@ __acquires(fi->lock)
 
  out_free:
 	fi->writectr--;
-	rb_erase(&wpa->writepages_entry, &fi->writepages);
 	fuse_writepage_finish(wpa);
 	spin_unlock(&fi->lock);
-
-	/* After rb_erase() aux request list is private */
-	for (aux = wpa->next; aux; aux = next) {
-		next = aux->next;
-		aux->next = NULL;
-		fuse_writepage_finish_stat(aux->inode, aux->ia.ap.pages[0]);
-		fuse_writepage_free(aux);
-	}
-
 	fuse_writepage_free(wpa);
 	spin_lock(&fi->lock);
 }
@@ -1897,43 +1826,6 @@ __acquires(fi->lock)
 	}
 }
 
-static struct fuse_writepage_args *fuse_insert_writeback(struct rb_root *root,
-						struct fuse_writepage_args *wpa)
-{
-	pgoff_t idx_from = wpa->ia.write.in.offset >> PAGE_SHIFT;
-	pgoff_t idx_to = idx_from + wpa->ia.ap.num_pages - 1;
-	struct rb_node **p = &root->rb_node;
-	struct rb_node  *parent = NULL;
-
-	WARN_ON(!wpa->ia.ap.num_pages);
-	while (*p) {
-		struct fuse_writepage_args *curr;
-		pgoff_t curr_index;
-
-		parent = *p;
-		curr = rb_entry(parent, struct fuse_writepage_args,
-				writepages_entry);
-		WARN_ON(curr->inode != wpa->inode);
-		curr_index = curr->ia.write.in.offset >> PAGE_SHIFT;
-
-		if (idx_from >= curr_index + curr->ia.ap.num_pages)
-			p = &(*p)->rb_right;
-		else if (idx_to < curr_index)
-			p = &(*p)->rb_left;
-		else
-			return curr;
-	}
-
-	rb_link_node(&wpa->writepages_entry, parent, p);
-	rb_insert_color(&wpa->writepages_entry, root);
-	return NULL;
-}
-
-static void tree_insert(struct rb_root *root, struct fuse_writepage_args *wpa)
-{
-	WARN_ON(fuse_insert_writeback(root, wpa));
-}
-
 static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args,
 			       int error)
 {
@@ -1953,41 +1845,6 @@ static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args,
 	if (!fc->writeback_cache)
 		fuse_invalidate_attr_mask(inode, FUSE_STATX_MODIFY);
 	spin_lock(&fi->lock);
-	rb_erase(&wpa->writepages_entry, &fi->writepages);
-	while (wpa->next) {
-		struct fuse_mount *fm = get_fuse_mount(inode);
-		struct fuse_write_in *inarg = &wpa->ia.write.in;
-		struct fuse_writepage_args *next = wpa->next;
-
-		wpa->next = next->next;
-		next->next = NULL;
-		tree_insert(&fi->writepages, next);
-
-		/*
-		 * Skip fuse_flush_writepages() to make it easy to crop requests
-		 * based on primary request size.
-		 *
-		 * 1st case (trivial): there are no concurrent activities using
-		 * fuse_set/release_nowrite.  Then we're on safe side because
-		 * fuse_flush_writepages() would call fuse_send_writepage()
-		 * anyway.
-		 *
-		 * 2nd case: someone called fuse_set_nowrite and it is waiting
-		 * now for completion of all in-flight requests.  This happens
-		 * rarely and no more than once per page, so this should be
-		 * okay.
-		 *
-		 * 3rd case: someone (e.g. fuse_do_setattr()) is in the middle
-		 * of fuse_set_nowrite..fuse_release_nowrite section.  The fact
-		 * that fuse_set_nowrite returned implies that all in-flight
-		 * requests were completed along with all of their secondary
-		 * requests.  Further primary requests are blocked by negative
-		 * writectr.  Hence there cannot be any in-flight requests and
-		 * no invocations of fuse_writepage_end() while we're in
-		 * fuse_set_nowrite..fuse_release_nowrite section.
-		 */
-		fuse_send_writepage(fm, next, inarg->offset + inarg->size);
-	}
 	fi->writectr--;
 	fuse_writepage_finish(wpa);
 	spin_unlock(&fi->lock);
@@ -2074,19 +1931,18 @@ static void fuse_writepage_add_to_bucket(struct fuse_conn *fc,
 }
 
 static void fuse_writepage_args_page_fill(struct fuse_writepage_args *wpa, struct folio *folio,
-					  struct folio *tmp_folio, uint32_t page_index)
+					  uint32_t page_index)
 {
 	struct inode *inode = folio->mapping->host;
 	struct fuse_args_pages *ap = &wpa->ia.ap;
 
-	folio_copy(tmp_folio, folio);
-
-	ap->pages[page_index] = &tmp_folio->page;
+	folio_get(folio);
+	ap->pages[page_index] = &folio->page;
 	ap->descs[page_index].offset = 0;
 	ap->descs[page_index].length = PAGE_SIZE;
 
 	inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
-	inc_node_page_state(&tmp_folio->page, NR_WRITEBACK_TEMP);
+	inc_node_page_state(&folio->page, NR_WRITEBACK);
 }
 
 static struct fuse_writepage_args *fuse_writepage_args_setup(struct folio *folio,
@@ -2121,18 +1977,12 @@ static int fuse_writepage_locked(struct folio *folio)
 	struct fuse_inode *fi = get_fuse_inode(inode);
 	struct fuse_writepage_args *wpa;
 	struct fuse_args_pages *ap;
-	struct folio *tmp_folio;
 	struct fuse_file *ff;
-	int error = -ENOMEM;
-
-	tmp_folio = folio_alloc(GFP_NOFS | __GFP_HIGHMEM, 0);
-	if (!tmp_folio)
-		goto err;
+	int error = -EIO;
 
-	error = -EIO;
 	ff = fuse_write_file_get(fi);
 	if (!ff)
-		goto err_nofile;
+		goto err;
 
 	wpa = fuse_writepage_args_setup(folio, ff);
 	error = -ENOMEM;
@@ -2143,22 +1993,17 @@ static int fuse_writepage_locked(struct folio *folio)
 	ap->num_pages = 1;
 
 	folio_start_writeback(folio);
-	fuse_writepage_args_page_fill(wpa, folio, tmp_folio, 0);
+	fuse_writepage_args_page_fill(wpa, folio, 0);
 
 	spin_lock(&fi->lock);
-	tree_insert(&fi->writepages, wpa);
 	list_add_tail(&wpa->queue_entry, &fi->queued_writes);
 	fuse_flush_writepages(inode);
 	spin_unlock(&fi->lock);
 
-	folio_end_writeback(folio);
-
 	return 0;
 
 err_writepage_args:
 	fuse_file_put(ff, false);
-err_nofile:
-	folio_put(tmp_folio);
 err:
 	mapping_set_error(folio->mapping, error);
 	return error;
@@ -2168,7 +2013,6 @@ struct fuse_fill_wb_data {
 	struct fuse_writepage_args *wpa;
 	struct fuse_file *ff;
 	struct inode *inode;
-	struct page **orig_pages;
 	unsigned int max_pages;
 };
 
@@ -2203,68 +2047,11 @@ static void fuse_writepages_send(struct fuse_fill_wb_data *data)
 	struct fuse_writepage_args *wpa = data->wpa;
 	struct inode *inode = data->inode;
 	struct fuse_inode *fi = get_fuse_inode(inode);
-	int num_pages = wpa->ia.ap.num_pages;
-	int i;
 
 	spin_lock(&fi->lock);
 	list_add_tail(&wpa->queue_entry, &fi->queued_writes);
 	fuse_flush_writepages(inode);
 	spin_unlock(&fi->lock);
-
-	for (i = 0; i < num_pages; i++)
-		end_page_writeback(data->orig_pages[i]);
-}
-
-/*
- * Check under fi->lock if the page is under writeback, and insert it onto the
- * rb_tree if not. Otherwise iterate auxiliary write requests, to see if there's
- * one already added for a page at this offset.  If there's none, then insert
- * this new request onto the auxiliary list, otherwise reuse the existing one by
- * swapping the new temp page with the old one.
- */
-static bool fuse_writepage_add(struct fuse_writepage_args *new_wpa,
-			       struct page *page)
-{
-	struct fuse_inode *fi = get_fuse_inode(new_wpa->inode);
-	struct fuse_writepage_args *tmp;
-	struct fuse_writepage_args *old_wpa;
-	struct fuse_args_pages *new_ap = &new_wpa->ia.ap;
-
-	WARN_ON(new_ap->num_pages != 0);
-	new_ap->num_pages = 1;
-
-	spin_lock(&fi->lock);
-	old_wpa = fuse_insert_writeback(&fi->writepages, new_wpa);
-	if (!old_wpa) {
-		spin_unlock(&fi->lock);
-		return true;
-	}
-
-	for (tmp = old_wpa->next; tmp; tmp = tmp->next) {
-		pgoff_t curr_index;
-
-		WARN_ON(tmp->inode != new_wpa->inode);
-		curr_index = tmp->ia.write.in.offset >> PAGE_SHIFT;
-		if (curr_index == page->index) {
-			WARN_ON(tmp->ia.ap.num_pages != 1);
-			swap(tmp->ia.ap.pages[0], new_ap->pages[0]);
-			break;
-		}
-	}
-
-	if (!tmp) {
-		new_wpa->next = old_wpa->next;
-		old_wpa->next = new_wpa;
-	}
-
-	spin_unlock(&fi->lock);
-
-	if (tmp) {
-		fuse_writepage_finish_stat(new_wpa->inode, new_ap->pages[0]);
-		fuse_writepage_free(new_wpa);
-	}
-
-	return false;
 }
 
