[PATCHSET 0/2] Add XFS support for RWF_DONTCACHE

[PATCHSET 0/2] Add XFS support for RWF_DONTCACHE

[Jens Axboe]

Hi,

Now that the main bits are in mainline, here are the XFS patches for
adding RWF_DONTCACHE support. It's pretty trivial - patch 1 adds the
basic iomap flag and check, and patch 2 flags FOP_DONTCACHE support
in the file_operations struct. Could be folded into a single patch
at this point, I'll leave that up to you guys what you prefer.

Patches are aginst 6.14-rc1.

-- 
Jens Axboe

[PATCH 1/2] iomap: make buffered writes work with RWF_DONTCACHE

[Jens Axboe]

Add iomap buffered write support for RWF_DONTCACHE. If RWF_DONTCACHE is
set for a write, mark the folios being written as uncached. Then
writeback completion will drop the pages. The write_iter handler simply
kicks off writeback for the pages, and writeback completion will take
care of the rest.

This still needs the user of the iomap buffered write helpers to call
folio_end_dropbehind_write() upon successful issue of the writes.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/iomap/buffered-io.c | 4 ++++
 include/linux/iomap.h  | 1 +
 2 files changed, 5 insertions(+)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index d303e6c8900c..ea863c3cf510 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -603,6 +603,8 @@ struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len)
 
 	if (iter->flags & IOMAP_NOWAIT)
 		fgp |= FGP_NOWAIT;
+	if (iter->flags & IOMAP_DONTCACHE)
+		fgp |= FGP_DONTCACHE;
 	fgp |= fgf_set_order(len);
 
 	return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
@@ -1034,6 +1036,8 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
 
 	if (iocb->ki_flags & IOCB_NOWAIT)
 		iter.flags |= IOMAP_NOWAIT;
+	if (iocb->ki_flags & IOCB_DONTCACHE)
+		iter.flags |= IOMAP_DONTCACHE;
 
 	while ((ret = iomap_iter(&iter, ops)) > 0)
 		iter.processed = iomap_write_iter(&iter, i);
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 75bf54e76f3b..26b0dbe23e62 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -183,6 +183,7 @@ struct iomap_folio_ops {
 #define IOMAP_DAX		0
 #endif /* CONFIG_FS_DAX */
 #define IOMAP_ATOMIC		(1 << 9)
+#define IOMAP_DONTCACHE		(1 << 10)
 
 struct iomap_ops {
 	/*
-- 
2.47.2

Re: [PATCH 1/2] iomap: make buffered writes work with RWF_DONTCACHE

[Darrick J. Wong]

On Mon, Feb 03, 2025 at 09:32:38AM -0700, Jens Axboe wrote:
> Add iomap buffered write support for RWF_DONTCACHE. If RWF_DONTCACHE is
> set for a write, mark the folios being written as uncached. Then
> writeback completion will drop the pages. The write_iter handler simply
> kicks off writeback for the pages, and writeback completion will take
> care of the rest.
> 
> This still needs the user of the iomap buffered write helpers to call
> folio_end_dropbehind_write() upon successful issue of the writes.

I thought iomap calls folio_end_writeback, which cares of that?  So xfs
doesn't itself have to call folio_end_dropbehind_write?

> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  fs/iomap/buffered-io.c | 4 ++++
>  include/linux/iomap.h  | 1 +
>  2 files changed, 5 insertions(+)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d303e6c8900c..ea863c3cf510 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -603,6 +603,8 @@ struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len)
>  
>  	if (iter->flags & IOMAP_NOWAIT)
>  		fgp |= FGP_NOWAIT;
> +	if (iter->flags & IOMAP_DONTCACHE)
> +		fgp |= FGP_DONTCACHE;
>  	fgp |= fgf_set_order(len);
>  
>  	return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
> @@ -1034,6 +1036,8 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
>  
>  	if (iocb->ki_flags & IOCB_NOWAIT)
>  		iter.flags |= IOMAP_NOWAIT;
> +	if (iocb->ki_flags & IOCB_DONTCACHE)
> +		iter.flags |= IOMAP_DONTCACHE;
>  
>  	while ((ret = iomap_iter(&iter, ops)) > 0)
>  		iter.processed = iomap_write_iter(&iter, i);
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 75bf54e76f3b..26b0dbe23e62 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -183,6 +183,7 @@ struct iomap_folio_ops {
>  #define IOMAP_DAX		0
>  #endif /* CONFIG_FS_DAX */
>  #define IOMAP_ATOMIC		(1 << 9)
> +#define IOMAP_DONTCACHE		(1 << 10)

