[PATCH] mm: support anonymous stable page

[PATCH] mm: support anonymous stable page

[Minchan Kim]

For developemnt for zram-swap asynchronous writeback, I found
strange corruption of compressed page. With investigation, it
reveals currently stable page doesn't support anonymous page.
IOW, reuse_swap_page can reuse the page without waiting
writeback completion so that it can corrupt data during
zram compression. It can affect every swap device which supports
asynchronous writeback and CRC checking as well as zRAM.

Unfortunately, reuse_swap_page should be atomic so that we
cannot wait on writeback in there so the approach in this patch
is simply return false if we found it needs stable page.
Although it increases memory footprint temporarily, it happens
rarely and it should be reclaimed easily althoug it happened.
Also, It would be better than waiting of IO completion, which
is critial path for application latency.

Cc: Hugh Dickins <hughd@google.com>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 mm/swapfile.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 816ca8663ae5..ea591435d8e0 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -949,7 +949,12 @@ bool reuse_swap_page(struct page *page, int *total_mapcount)
 			delete_from_swap_cache(page);
 			SetPageDirty(page);
 		} else {
-			wait_for_stable_page(page);
+			struct address_space *mapping;
+
+			mapping = page_mapping(page);
+			if (bdi_cap_stable_pages_required(
+					inode_to_bdi(mapping->host)))
+				return false;
 		}
 	}
 out:
@@ -2185,6 +2190,7 @@ static struct swap_info_struct *alloc_swap_info(void)
 static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
 {
 	int error;
+	struct address_space *swapper_space;
 
 	if (S_ISBLK(inode->i_mode)) {
 		p->bdev = bdgrab(I_BDEV(inode));
@@ -2207,6 +2213,9 @@ static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
 	} else
 		return -EINVAL;
 
+	swapper_space = &swapper_spaces[p->type];
+	swapper_space->host = inode;
+
 	return 0;
 }
 
-- 
2.7.4

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Re: [PATCH] mm: support anonymous stable page

[Minchan Kim]

Sorry for sending a wrong version. Here is new one.

Re: [PATCH] mm: support anonymous stable page

[Hugh Dickins]

On Fri, 11 Nov 2016, Minchan Kim wrote:
> Sorry for sending a wrong version. Here is new one.
> 
> From 2d42ead9335cde51fd58d6348439ca03cf359ba2 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@kernel.org>
> Date: Fri, 11 Nov 2016 15:02:57 +0900
> Subject: [PATCH] mm: support anonymous stable page
> 
> For developemnt for zram-swap asynchronous writeback, I found
> strange corruption of compressed page. With investigation, it
> reveals currently stable page doesn't support anonymous page.
> IOW, reuse_swap_page can reuse the page without waiting
> writeback completion so that it can corrupt data during
> zram compression. It can affect every swap device which supports
> asynchronous writeback and CRC checking as well as zRAM.
> 
> Unfortunately, reuse_swap_page should be atomic so that we
> cannot wait on writeback in there so the approach in this patch
> is simply return false if we found it needs stable page.
> Although it increases memory footprint temporarily, it happens
> rarely and it should be reclaimed easily althoug it happened.
> Also, It would be better than waiting of IO completion, which
> is critial path for application latency.
> 
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Darrick J. Wong <darrick.wong@oracle.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>

Ack to your intention (we discussed this together years ago, but saw
no actual demand for it before now), and I like what you're doing;
but it has to be NAK to this implementation.

I sensed there was an problem when you posted; but only now, after
searching through the uses of mapping->host, do I see that problem.

You're setting swap's mapping->host = inode when it used to be NULL:
which seems like a very good way to get what you need, but I'm afraid
it's a change which goes way beyond your intention.

See inode_to_bdi(): for ordinary disk-based swap, it will now pick
up the bdi of the block device instead of noop_backing_dev_info, so
swap would then pass the mapping_cap_account_dirty() and similar
tests (mostly in mm/page-writeback.c), and go down codepaths it
has never gone down before.