 static bool fuse_writepage_need_send(struct fuse_conn *fc, struct page *page,
@@ -2273,15 +2060,6 @@ static bool fuse_writepage_need_send(struct fuse_conn *fc, struct page *page,
 {
 	WARN_ON(!ap->num_pages);
 
-	/*
-	 * Being under writeback is unlikely but possible.  For example direct
-	 * read to an mmaped fuse file will set the page dirty twice; once when
-	 * the pages are faulted with get_user_pages(), and then after the read
-	 * completed.
-	 */
-	if (fuse_page_is_writeback(data->inode, page->index))
-		return true;
-
 	/* Reached max pages */
 	if (ap->num_pages == fc->max_pages)
 		return true;
@@ -2291,7 +2069,7 @@ static bool fuse_writepage_need_send(struct fuse_conn *fc, struct page *page,
 		return true;
 
 	/* Discontinuity */
-	if (data->orig_pages[ap->num_pages - 1]->index + 1 != page->index)
+	if (ap->pages[ap->num_pages - 1]->index + 1 != page->index)
 		return true;
 
 	/* Need to grow the pages array?  If so, did the expansion fail? */
@@ -2308,9 +2086,7 @@ static int fuse_writepages_fill(struct folio *folio,
 	struct fuse_writepage_args *wpa = data->wpa;
 	struct fuse_args_pages *ap = &wpa->ia.ap;
 	struct inode *inode = data->inode;
-	struct fuse_inode *fi = get_fuse_inode(inode);
 	struct fuse_conn *fc = get_fuse_conn(inode);
-	struct folio *tmp_folio;
 	int err;
 
 	if (wpa && fuse_writepage_need_send(fc, &folio->page, ap, data)) {
@@ -2318,54 +2094,23 @@ static int fuse_writepages_fill(struct folio *folio,
 		data->wpa = NULL;
 	}
 
-	err = -ENOMEM;
-	tmp_folio = folio_alloc(GFP_NOFS | __GFP_HIGHMEM, 0);
-	if (!tmp_folio)
-		goto out_unlock;
-
-	/*
-	 * The page must not be redirtied until the writeout is completed
-	 * (i.e. userspace has sent a reply to the write request).  Otherwise
-	 * there could be more than one temporary page instance for each real
-	 * page.
-	 *
-	 * This is ensured by holding the page lock in page_mkwrite() while
-	 * checking fuse_page_is_writeback().  We already hold the page lock
-	 * since clear_page_dirty_for_io() and keep it held until we add the
-	 * request to the fi->writepages list and increment ap->num_pages.
-	 * After this fuse_page_is_writeback() will indicate that the page is
-	 * under writeback, so we can release the page lock.
-	 */
 	if (data->wpa == NULL) {
 		err = -ENOMEM;
 		wpa = fuse_writepage_args_setup(folio, data->ff);
-		if (!wpa) {
-			folio_put(tmp_folio);
+		if (!wpa)
 			goto out_unlock;
-		}
 		fuse_file_get(wpa->ia.ff);
 		data->max_pages = 1;
 		ap = &wpa->ia.ap;
 	}
 	folio_start_writeback(folio);
 
-	fuse_writepage_args_page_fill(wpa, folio, tmp_folio, ap->num_pages);
-	data->orig_pages[ap->num_pages] = &folio->page;
+	fuse_writepage_args_page_fill(wpa, folio, ap->num_pages);
 
 	err = 0;
-	if (data->wpa) {
-		/*
-		 * Protected by fi->lock against concurrent access by
-		 * fuse_page_is_writeback().
-		 */
-		spin_lock(&fi->lock);
-		ap->num_pages++;
-		spin_unlock(&fi->lock);
-	} else if (fuse_writepage_add(wpa, &folio->page)) {
+	ap->num_pages++;
+	if (!data->wpa)
 		data->wpa = wpa;
-	} else {
-		folio_end_writeback(folio);
-	}
 out_unlock:
 	folio_unlock(folio);
 
@@ -2394,21 +2139,12 @@ static int fuse_writepages(struct address_space *mapping,
 	if (!data.ff)
 		return -EIO;
 
-	err = -ENOMEM;
-	data.orig_pages = kcalloc(fc->max_pages,
-				  sizeof(struct page *),
-				  GFP_NOFS);
-	if (!data.orig_pages)
-		goto out;
-
 	err = write_cache_pages(mapping, wbc, fuse_writepages_fill, &data);
 	if (data.wpa) {
 		WARN_ON(!data.wpa->ia.ap.num_pages);
 		fuse_writepages_send(&data);
 	}
 
-	kfree(data.orig_pages);
-out:
 	fuse_file_put(data.ff, false);
 	return err;
 }
@@ -2433,7 +2169,7 @@ static int fuse_write_begin(struct file *file, struct address_space *mapping,
 	if (IS_ERR(folio))
 		goto error;
 
-	fuse_wait_on_page_writeback(mapping->host, folio->index);
+	folio_wait_writeback(folio);
 
 	if (folio_test_uptodate(folio) || len >= folio_size(folio))
 		goto success;
@@ -2497,13 +2233,11 @@ static int fuse_launder_folio(struct folio *folio)
 {
 	int err = 0;
 	if (folio_clear_dirty_for_io(folio)) {
-		struct inode *inode = folio->mapping->host;
-
 		/* Serialize with pending writeback for the same page */
-		fuse_wait_on_page_writeback(inode, folio->index);
+		folio_wait_writeback(folio);
 		err = fuse_writepage_locked(folio);
 		if (!err)
-			fuse_wait_on_page_writeback(inode, folio->index);
+			folio_wait_writeback(folio);
 	}
 	return err;
 }
@@ -2547,7 +2281,7 @@ static vm_fault_t fuse_page_mkwrite(struct vm_fault *vmf)
 		return VM_FAULT_NOPAGE;
 	}
 
-	fuse_wait_on_page_writeback(inode, page->index);
+	folio_wait_writeback(page_folio(page));
 	return VM_FAULT_LOCKED;
 }
 
@@ -3375,7 +3109,6 @@ void fuse_init_file_inode(struct inode *inode, unsigned int flags)
 	fi->iocachectr = 0;
 	init_waitqueue_head(&fi->page_waitq);
 	init_waitqueue_head(&fi->direct_io_waitq);
-	fi->writepages = RB_ROOT;
 
 	if (IS_ENABLED(CONFIG_FUSE_DAX))
 		fuse_dax_inode_init(inode, flags);
-- 
2.43.5

Re: [PATCH 2/2] fuse: remove tmp folio for writebacks and internal rb tree

[Shakeel Butt]

On Fri, Oct 11, 2024 at 03:34:34PM GMT, Joanne Koong wrote:
> Currently, we allocate and copy data to a temporary folio when
> handling writeback in order to mitigate the following deadlock scenario
> that may arise if reclaim waits on writeback to complete:
> * single-threaded FUSE server is in the middle of handling a request
>   that needs a memory allocation
> * memory allocation triggers direct reclaim
> * direct reclaim waits on a folio under writeback
> * the FUSE server can't write back the folio since it's stuck in
>   direct reclaim
> 
> To work around this, we allocate a temporary folio and copy over the
> original folio to the temporary folio so that writeback can be
> immediately cleared on the original folio. This additionally requires us
> to maintain an internal rb tree to keep track of writeback state on the
> temporary folios.
> 
> This change sets the address space operations flag
> ASOP_NO_RECLAIM_IN_WRITEBACK so that FUSE folios are not reclaimed and

I couldn't find where ASOP_NO_RECLAIM_IN_WRITEBACK is being set for
fuse.