This needs a mention in the iomap documentation.  If the patch below
accurately summarizes what it does nowadays, then you can add it to the
series with a:

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>

--D

diff --git a/Documentation/filesystems/iomap/design.rst b/Documentation/filesystems/iomap/design.rst
index b0d0188a095e55..7b91546750f59e 100644
--- a/Documentation/filesystems/iomap/design.rst
+++ b/Documentation/filesystems/iomap/design.rst
@@ -352,6 +352,11 @@ operations:
    ``IOMAP_NOWAIT`` is often set on behalf of ``IOCB_NOWAIT`` or
    ``RWF_NOWAIT``.
 
+ * ``IOMAP_DONTCACHE`` is set when the caller wishes to perform a
+   buffered file I/O and would like the kernel to drop the pagecache
+   after the I/O completes, if it isn't already being used by another
+   thread.
+
 If it is necessary to read existing file contents from a `different
 <https://lore.kernel.org/all/20191008071527.29304-9-hch@lst.de/>`_
 device or address range on a device, the filesystem should return that
diff --git a/Documentation/filesystems/iomap/operations.rst b/Documentation/filesystems/iomap/operations.rst
index 2c7f5df9d8b037..584ff549f9a659 100644
--- a/Documentation/filesystems/iomap/operations.rst
+++ b/Documentation/filesystems/iomap/operations.rst
@@ -131,6 +131,8 @@ These ``struct kiocb`` flags are significant for buffered I/O with iomap:
 
  * ``IOCB_NOWAIT``: Turns on ``IOMAP_NOWAIT``.
 
+ * ``IOCB_DONTCACHE``: Turns on ``IOMAP_DONTCACHE``.
+
 Internal per-Folio State
 ------------------------
 

Re: [PATCH 1/2] iomap: make buffered writes work with RWF_DONTCACHE

[Jens Axboe]

On 2/3/25 10:19 AM, Darrick J. Wong wrote:
> On Mon, Feb 03, 2025 at 09:32:38AM -0700, Jens Axboe wrote:
>> Add iomap buffered write support for RWF_DONTCACHE. If RWF_DONTCACHE is
>> set for a write, mark the folios being written as uncached. Then
>> writeback completion will drop the pages. The write_iter handler simply
>> kicks off writeback for the pages, and writeback completion will take
>> care of the rest.
>>
>> This still needs the user of the iomap buffered write helpers to call
>> folio_end_dropbehind_write() upon successful issue of the writes.
> 
> I thought iomap calls folio_end_writeback, which cares of that?  So xfs
> doesn't itself have to call folio_end_dropbehind_write?

Yep it does, stale commit message! I'll fix it up and send out a v2.