It's possible that swap (and shmem) would be better off going down
those paths, to be throttled in a similar way to files; but that's
debatable, and a much bigger change than you want to get into for
zram stable pages.

Maybe add SWP_STABLE_WRITES in include/linux/swap.h, and set that
in swap_info->flags according to bdi_cap_stable_pages_required(),
leaving mapping->host itself NULL as before?

Hugh

> ---
>  mm/swapfile.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 2210de290b54..ea591435d8e0 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -943,11 +943,21 @@ bool reuse_swap_page(struct page *page, int *total_mapcount)
>  	count = page_trans_huge_mapcount(page, total_mapcount);
>  	if (count <= 1 && PageSwapCache(page)) {
>  		count += page_swapcount(page);
> -		if (count == 1 && !PageWriteback(page)) {
> +		if (count != 1)
> +			goto out;
> +		if (!PageWriteback(page)) {
>  			delete_from_swap_cache(page);
>  			SetPageDirty(page);
> +		} else {
> +			struct address_space *mapping;
> +
> +			mapping = page_mapping(page);
> +			if (bdi_cap_stable_pages_required(
> +					inode_to_bdi(mapping->host)))
> +				return false;
>  		}
>  	}
> +out:
>  	return count <= 1;
>  }
>  
> @@ -2180,6 +2190,7 @@ static struct swap_info_struct *alloc_swap_info(void)
>  static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
>  {
>  	int error;
> +	struct address_space *swapper_space;
>  
>  	if (S_ISBLK(inode->i_mode)) {
>  		p->bdev = bdgrab(I_BDEV(inode));
> @@ -2202,6 +2213,9 @@ static int claim_swapfile(struct swap_info_struct *p, struct inode *inode)
>  	} else
>  		return -EINVAL;
>  
> +	swapper_space = &swapper_spaces[p->type];
> +	swapper_space->host = inode;
> +
>  	return 0;
>  }
>  
> -- 
> 2.7.4

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Re: [PATCH] mm: support anonymous stable page

[Minchan Kim]

Hi Hugh,

On Thu, Nov 17, 2016 at 08:35:10PM -0800, Hugh Dickins wrote:
> On Fri, 11 Nov 2016, Minchan Kim wrote:
> > Sorry for sending a wrong version. Here is new one.
> > 
> > From 2d42ead9335cde51fd58d6348439ca03cf359ba2 Mon Sep 17 00:00:00 2001
> > From: Minchan Kim <minchan@kernel.org>
> > Date: Fri, 11 Nov 2016 15:02:57 +0900
> > Subject: [PATCH] mm: support anonymous stable page
> > 
> > For developemnt for zram-swap asynchronous writeback, I found
> > strange corruption of compressed page. With investigation, it
> > reveals currently stable page doesn't support anonymous page.
> > IOW, reuse_swap_page can reuse the page without waiting
> > writeback completion so that it can corrupt data during
> > zram compression. It can affect every swap device which supports
> > asynchronous writeback and CRC checking as well as zRAM.
> > 
> > Unfortunately, reuse_swap_page should be atomic so that we
> > cannot wait on writeback in there so the approach in this patch
> > is simply return false if we found it needs stable page.
> > Although it increases memory footprint temporarily, it happens
> > rarely and it should be reclaimed easily althoug it happened.
> > Also, It would be better than waiting of IO completion, which
> > is critial path for application latency.
> > 
> > Cc: Hugh Dickins <hughd@google.com>
> > Cc: Darrick J. Wong <darrick.wong@oracle.com>
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> 
> Ack to your intention (we discussed this together years ago, but saw
> no actual demand for it before now), and I like what you're doing;
> but it has to be NAK to this implementation.
> 
> I sensed there was an problem when you posted; but only now, after
> searching through the uses of mapping->host, do I see that problem.
> 
> You're setting swap's mapping->host = inode when it used to be NULL:
> which seems like a very good way to get what you need, but I'm afraid
> it's a change which goes way beyond your intention.
> 
> See inode_to_bdi(): for ordinary disk-based swap, it will now pick
> up the bdi of the block device instead of noop_backing_dev_info, so
> swap would then pass the mapping_cap_account_dirty() and similar
> tests (mostly in mm/page-writeback.c), and go down codepaths it
> has never gone down before.
> 
> It's possible that swap (and shmem) would be better off going down
> those paths, to be throttled in a similar way to files; but that's
> debatable, and a much bigger change than you want to get into for
> zram stable pages.