> waited on while in writeback, and removes the temporary folio +
> extra copying and the internal rb tree.
> 
> fio benchmarks --
> (using averages observed from 10 runs, throwing away outliers)
> 
> Setup:
> sudo mount -t tmpfs -o size=30G tmpfs ~/tmp_mount
>  ./libfuse/build/example/passthrough_ll -o writeback -o max_threads=4 -o source=~/tmp_mount ~/fuse_mount
> 
> fio --name=writeback --ioengine=sync --rw=write --bs={1k,4k,1M} --size=2G
> --numjobs=2 --ramp_time=30 --group_reporting=1 --directory=/root/fuse_mount
> 
>         bs =  1k          4k            1M
> Before  351 MiB/s     1818 MiB/s     1851 MiB/s
> After   341 MiB/s     2246 MiB/s     2685 MiB/s
> % diff        -3%          23%         45%
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  fs/fuse/file.c | 321 +++++--------------------------------------------
>  1 file changed, 27 insertions(+), 294 deletions(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 4304e44f32e6..9fee9f3062db 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -415,74 +415,11 @@ u64 fuse_lock_owner_id(struct fuse_conn *fc, fl_owner_t id)
>  
>  struct fuse_writepage_args {
>  	struct fuse_io_args ia;
> -	struct rb_node writepages_entry;
>  	struct list_head queue_entry;
> -	struct fuse_writepage_args *next;
>  	struct inode *inode;
>  	struct fuse_sync_bucket *bucket;
>  };
>  
> -static struct fuse_writepage_args *fuse_find_writeback(struct fuse_inode *fi,
> -					    pgoff_t idx_from, pgoff_t idx_to)
> -{
> -	struct rb_node *n;
> -
> -	n = fi->writepages.rb_node;
> -
> -	while (n) {
> -		struct fuse_writepage_args *wpa;
> -		pgoff_t curr_index;
> -
> -		wpa = rb_entry(n, struct fuse_writepage_args, writepages_entry);
> -		WARN_ON(get_fuse_inode(wpa->inode) != fi);
> -		curr_index = wpa->ia.write.in.offset >> PAGE_SHIFT;
> -		if (idx_from >= curr_index + wpa->ia.ap.num_pages)
> -			n = n->rb_right;
> -		else if (idx_to < curr_index)
> -			n = n->rb_left;
> -		else
> -			return wpa;
> -	}
> -	return NULL;
> -}
> -
> -/*
> - * Check if any page in a range is under writeback
> - */
> -static bool fuse_range_is_writeback(struct inode *inode, pgoff_t idx_from,
> -				   pgoff_t idx_to)
> -{
> -	struct fuse_inode *fi = get_fuse_inode(inode);
> -	bool found;
> -
> -	if (RB_EMPTY_ROOT(&fi->writepages))
> -		return false;
> -
> -	spin_lock(&fi->lock);
> -	found = fuse_find_writeback(fi, idx_from, idx_to);
> -	spin_unlock(&fi->lock);
> -
> -	return found;
> -}
> -
> -static inline bool fuse_page_is_writeback(struct inode *inode, pgoff_t index)
> -{
> -	return fuse_range_is_writeback(inode, index, index);
> -}
> -
> -/*
> - * Wait for page writeback to be completed.
> - *
> - * Since fuse doesn't rely on the VM writeback tracking, this has to
> - * use some other means.
> - */
> -static void fuse_wait_on_page_writeback(struct inode *inode, pgoff_t index)
> -{
> -	struct fuse_inode *fi = get_fuse_inode(inode);
> -
> -	wait_event(fi->page_waitq, !fuse_page_is_writeback(inode, index));
> -}
> -
>  /*
>   * Wait for all pending writepages on the inode to finish.
>   *
> @@ -876,7 +813,7 @@ static int fuse_do_readpage(struct file *file, struct page *page)
>  	 * page-cache page, so make sure we read a properly synced
>  	 * page.
>  	 */
> -	fuse_wait_on_page_writeback(inode, page->index);
> +	folio_wait_writeback(page_folio(page));
>  
>  	attr_ver = fuse_get_attr_version(fm->fc);
>  
> @@ -1024,8 +961,7 @@ static void fuse_readahead(struct readahead_control *rac)
>  		ap = &ia->ap;
>  		nr_pages = __readahead_batch(rac, ap->pages, nr_pages);
>  		for (i = 0; i < nr_pages; i++) {
> -			fuse_wait_on_page_writeback(inode,
> -						    readahead_index(rac) + i);
> +			folio_wait_writeback(page_folio(ap->pages[i]));
>  			ap->descs[i].length = PAGE_SIZE;
>  		}
>  		ap->num_pages = nr_pages;
> @@ -1147,7 +1083,7 @@ static ssize_t fuse_send_write_pages(struct fuse_io_args *ia,
>  	int err;
>  
>  	for (i = 0; i < ap->num_pages; i++)
> -		fuse_wait_on_page_writeback(inode, ap->pages[i]->index);
> +		folio_wait_writeback(page_folio(ap->pages[i]));
>  
>  	fuse_write_args_fill(ia, ff, pos, count);
>  	ia->write.in.flags = fuse_write_flags(iocb);
> @@ -1583,7 +1519,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
>  			return res;
>  		}
>  	}
> -	if (!cuse && fuse_range_is_writeback(inode, idx_from, idx_to)) {
> +	if (!cuse && filemap_range_has_writeback(mapping, pos, (pos + count - 1))) {
>  		if (!write)
>  			inode_lock(inode);
>  		fuse_sync_writes(inode);
> @@ -1780,13 +1716,17 @@ static ssize_t fuse_splice_write(struct pipe_inode_info *pipe, struct file *out,
>  static void fuse_writepage_free(struct fuse_writepage_args *wpa)
>  {
>  	struct fuse_args_pages *ap = &wpa->ia.ap;
> +	struct folio *folio;
>  	int i;
>  
>  	if (wpa->bucket)
>  		fuse_sync_bucket_dec(wpa->bucket);
>  
> -	for (i = 0; i < ap->num_pages; i++)
> -		__free_page(ap->pages[i]);
> +	for (i = 0; i < ap->num_pages; i++) {
> +		folio = page_folio(ap->pages[i]);
> +		folio_end_writeback(folio);
> +		folio_put(folio);
> +	}
>  
>  	fuse_file_put(wpa->ia.ff, false);
>  
> @@ -1799,7 +1739,7 @@ static void fuse_writepage_finish_stat(struct inode *inode, struct page *page)
>  	struct backing_dev_info *bdi = inode_to_bdi(inode);
>  
>  	dec_wb_stat(&bdi->wb, WB_WRITEBACK);
> -	dec_node_page_state(page, NR_WRITEBACK_TEMP);
> +	dec_node_page_state(page, NR_WRITEBACK);
>  	wb_writeout_inc(&bdi->wb);
>  }
>  
> @@ -1822,7 +1762,6 @@ static void fuse_send_writepage(struct fuse_mount *fm,
>  __releases(fi->lock)
>  __acquires(fi->lock)
>  {
> -	struct fuse_writepage_args *aux, *next;
>  	struct fuse_inode *fi = get_fuse_inode(wpa->inode);
>  	struct fuse_write_in *inarg = &wpa->ia.write.in;
>  	struct fuse_args *args = &wpa->ia.ap.args;
> @@ -1858,18 +1797,8 @@ __acquires(fi->lock)
>  
>   out_free:
>  	fi->writectr--;
> -	rb_erase(&wpa->writepages_entry, &fi->writepages);
>  	fuse_writepage_finish(wpa);
>  	spin_unlock(&fi->lock);
> -
> -	/* After rb_erase() aux request list is private */
> -	for (aux = wpa->next; aux; aux = next) {
> -		next = aux->next;
> -		aux->next = NULL;
> -		fuse_writepage_finish_stat(aux->inode, aux->ia.ap.pages[0]);
> -		fuse_writepage_free(aux);
> -	}
> -
>  	fuse_writepage_free(wpa);
>  	spin_lock(&fi->lock);
>  }
> @@ -1897,43 +1826,6 @@ __acquires(fi->lock)
>  	}
>  }
>  
> -static struct fuse_writepage_args *fuse_insert_writeback(struct rb_root *root,
> -						struct fuse_writepage_args *wpa)
> -{
> -	pgoff_t idx_from = wpa->ia.write.in.offset >> PAGE_SHIFT;
> -	pgoff_t idx_to = idx_from + wpa->ia.ap.num_pages - 1;
> -	struct rb_node **p = &root->rb_node;
> -	struct rb_node  *parent = NULL;
> -
> -	WARN_ON(!wpa->ia.ap.num_pages);
> -	while (*p) {
> -		struct fuse_writepage_args *curr;
> -		pgoff_t curr_index;
> -
> -		parent = *p;
> -		curr = rb_entry(parent, struct fuse_writepage_args,
> -				writepages_entry);
> -		WARN_ON(curr->inode != wpa->inode);
> -		curr_index = curr->ia.write.in.offset >> PAGE_SHIFT;
> -
> -		if (idx_from >= curr_index + curr->ia.ap.num_pages)
> -			p = &(*p)->rb_right;
> -		else if (idx_to < curr_index)
> -			p = &(*p)->rb_left;
> -		else
> -			return curr;
> -	}
> -
> -	rb_link_node(&wpa->writepages_entry, parent, p);
> -	rb_insert_color(&wpa->writepages_entry, root);
> -	return NULL;
> -}
> -
> -static void tree_insert(struct rb_root *root, struct fuse_writepage_args *wpa)
> -{
> -	WARN_ON(fuse_insert_writeback(root, wpa));
> -}
> -
>  static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args,
>  			       int error)
>  {
> @@ -1953,41 +1845,6 @@ static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args,
>  	if (!fc->writeback_cache)
>  		fuse_invalidate_attr_mask(inode, FUSE_STATX_MODIFY);
>  	spin_lock(&fi->lock);
> -	rb_erase(&wpa->writepages_entry, &fi->writepages);
> -	while (wpa->next) {
> -		struct fuse_mount *fm = get_fuse_mount(inode);
> -		struct fuse_write_in *inarg = &wpa->ia.write.in;
> -		struct fuse_writepage_args *next = wpa->next;
> -
> -		wpa->next = next->next;
> -		next->next = NULL;
> -		tree_insert(&fi->writepages, next);
> -
> -		/*
> -		 * Skip fuse_flush_writepages() to make it easy to crop requests
> -		 * based on primary request size.
> -		 *
> -		 * 1st case (trivial): there are no concurrent activities using
> -		 * fuse_set/release_nowrite.  Then we're on safe side because
> -		 * fuse_flush_writepages() would call fuse_send_writepage()
> -		 * anyway.
> -		 *
> -		 * 2nd case: someone called fuse_set_nowrite and it is waiting
> -		 * now for completion of all in-flight requests.  This happens
> -		 * rarely and no more than once per page, so this should be
> -		 * okay.
> -		 *
> -		 * 3rd case: someone (e.g. fuse_do_setattr()) is in the middle
> -		 * of fuse_set_nowrite..fuse_release_nowrite section.  The fact
> -		 * that fuse_set_nowrite returned implies that all in-flight
> -		 * requests were completed along with all of their secondary
> -		 * requests.  Further primary requests are blocked by negative
> -		 * writectr.  Hence there cannot be any in-flight requests and
> -		 * no invocations of fuse_writepage_end() while we're in
> -		 * fuse_set_nowrite..fuse_release_nowrite section.
> -		 */
> -		fuse_send_writepage(fm, next, inarg->offset + inarg->size);
> -	}
>  	fi->writectr--;
>  	fuse_writepage_finish(wpa);
>  	spin_unlock(&fi->lock);
> @@ -2074,19 +1931,18 @@ static void fuse_writepage_add_to_bucket(struct fuse_conn *fc,
>  }
>  
>  static void fuse_writepage_args_page_fill(struct fuse_writepage_args *wpa, struct folio *folio,