>>  fs/iomap/buffered-io.c | 4 ++++
>>  include/linux/iomap.h  | 1 +
>>  2 files changed, 5 insertions(+)
>>
>> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
>> index d303e6c8900c..ea863c3cf510 100644
>> --- a/fs/iomap/buffered-io.c
>> +++ b/fs/iomap/buffered-io.c
>> @@ -603,6 +603,8 @@ struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len)
>>  
>>  	if (iter->flags & IOMAP_NOWAIT)
>>  		fgp |= FGP_NOWAIT;
>> +	if (iter->flags & IOMAP_DONTCACHE)
>> +		fgp |= FGP_DONTCACHE;
>>  	fgp |= fgf_set_order(len);
>>  
>>  	return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
>> @@ -1034,6 +1036,8 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
>>  
>>  	if (iocb->ki_flags & IOCB_NOWAIT)
>>  		iter.flags |= IOMAP_NOWAIT;
>> +	if (iocb->ki_flags & IOCB_DONTCACHE)
>> +		iter.flags |= IOMAP_DONTCACHE;
>>  
>>  	while ((ret = iomap_iter(&iter, ops)) > 0)
>>  		iter.processed = iomap_write_iter(&iter, i);
>> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
>> index 75bf54e76f3b..26b0dbe23e62 100644
>> --- a/include/linux/iomap.h
>> +++ b/include/linux/iomap.h
>> @@ -183,6 +183,7 @@ struct iomap_folio_ops {
>>  #define IOMAP_DAX		0
>>  #endif /* CONFIG_FS_DAX */
>>  #define IOMAP_ATOMIC		(1 << 9)
>> +#define IOMAP_DONTCACHE		(1 << 10)
> 
> This needs a mention in the iomap documentation.  If the patch below
> accurately summarizes what it does nowadays, then you can add it to the
> series with a:

Thanks Darrick, I'll add that and your SOB as well.

-- 
Jens Axboe

[PATCH 2/2] xfs: flag as supporting FOP_DONTCACHE

[Jens Axboe]

Read side was already fully supported, and with the write side
appropriately punted to the worker queue, all that's needed now is
setting FOP_DONTCACHE in the file_operations structure to enable full
support for read and write uncached IO.

This provides similar benefits to using RWF_DONTCACHE with reads. Testing
buffered writes on 32 files:

writing bs 65536, uncached 0
  1s: 196035MB/sec
  2s: 132308MB/sec
  3s: 132438MB/sec
  4s: 116528MB/sec
  5s: 103898MB/sec
  6s: 108893MB/sec
  7s: 99678MB/sec
  8s: 106545MB/sec
  9s: 106826MB/sec
 10s: 101544MB/sec
 11s: 111044MB/sec
 12s: 124257MB/sec
 13s: 116031MB/sec
 14s: 114540MB/sec
 15s: 115011MB/sec
 16s: 115260MB/sec
 17s: 116068MB/sec
 18s: 116096MB/sec

where it's quite obvious where the page cache filled, and performance
dropped from to about half of where it started, settling in at around
115GB/sec. Meanwhile, 32 kswapds were running full steam trying to
reclaim pages.

Running the same test with uncached buffered writes:

writing bs 65536, uncached 1
  1s: 198974MB/sec
  2s: 189618MB/sec
  3s: 193601MB/sec
  4s: 188582MB/sec
  5s: 193487MB/sec
  6s: 188341MB/sec
  7s: 194325MB/sec
  8s: 188114MB/sec
  9s: 192740MB/sec
 10s: 189206MB/sec
 11s: 193442MB/sec
 12s: 189659MB/sec
 13s: 191732MB/sec
 14s: 190701MB/sec
 15s: 191789MB/sec
 16s: 191259MB/sec
 17s: 190613MB/sec
 18s: 191951MB/sec

and the behavior is fully predictable, performing the same throughout
even after the page cache would otherwise have fully filled with dirty
data. It's also about 65% faster, and using half the CPU of the system
compared to the normal buffered write.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/xfs/xfs_file.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index f7a7d89c345e..358987b6e2f8 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -1626,7 +1626,8 @@ const struct file_operations xfs_file_operations = {
 	.fadvise	= xfs_file_fadvise,
 	.remap_file_range = xfs_file_remap_range,
 	.fop_flags	= FOP_MMAP_SYNC | FOP_BUFFER_RASYNC |
-			  FOP_BUFFER_WASYNC | FOP_DIO_PARALLEL_WRITE,
+			  FOP_BUFFER_WASYNC | FOP_DIO_PARALLEL_WRITE |
+			  FOP_DONTCACHE,
 };
 
 const struct file_operations xfs_dir_file_operations = {
-- 
2.47.2