> -					  struct folio *tmp_folio, uint32_t page_index)
> +					  uint32_t page_index)
>  {
>  	struct inode *inode = folio->mapping->host;
>  	struct fuse_args_pages *ap = &wpa->ia.ap;
>  
> -	folio_copy(tmp_folio, folio);
> -
> -	ap->pages[page_index] = &tmp_folio->page;
> +	folio_get(folio);
> +	ap->pages[page_index] = &folio->page;
>  	ap->descs[page_index].offset = 0;
>  	ap->descs[page_index].length = PAGE_SIZE;
>  
>  	inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
> -	inc_node_page_state(&tmp_folio->page, NR_WRITEBACK_TEMP);
> +	inc_node_page_state(&folio->page, NR_WRITEBACK);
>  }
>  
>  static struct fuse_writepage_args *fuse_writepage_args_setup(struct folio *folio,
> @@ -2121,18 +1977,12 @@ static int fuse_writepage_locked(struct folio *folio)
>  	struct fuse_inode *fi = get_fuse_inode(inode);
>  	struct fuse_writepage_args *wpa;
>  	struct fuse_args_pages *ap;
> -	struct folio *tmp_folio;
>  	struct fuse_file *ff;
> -	int error = -ENOMEM;
> -
> -	tmp_folio = folio_alloc(GFP_NOFS | __GFP_HIGHMEM, 0);
> -	if (!tmp_folio)
> -		goto err;
> +	int error = -EIO;
>  
> -	error = -EIO;
>  	ff = fuse_write_file_get(fi);
>  	if (!ff)
> -		goto err_nofile;
> +		goto err;
>  
>  	wpa = fuse_writepage_args_setup(folio, ff);
>  	error = -ENOMEM;
> @@ -2143,22 +1993,17 @@ static int fuse_writepage_locked(struct folio *folio)
>  	ap->num_pages = 1;
>  
>  	folio_start_writeback(folio);
> -	fuse_writepage_args_page_fill(wpa, folio, tmp_folio, 0);
> +	fuse_writepage_args_page_fill(wpa, folio, 0);
>  
>  	spin_lock(&fi->lock);
> -	tree_insert(&fi->writepages, wpa);
>  	list_add_tail(&wpa->queue_entry, &fi->queued_writes);
>  	fuse_flush_writepages(inode);
>  	spin_unlock(&fi->lock);
>  
> -	folio_end_writeback(folio);
> -
>  	return 0;
>  
>  err_writepage_args:
>  	fuse_file_put(ff, false);
> -err_nofile:
> -	folio_put(tmp_folio);
>  err:
>  	mapping_set_error(folio->mapping, error);
>  	return error;
> @@ -2168,7 +2013,6 @@ struct fuse_fill_wb_data {
>  	struct fuse_writepage_args *wpa;
>  	struct fuse_file *ff;
>  	struct inode *inode;
> -	struct page **orig_pages;
>  	unsigned int max_pages;
>  };
>  
> @@ -2203,68 +2047,11 @@ static void fuse_writepages_send(struct fuse_fill_wb_data *data)
>  	struct fuse_writepage_args *wpa = data->wpa;
>  	struct inode *inode = data->inode;
>  	struct fuse_inode *fi = get_fuse_inode(inode);
> -	int num_pages = wpa->ia.ap.num_pages;
> -	int i;
>  
>  	spin_lock(&fi->lock);
>  	list_add_tail(&wpa->queue_entry, &fi->queued_writes);
>  	fuse_flush_writepages(inode);
>  	spin_unlock(&fi->lock);
> -
> -	for (i = 0; i < num_pages; i++)
> -		end_page_writeback(data->orig_pages[i]);
> -}
> -
> -/*
> - * Check under fi->lock if the page is under writeback, and insert it onto the
> - * rb_tree if not. Otherwise iterate auxiliary write requests, to see if there's
> - * one already added for a page at this offset.  If there's none, then insert
> - * this new request onto the auxiliary list, otherwise reuse the existing one by
> - * swapping the new temp page with the old one.
> - */
> -static bool fuse_writepage_add(struct fuse_writepage_args *new_wpa,
> -			       struct page *page)
> -{
> -	struct fuse_inode *fi = get_fuse_inode(new_wpa->inode);
> -	struct fuse_writepage_args *tmp;
> -	struct fuse_writepage_args *old_wpa;
> -	struct fuse_args_pages *new_ap = &new_wpa->ia.ap;
> -
> -	WARN_ON(new_ap->num_pages != 0);
> -	new_ap->num_pages = 1;
> -
> -	spin_lock(&fi->lock);
> -	old_wpa = fuse_insert_writeback(&fi->writepages, new_wpa);
> -	if (!old_wpa) {
> -		spin_unlock(&fi->lock);
> -		return true;
> -	}
> -
> -	for (tmp = old_wpa->next; tmp; tmp = tmp->next) {
> -		pgoff_t curr_index;
> -
> -		WARN_ON(tmp->inode != new_wpa->inode);
> -		curr_index = tmp->ia.write.in.offset >> PAGE_SHIFT;
> -		if (curr_index == page->index) {
> -			WARN_ON(tmp->ia.ap.num_pages != 1);
> -			swap(tmp->ia.ap.pages[0], new_ap->pages[0]);
> -			break;
> -		}
> -	}
> -
> -	if (!tmp) {
> -		new_wpa->next = old_wpa->next;
> -		old_wpa->next = new_wpa;
> -	}
> -
> -	spin_unlock(&fi->lock);
> -
> -	if (tmp) {
> -		fuse_writepage_finish_stat(new_wpa->inode, new_ap->pages[0]);
> -		fuse_writepage_free(new_wpa);
> -	}
> -
> -	return false;
>  }
>  
>  static bool fuse_writepage_need_send(struct fuse_conn *fc, struct page *page,
> @@ -2273,15 +2060,6 @@ static bool fuse_writepage_need_send(struct fuse_conn *fc, struct page *page,
>  {
>  	WARN_ON(!ap->num_pages);
>  
> -	/*
> -	 * Being under writeback is unlikely but possible.  For example direct
> -	 * read to an mmaped fuse file will set the page dirty twice; once when
> -	 * the pages are faulted with get_user_pages(), and then after the read
> -	 * completed.
> -	 */
> -	if (fuse_page_is_writeback(data->inode, page->index))
> -		return true;
> -
>  	/* Reached max pages */
>  	if (ap->num_pages == fc->max_pages)
>  		return true;
> @@ -2291,7 +2069,7 @@ static bool fuse_writepage_need_send(struct fuse_conn *fc, struct page *page,
>  		return true;
>  
>  	/* Discontinuity */
> -	if (data->orig_pages[ap->num_pages - 1]->index + 1 != page->index)
> +	if (ap->pages[ap->num_pages - 1]->index + 1 != page->index)
>  		return true;
>  
>  	/* Need to grow the pages array?  If so, did the expansion fail? */
> @@ -2308,9 +2086,7 @@ static int fuse_writepages_fill(struct folio *folio,
>  	struct fuse_writepage_args *wpa = data->wpa;
>  	struct fuse_args_pages *ap = &wpa->ia.ap;
>  	struct inode *inode = data->inode;
> -	struct fuse_inode *fi = get_fuse_inode(inode);
>  	struct fuse_conn *fc = get_fuse_conn(inode);
> -	struct folio *tmp_folio;
>  	int err;
>  
>  	if (wpa && fuse_writepage_need_send(fc, &folio->page, ap, data)) {
> @@ -2318,54 +2094,23 @@ static int fuse_writepages_fill(struct folio *folio,
>  		data->wpa = NULL;
>  	}
>  
> -	err = -ENOMEM;
> -	tmp_folio = folio_alloc(GFP_NOFS | __GFP_HIGHMEM, 0);
> -	if (!tmp_folio)
> -		goto out_unlock;
> -
> -	/*
> -	 * The page must not be redirtied until the writeout is completed
> -	 * (i.e. userspace has sent a reply to the write request).  Otherwise
> -	 * there could be more than one temporary page instance for each real
> -	 * page.
> -	 *
> -	 * This is ensured by holding the page lock in page_mkwrite() while
> -	 * checking fuse_page_is_writeback().  We already hold the page lock
> -	 * since clear_page_dirty_for_io() and keep it held until we add the
> -	 * request to the fi->writepages list and increment ap->num_pages.
> -	 * After this fuse_page_is_writeback() will indicate that the page is
> -	 * under writeback, so we can release the page lock.
> -	 */
>  	if (data->wpa == NULL) {
>  		err = -ENOMEM;
>  		wpa = fuse_writepage_args_setup(folio, data->ff);
> -		if (!wpa) {
> -			folio_put(tmp_folio);
> +		if (!wpa)
>  			goto out_unlock;
> -		}
>  		fuse_file_get(wpa->ia.ff);
>  		data->max_pages = 1;
>  		ap = &wpa->ia.ap;
>  	}
>  	folio_start_writeback(folio);
>  
> -	fuse_writepage_args_page_fill(wpa, folio, tmp_folio, ap->num_pages);
> -	data->orig_pages[ap->num_pages] = &folio->page;
> +	fuse_writepage_args_page_fill(wpa, folio, ap->num_pages);
>  
>  	err = 0;
> -	if (data->wpa) {
> -		/*
> -		 * Protected by fi->lock against concurrent access by
> -		 * fuse_page_is_writeback().
> -		 */
> -		spin_lock(&fi->lock);
> -		ap->num_pages++;
> -		spin_unlock(&fi->lock);
> -	} else if (fuse_writepage_add(wpa, &folio->page)) {
> +	ap->num_pages++;
> +	if (!data->wpa)
>  		data->wpa = wpa;
> -	} else {
> -		folio_end_writeback(folio);
> -	}
>  out_unlock:
>  	folio_unlock(folio);
>  
> @@ -2394,21 +2139,12 @@ static int fuse_writepages(struct address_space *mapping,
>  	if (!data.ff)
>  		return -EIO;
>  
> -	err = -ENOMEM;
> -	data.orig_pages = kcalloc(fc->max_pages,
> -				  sizeof(struct page *),
> -				  GFP_NOFS);
> -	if (!data.orig_pages)
> -		goto out;
> -
>  	err = write_cache_pages(mapping, wbc, fuse_writepages_fill, &data);
>  	if (data.wpa) {
>  		WARN_ON(!data.wpa->ia.ap.num_pages);
>  		fuse_writepages_send(&data);
>  	}
>  
> -	kfree(data.orig_pages);
> -out:
>  	fuse_file_put(data.ff, false);
>  	return err;
>  }
> @@ -2433,7 +2169,7 @@ static int fuse_write_begin(struct file *file, struct address_space *mapping,
>  	if (IS_ERR(folio))
>  		goto error;
>  
> -	fuse_wait_on_page_writeback(mapping->host, folio->index);
> +	folio_wait_writeback(folio);
>  
>  	if (folio_test_uptodate(folio) || len >= folio_size(folio))
>  		goto success;
> @@ -2497,13 +2233,11 @@ static int fuse_launder_folio(struct folio *folio)
>  {
>  	int err = 0;
>  	if (folio_clear_dirty_for_io(folio)) {
> -		struct inode *inode = folio->mapping->host;
> -
>  		/* Serialize with pending writeback for the same page */
> -		fuse_wait_on_page_writeback(inode, folio->index);
> +		folio_wait_writeback(folio);
>  		err = fuse_writepage_locked(folio);
>  		if (!err)
> -			fuse_wait_on_page_writeback(inode, folio->index);
> +			folio_wait_writeback(folio);
>  	}
>  	return err;
>  }
> @@ -2547,7 +2281,7 @@ static vm_fault_t fuse_page_mkwrite(struct vm_fault *vmf)
>  		return VM_FAULT_NOPAGE;
>  	}
>  
> -	fuse_wait_on_page_writeback(inode, page->index);
> +	folio_wait_writeback(page_folio(page));
>  	return VM_FAULT_LOCKED;
>  }
>  
> @@ -3375,7 +3109,6 @@ void fuse_init_file_inode(struct inode *inode, unsigned int flags)
>  	fi->iocachectr = 0;
>  	init_waitqueue_head(&fi->page_waitq);
>  	init_waitqueue_head(&fi->direct_io_waitq);
> -	fi->writepages = RB_ROOT;
>  
>  	if (IS_ENABLED(CONFIG_FUSE_DAX))
>  		fuse_dax_inode_init(inode, flags);
> -- 
> 2.43.5
> 

Re: [PATCH 2/2] fuse: remove tmp folio for writebacks and internal rb tree

[Joanne Koong]

On Sat, Oct 12, 2024 at 9:56 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Fri, Oct 11, 2024 at 03:34:34PM GMT, Joanne Koong wrote:
> > Currently, we allocate and copy data to a temporary folio when
> > handling writeback in order to mitigate the following deadlock scenario
> > that may arise if reclaim waits on writeback to complete:
> > * single-threaded FUSE server is in the middle of handling a request
> >   that needs a memory allocation
> > * memory allocation triggers direct reclaim
> > * direct reclaim waits on a folio under writeback
> > * the FUSE server can't write back the folio since it's stuck in
> >   direct reclaim
> >
> > To work around this, we allocate a temporary folio and copy over the
> > original folio to the temporary folio so that writeback can be
> > immediately cleared on the original folio. This additionally requires us
> > to maintain an internal rb tree to keep track of writeback state on the
> > temporary folios.
> >
> > This change sets the address space operations flag
> > ASOP_NO_RECLAIM_IN_WRITEBACK so that FUSE folios are not reclaimed and
>
> I couldn't find where ASOP_NO_RECLAIM_IN_WRITEBACK is being set for
> fuse.

Agh this patch was from an earlier branch in my tree and I forgot to
add this line when I patched it in:

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 9fee9f3062db..192b9f5f6b25 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -3094,6 +3094,7 @@ static const struct address_space_operations
fuse_file_aops  = {
        .direct_IO      = fuse_direct_IO,
        .write_begin    = fuse_write_begin,
        .write_end      = fuse_write_end,
+       .asop_flags     = ASOP_NO_RECLAIM_IN_WRITEBACK,
 };

sorry about that. With your suggestion of adding the flag instead to
the "enum mapping_flags", I'll be setting it in v2 in the
fuse_init_file_inode() function.


Thanks,
Joanne

>
> > waited on while in writeback, and removes the temporary folio +
> > extra copying and the internal rb tree.
> >
> > fio benchmarks --
> > (using averages observed from 10 runs, throwing away outliers)
> >
> > Setup:
> > sudo mount -t tmpfs -o size=30G tmpfs ~/tmp_mount
> >  ./libfuse/build/example/passthrough_ll -o writeback -o max_threads=4 -o source=~/tmp_mount ~/fuse_mount
> >
> > fio --name=writeback --ioengine=sync --rw=write --bs={1k,4k,1M} --size=2G
> > --numjobs=2 --ramp_time=30 --group_reporting=1 --directory=/root/fuse_mount
> >
> >         bs =  1k          4k            1M
> > Before  351 MiB/s     1818 MiB/s     1851 MiB/s
> > After   341 MiB/s     2246 MiB/s     2685 MiB/s
> > % diff        -3%          23%         45%
> >
> > Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> > ---
> >  fs/fuse/file.c | 321 +++++--------------------------------------------
> >  1 file changed, 27 insertions(+), 294 deletions(-)
> >
> > diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> > index 4304e44f32e6..9fee9f3062db 100644
> > --- a/fs/fuse/file.c
> > +++ b/fs/fuse/file.c
> > @@ -415,74 +415,11 @@ u64 fuse_lock_owner_id(struct fuse_conn *fc, fl_owner_t id)
> >
> >  struct fuse_writepage_args {
> >       struct fuse_io_args ia;
> > -     struct rb_node writepages_entry;
> >       struct list_head queue_entry;
> > -     struct fuse_writepage_args *next;
> >       struct inode *inode;
> >       struct fuse_sync_bucket *bucket;
> >  };
> >
> > -static struct fuse_writepage_args *fuse_find_writeback(struct fuse_inode *fi,
> > -                                         pgoff_t idx_from, pgoff_t idx_to)
> > -{
> > -     struct rb_node *n;
> > -
> > -     n = fi->writepages.rb_node;
> > -
> > -     while (n) {
> > -             struct fuse_writepage_args *wpa;
> > -             pgoff_t curr_index;
> > -
> > -             wpa = rb_entry(n, struct fuse_writepage_args, writepages_entry);
> > -             WARN_ON(get_fuse_inode(wpa->inode) != fi);
> > -             curr_index = wpa->ia.write.in.offset >> PAGE_SHIFT;
> > -             if (idx_from >= curr_index + wpa->ia.ap.num_pages)
> > -                     n = n->rb_right;
> > -             else if (idx_to < curr_index)
> > -                     n = n->rb_left;
> > -             else
> > -                     return wpa;
> > -     }
> > -     return NULL;
> > -}
> > -
> > -/*
> > - * Check if any page in a range is under writeback
> > - */
> > -static bool fuse_range_is_writeback(struct inode *inode, pgoff_t idx_from,
> > -                                pgoff_t idx_to)
> > -{
> > -     struct fuse_inode *fi = get_fuse_inode(inode);
> > -     bool found;
> > -
> > -     if (RB_EMPTY_ROOT(&fi->writepages))
> > -             return false;
> > -
> > -     spin_lock(&fi->lock);
> > -     found = fuse_find_writeback(fi, idx_from, idx_to);
> > -     spin_unlock(&fi->lock);
> > -
> > -     return found;
> > -}
> > -
> > -static inline bool fuse_page_is_writeback(struct inode *inode, pgoff_t index)
> > -{
> > -     return fuse_range_is_writeback(inode, index, index);
> > -}
> > -
> > -/*
> > - * Wait for page writeback to be completed.
> > - *
> > - * Since fuse doesn't rely on the VM writeback tracking, this has to
> > - * use some other means.
> > - */
> > -static void fuse_wait_on_page_writeback(struct inode *inode, pgoff_t index)
> > -{
> > -     struct fuse_inode *fi = get_fuse_inode(inode);
> > -
> > -     wait_event(fi->page_waitq, !fuse_page_is_writeback(inode, index));
> > -}
> > -
> >  /*
> >   * Wait for all pending writepages on the inode to finish.
> >   *
> > @@ -876,7 +813,7 @@ static int fuse_do_readpage(struct file *file, struct page *page)
> >        * page-cache page, so make sure we read a properly synced
> >        * page.
> >        */
> > -     fuse_wait_on_page_writeback(inode, page->index);
> > +     folio_wait_writeback(page_folio(page));
> >
> >       attr_ver = fuse_get_attr_version(fm->fc);
> >
> > @@ -1024,8 +961,7 @@ static void fuse_readahead(struct readahead_control *rac)
> >               ap = &ia->ap;
> >               nr_pages = __readahead_batch(rac, ap->pages, nr_pages);
> >               for (i = 0; i < nr_pages; i++) {
> > -                     fuse_wait_on_page_writeback(inode,
> > -                                                 readahead_index(rac) + i);
> > +                     folio_wait_writeback(page_folio(ap->pages[i]));
> >                       ap->descs[i].length = PAGE_SIZE;
> >               }
> >               ap->num_pages = nr_pages;
> > @@ -1147,7 +1083,7 @@ static ssize_t fuse_send_write_pages(struct fuse_io_args *ia,
> >       int err;
> >
> >       for (i = 0; i < ap->num_pages; i++)
> > -             fuse_wait_on_page_writeback(inode, ap->pages[i]->index);
> > +             folio_wait_writeback(page_folio(ap->pages[i]));
> >
> >       fuse_write_args_fill(ia, ff, pos, count);
> >       ia->write.in.flags = fuse_write_flags(iocb);
> > @@ -1583,7 +1519,7 @@ ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,
> >                       return res;
> >               }
> >       }
> > -     if (!cuse && fuse_range_is_writeback(inode, idx_from, idx_to)) {
> > +     if (!cuse && filemap_range_has_writeback(mapping, pos, (pos + count - 1))) {
> >               if (!write)
> >                       inode_lock(inode);
> >               fuse_sync_writes(inode);
> > @@ -1780,13 +1716,17 @@ static ssize_t fuse_splice_write(struct pipe_inode_info *pipe, struct file *out,
> >  static void fuse_writepage_free(struct fuse_writepage_args *wpa)
> >  {
> >       struct fuse_args_pages *ap = &wpa->ia.ap;
> > +     struct folio *folio;
> >       int i;
> >
> >       if (wpa->bucket)
> >               fuse_sync_bucket_dec(wpa->bucket);
> >
> > -     for (i = 0; i < ap->num_pages; i++)
> > -             __free_page(ap->pages[i]);
> > +     for (i = 0; i < ap->num_pages; i++) {
> > +             folio = page_folio(ap->pages[i]);
> > +             folio_end_writeback(folio);
> > +             folio_put(folio);
> > +     }
> >
> >       fuse_file_put(wpa->ia.ff, false);
> >
> > @@ -1799,7 +1739,7 @@ static void fuse_writepage_finish_stat(struct inode *inode, struct page *page)
> >       struct backing_dev_info *bdi = inode_to_bdi(inode);
> >
> >       dec_wb_stat(&bdi->wb, WB_WRITEBACK);
> > -     dec_node_page_state(page, NR_WRITEBACK_TEMP);
> > +     dec_node_page_state(page, NR_WRITEBACK);
> >       wb_writeout_inc(&bdi->wb);
> >  }
> >
> > @@ -1822,7 +1762,6 @@ static void fuse_send_writepage(struct fuse_mount *fm,
> >  __releases(fi->lock)
> >  __acquires(fi->lock)
> >  {
> > -     struct fuse_writepage_args *aux, *next;
> >       struct fuse_inode *fi = get_fuse_inode(wpa->inode);
> >       struct fuse_write_in *inarg = &wpa->ia.write.in;
> >       struct fuse_args *args = &wpa->ia.ap.args;
> > @@ -1858,18 +1797,8 @@ __acquires(fi->lock)
> >
> >   out_free:
> >       fi->writectr--;
> > -     rb_erase(&wpa->writepages_entry, &fi->writepages);
> >       fuse_writepage_finish(wpa);
> >       spin_unlock(&fi->lock);
> > -
> > -     /* After rb_erase() aux request list is private */
> > -     for (aux = wpa->next; aux; aux = next) {
> > -             next = aux->next;
> > -             aux->next = NULL;
> > -             fuse_writepage_finish_stat(aux->inode, aux->ia.ap.pages[0]);
> > -             fuse_writepage_free(aux);
> > -     }
> > -
> >       fuse_writepage_free(wpa);
> >       spin_lock(&fi->lock);
> >  }
> > @@ -1897,43 +1826,6 @@ __acquires(fi->lock)
> >       }
> >  }
> >
> > -static struct fuse_writepage_args *fuse_insert_writeback(struct rb_root *root,
> > -                                             struct fuse_writepage_args *wpa)
> > -{
> > -     pgoff_t idx_from = wpa->ia.write.in.offset >> PAGE_SHIFT;
> > -     pgoff_t idx_to = idx_from + wpa->ia.ap.num_pages - 1;
> > -     struct rb_node **p = &root->rb_node;
> > -     struct rb_node  *parent = NULL;
> > -
> > -     WARN_ON(!wpa->ia.ap.num_pages);
> > -     while (*p) {
> > -             struct fuse_writepage_args *curr;
> > -             pgoff_t curr_index;
> > -
> > -             parent = *p;
> > -             curr = rb_entry(parent, struct fuse_writepage_args,
> > -                             writepages_entry);
> > -             WARN_ON(curr->inode != wpa->inode);
> > -             curr_index = curr->ia.write.in.offset >> PAGE_SHIFT;
> > -
> > -             if (idx_from >= curr_index + curr->ia.ap.num_pages)
> > -                     p = &(*p)->rb_right;
> > -             else if (idx_to < curr_index)
> > -                     p = &(*p)->rb_left;
> > -             else
> > -                     return curr;
> > -     }
> > -
> > -     rb_link_node(&wpa->writepages_entry, parent, p);
> > -     rb_insert_color(&wpa->writepages_entry, root);
> > -     return NULL;
> > -}
> > -
> > -static void tree_insert(struct rb_root *root, struct fuse_writepage_args *wpa)
> > -{
> > -     WARN_ON(fuse_insert_writeback(root, wpa));
> > -}
> > -
> >  static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args,
> >                              int error)
> >  {
> > @@ -1953,41 +1845,6 @@ static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args,
> >       if (!fc->writeback_cache)
> >               fuse_invalidate_attr_mask(inode, FUSE_STATX_MODIFY);
> >       spin_lock(&fi->lock);
> > -     rb_erase(&wpa->writepages_entry, &fi->writepages);
> > -     while (wpa->next) {
> > -             struct fuse_mount *fm = get_fuse_mount(inode);
> > -             struct fuse_write_in *inarg = &wpa->ia.write.in;
> > -             struct fuse_writepage_args *next = wpa->next;
> > -
> > -             wpa->next = next->next;
> > -             next->next = NULL;
> > -             tree_insert(&fi->writepages, next);
> > -
> > -             /*
> > -              * Skip fuse_flush_writepages() to make it easy to crop requests
> > -              * based on primary request size.
> > -              *
> > -              * 1st case (trivial): there are no concurrent activities using
> > -              * fuse_set/release_nowrite.  Then we're on safe side because
> > -              * fuse_flush_writepages() would call fuse_send_writepage()
> > -              * anyway.
> > -              *
> > -              * 2nd case: someone called fuse_set_nowrite and it is waiting
> > -              * now for completion of all in-flight requests.  This happens
> > -              * rarely and no more than once per page, so this should be
> > -              * okay.
> > -              *
> > -              * 3rd case: someone (e.g. fuse_do_setattr()) is in the middle
> > -              * of fuse_set_nowrite..fuse_release_nowrite section.  The fact
> > -              * that fuse_set_nowrite returned implies that all in-flight
> > -              * requests were completed along with all of their secondary
> > -              * requests.  Further primary requests are blocked by negative
> > -              * writectr.  Hence there cannot be any in-flight requests and
> > -              * no invocations of fuse_writepage_end() while we're in
> > -              * fuse_set_nowrite..fuse_release_nowrite section.
> > -              */
> > -             fuse_send_writepage(fm, next, inarg->offset + inarg->size);
> > -     }
> >       fi->writectr--;
> >       fuse_writepage_finish(wpa);
> >       spin_unlock(&fi->lock);
> > @@ -2074,19 +1931,18 @@ static void fuse_writepage_add_to_bucket(struct fuse_conn *fc,
> >  }
> >
> >  static void fuse_writepage_args_page_fill(struct fuse_writepage_args *wpa, struct folio *folio,
> > -                                       struct folio *tmp_folio, uint32_t page_index)
> > +                                       uint32_t page_index)
> >  {
> >       struct inode *inode = folio->mapping->host;
> >       struct fuse_args_pages *ap = &wpa->ia.ap;
> >
> > -     folio_copy(tmp_folio, folio);
> > -
> > -     ap->pages[page_index] = &tmp_folio->page;
> > +     folio_get(folio);
> > +     ap->pages[page_index] = &folio->page;
> >       ap->descs[page_index].offset = 0;
> >       ap->descs[page_index].length = PAGE_SIZE;
> >
> >       inc_wb_stat(&inode_to_bdi(inode)->wb, WB_WRITEBACK);
> > -     inc_node_page_state(&tmp_folio->page, NR_WRITEBACK_TEMP);
> > +     inc_node_page_state(&folio->page, NR_WRITEBACK);
> >  }
> >
> >  static struct fuse_writepage_args *fuse_writepage_args_setup(struct folio *folio,
> > @@ -2121,18 +1977,12 @@ static int fuse_writepage_locked(struct folio *folio)
> >       struct fuse_inode *fi = get_fuse_inode(inode);
> >       struct fuse_writepage_args *wpa;
> >       struct fuse_args_pages *ap;
> > -     struct folio *tmp_folio;
> >       struct fuse_file *ff;
> > -     int error = -ENOMEM;
> > -
> > -     tmp_folio = folio_alloc(GFP_NOFS | __GFP_HIGHMEM, 0);
> > -     if (!tmp_folio)
> > -             goto err;
> > +     int error = -EIO;
> >
> > -     error = -EIO;
> >       ff = fuse_write_file_get(fi);
> >       if (!ff)
> > -             goto err_nofile;
> > +             goto err;
> >
> >       wpa = fuse_writepage_args_setup(folio, ff);
> >       error = -ENOMEM;
> > @@ -2143,22 +1993,17 @@ static int fuse_writepage_locked(struct folio *folio)
> >       ap->num_pages = 1;
> >
> >       folio_start_writeback(folio);
> > -     fuse_writepage_args_page_fill(wpa, folio, tmp_folio, 0);
> > +     fuse_writepage_args_page_fill(wpa, folio, 0);
> >
> >       spin_lock(&fi->lock);
> > -     tree_insert(&fi->writepages, wpa);
> >       list_add_tail(&wpa->queue_entry, &fi->queued_writes);
> >       fuse_flush_writepages(inode);
> >       spin_unlock(&fi->lock);
> >
> > -     folio_end_writeback(folio);
> > -
> >       return 0;
> >
> >  err_writepage_args:
> >       fuse_file_put(ff, false);
> > -err_nofile:
> > -     folio_put(tmp_folio);
> >  err:
> >       mapping_set_error(folio->mapping, error);
> >       return error;
> > @@ -2168,7 +2013,6 @@ struct fuse_fill_wb_data {
> >       struct fuse_writepage_args *wpa;
> >       struct fuse_file *ff;
> >       struct inode *inode;
> > -     struct page **orig_pages;
> >       unsigned int max_pages;
> >  };
> >
> > @@ -2203,68 +2047,11 @@ static void fuse_writepages_send(struct fuse_fill_wb_data *data)
> >       struct fuse_writepage_args *wpa = data->wpa;
> >       struct inode *inode = data->inode;
> >       struct fuse_inode *fi = get_fuse_inode(inode);
> > -     int num_pages = wpa->ia.ap.num_pages;
> > -     int i;
> >
> >       spin_lock(&fi->lock);
> >       list_add_tail(&wpa->queue_entry, &fi->queued_writes);
> >       fuse_flush_writepages(inode);
> >       spin_unlock(&fi->lock);
> > -
> > -     for (i = 0; i < num_pages; i++)
> > -             end_page_writeback(data->orig_pages[i]);
> > -}
> > -
> > -/*
> > - * Check under fi->lock if the page is under writeback, and insert it onto the
> > - * rb_tree if not. Otherwise iterate auxiliary write requests, to see if there's
> > - * one already added for a page at this offset.  If there's none, then insert
> > - * this new request onto the auxiliary list, otherwise reuse the existing one by
> > - * swapping the new temp page with the old one.
> > - */
> > -static bool fuse_writepage_add(struct fuse_writepage_args *new_wpa,
> > -                            struct page *page)
> > -{
> > -     struct fuse_inode *fi = get_fuse_inode(new_wpa->inode);
> > -     struct fuse_writepage_args *tmp;
> > -     struct fuse_writepage_args *old_wpa;
> > -     struct fuse_args_pages *new_ap = &new_wpa->ia.ap;
> > -
> > -     WARN_ON(new_ap->num_pages != 0);
> > -     new_ap->num_pages = 1;
> > -
> > -     spin_lock(&fi->lock);
> > -     old_wpa = fuse_insert_writeback(&fi->writepages, new_wpa);
> > -     if (!old_wpa) {
> > -             spin_unlock(&fi->lock);
> > -             return true;
> > -     }
> > -
> > -     for (tmp = old_wpa->next; tmp; tmp = tmp->next) {
> > -             pgoff_t curr_index;
> > -
> > -             WARN_ON(tmp->inode != new_wpa->inode);
> > -             curr_index = tmp->ia.write.in.offset >> PAGE_SHIFT;
> > -             if (curr_index == page->index) {
> > -                     WARN_ON(tmp->ia.ap.num_pages != 1);
> > -                     swap(tmp->ia.ap.pages[0], new_ap->pages[0]);
> > -                     break;
> > -             }
> > -     }
> > -
> > -     if (!tmp) {
> > -             new_wpa->next = old_wpa->next;
> > -             old_wpa->next = new_wpa;
> > -     }
> > -
> > -     spin_unlock(&fi->lock);
> > -
> > -     if (tmp) {
> > -             fuse_writepage_finish_stat(new_wpa->inode, new_ap->pages[0]);
> > -             fuse_writepage_free(new_wpa);
> > -     }
> > -
> > -     return false;
> >  }
> >
> >  static bool fuse_writepage_need_send(struct fuse_conn *fc, struct page *page,
> > @@ -2273,15 +2060,6 @@ static bool fuse_writepage_need_send(struct fuse_conn *fc, struct page *page,
> >  {
> >       WARN_ON(!ap->num_pages);
> >
> > -     /*
> > -      * Being under writeback is unlikely but possible.  For example direct
> > -      * read to an mmaped fuse file will set the page dirty twice; once when
> > -      * the pages are faulted with get_user_pages(), and then after the read
> > -      * completed.
> > -      */
> > -     if (fuse_page_is_writeback(data->inode, page->index))
> > -             return true;
> > -
> >       /* Reached max pages */
> >       if (ap->num_pages == fc->max_pages)
> >               return true;
> > @@ -2291,7 +2069,7 @@ static bool fuse_writepage_need_send(struct fuse_conn *fc, struct page *page,
> >               return true;
> >
> >       /* Discontinuity */
> > -     if (data->orig_pages[ap->num_pages - 1]->index + 1 != page->index)
> > +     if (ap->pages[ap->num_pages - 1]->index + 1 != page->index)
> >               return true;
> >
> >       /* Need to grow the pages array?  If so, did the expansion fail? */
> > @@ -2308,9 +2086,7 @@ static int fuse_writepages_fill(struct folio *folio,
> >       struct fuse_writepage_args *wpa = data->wpa;
> >       struct fuse_args_pages *ap = &wpa->ia.ap;
> >       struct inode *inode = data->inode;
> > -     struct fuse_inode *fi = get_fuse_inode(inode);
> >       struct fuse_conn *fc = get_fuse_conn(inode);
> > -     struct folio *tmp_folio;
> >       int err;
> >
> >       if (wpa && fuse_writepage_need_send(fc, &folio->page, ap, data)) {
> > @@ -2318,54 +2094,23 @@ static int fuse_writepages_fill(struct folio *folio,
> >               data->wpa = NULL;
> >       }
> >
> > -     err = -ENOMEM;
> > -     tmp_folio = folio_alloc(GFP_NOFS | __GFP_HIGHMEM, 0);
> > -     if (!tmp_folio)
> > -             goto out_unlock;
> > -
> > -     /*
> > -      * The page must not be redirtied until the writeout is completed
> > -      * (i.e. userspace has sent a reply to the write request).  Otherwise
> > -      * there could be more than one temporary page instance for each real
> > -      * page.
> > -      *
> > -      * This is ensured by holding the page lock in page_mkwrite() while
> > -      * checking fuse_page_is_writeback().  We already hold the page lock
> > -      * since clear_page_dirty_for_io() and keep it held until we add the
> > -      * request to the fi->writepages list and increment ap->num_pages.
> > -      * After this fuse_page_is_writeback() will indicate that the page is
> > -      * under writeback, so we can release the page lock.
> > -      */
> >       if (data->wpa == NULL) {
> >               err = -ENOMEM;
> >               wpa = fuse_writepage_args_setup(folio, data->ff);
> > -             if (!wpa) {
> > -                     folio_put(tmp_folio);
> > +             if (!wpa)
> >                       goto out_unlock;
> > -             }
> >               fuse_file_get(wpa->ia.ff);
> >               data->max_pages = 1;
> >               ap = &wpa->ia.ap;
> >       }
> >       folio_start_writeback(folio);
> >
> > -     fuse_writepage_args_page_fill(wpa, folio, tmp_folio, ap->num_pages);
> > -     data->orig_pages[ap->num_pages] = &folio->page;
> > +     fuse_writepage_args_page_fill(wpa, folio, ap->num_pages);
> >
> >       err = 0;
> > -     if (data->wpa) {
> > -             /*
> > -              * Protected by fi->lock against concurrent access by
> > -              * fuse_page_is_writeback().
> > -              */
> > -             spin_lock(&fi->lock);
> > -             ap->num_pages++;
> > -             spin_unlock(&fi->lock);
> > -     } else if (fuse_writepage_add(wpa, &folio->page)) {
> > +     ap->num_pages++;
> > +     if (!data->wpa)
> >               data->wpa = wpa;
> > -     } else {
> > -             folio_end_writeback(folio);
> > -     }
> >  out_unlock:
> >       folio_unlock(folio);
> >
> > @@ -2394,21 +2139,12 @@ static int fuse_writepages(struct address_space *mapping,
> >       if (!data.ff)
> >               return -EIO;
> >
> > -     err = -ENOMEM;
> > -     data.orig_pages = kcalloc(fc->max_pages,
> > -                               sizeof(struct page *),
> > -                               GFP_NOFS);
> > -     if (!data.orig_pages)
> > -             goto out;
> > -
> >       err = write_cache_pages(mapping, wbc, fuse_writepages_fill, &data);
> >       if (data.wpa) {
> >               WARN_ON(!data.wpa->ia.ap.num_pages);
> >               fuse_writepages_send(&data);
> >       }
> >
> > -     kfree(data.orig_pages);
> > -out:
> >       fuse_file_put(data.ff, false);
> >       return err;
> >  }
> > @@ -2433,7 +2169,7 @@ static int fuse_write_begin(struct file *file, struct address_space *mapping,
> >       if (IS_ERR(folio))
> >               goto error;
> >
> > -     fuse_wait_on_page_writeback(mapping->host, folio->index);
> > +     folio_wait_writeback(folio);
> >
> >       if (folio_test_uptodate(folio) || len >= folio_size(folio))
> >               goto success;
> > @@ -2497,13 +2233,11 @@ static int fuse_launder_folio(struct folio *folio)
> >  {
> >       int err = 0;
> >       if (folio_clear_dirty_for_io(folio)) {
> > -             struct inode *inode = folio->mapping->host;
> > -
> >               /* Serialize with pending writeback for the same page */
> > -             fuse_wait_on_page_writeback(inode, folio->index);
> > +             folio_wait_writeback(folio);
> >               err = fuse_writepage_locked(folio);
> >               if (!err)
> > -                     fuse_wait_on_page_writeback(inode, folio->index);
> > +                     folio_wait_writeback(folio);
> >       }
> >       return err;
> >  }
> > @@ -2547,7 +2281,7 @@ static vm_fault_t fuse_page_mkwrite(struct vm_fault *vmf)
> >               return VM_FAULT_NOPAGE;
> >       }
> >
> > -     fuse_wait_on_page_writeback(inode, page->index);
> > +     folio_wait_writeback(page_folio(page));
> >       return VM_FAULT_LOCKED;
> >  }
> >
> > @@ -3375,7 +3109,6 @@ void fuse_init_file_inode(struct inode *inode, unsigned int flags)
> >       fi->iocachectr = 0;
> >       init_waitqueue_head(&fi->page_waitq);
> >       init_waitqueue_head(&fi->direct_io_waitq);
> > -     fi->writepages = RB_ROOT;
> >
> >       if (IS_ENABLED(CONFIG_FUSE_DAX))
> >               fuse_dax_inode_init(inode, flags);
> > --
> > 2.43.5
> >