Good point.
Thanks for the review, Hugh.

> 
> Maybe add SWP_STABLE_WRITES in include/linux/swap.h, and set that
> in swap_info->flags according to bdi_cap_stable_pages_required(),
> leaving mapping->host itself NULL as before?

The problem with the approach is that we need to get swap_info_struct
in reuse_swap_page so maybe, every caller should pass swp_entry_t
into reuse_swap_page. It would be no problem if swap slot is really
referenced the page(IOW, pte is real swp_entry_t) but some cases
where swap slot is already empty but the page remains in only
swap cache, we cannot pass swp_entry_t which means that we cannot
get swap_info_struct.

So, if I didn't miss, another option I can imagine is to move
SWP_STABLE_WRITES to address_space->flags as AS_STABLE_WRITES.
With that, we can always get the information without passing
swp_entry_t. Is there any better idea?


diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index dd15d39e1985..5397e82bfd57 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -26,6 +26,8 @@ enum mapping_flags {
 	AS_EXITING	= 4, 	/* final truncate in progress */
 	/* writeback related tags are not used */
 	AS_NO_WRITEBACK_TAGS = 5,
+	/* need stable write for swap */
+	AS_STABLE_WRITES = 6,
 };
 
 static inline void mapping_set_error(struct address_space *mapping, int error)
@@ -55,6 +57,21 @@ static inline int mapping_unevictable(struct address_space *mapping)
 	return !!mapping;
 }
 
+static inline void mapping_set_stable(struct address_space *mapping)
+{
+	set_bit(AS_STABLE_WRITES, &mapping->flags);
+}
+
+static inline void mapping_clear_stable(struct address_space *mapping)
+{
+	clear_bit(AS_STABLE_WRITES, &mapping->flags);
+}
+
+static inline int mapping_stable(struct address_space *mapping)
+{
+	return test_bit(AS_STABLE_WRITES, &mapping->flags);
+}
+
 static inline void mapping_set_exiting(struct address_space *mapping)
 {
 	set_bit(AS_EXITING, &mapping->flags);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 2210de290b54..0c31fd814933 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -943,11 +943,20 @@ bool reuse_swap_page(struct page *page, int *total_mapcount)
 	count = page_trans_huge_mapcount(page, total_mapcount);
 	if (count <= 1 && PageSwapCache(page)) {
 		count += page_swapcount(page);
-		if (count == 1 && !PageWriteback(page)) {
+		if (count != 1)
+			goto out;
+		if (!PageWriteback(page)) {
 			delete_from_swap_cache(page);
 			SetPageDirty(page);
+		} else {
+			struct address_space *mapping;
+
+			mapping = page_mapping(page);
+			if (mapping_stable(mapping))
+				return false;
 		}
 	}
+out:
 	return count <= 1;
 }
 
@@ -2386,6 +2395,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	unsigned long *frontswap_map = NULL;
 	struct page *page = NULL;
 	struct inode *inode = NULL;
+	struct address_space *swapper_space;
 
 	if (swap_flags & ~SWAP_FLAGS_VALID)
 		return -EINVAL;
@@ -2447,6 +2457,13 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		error = -ENOMEM;
 		goto bad_swap;
 	}
+
+	swapper_space = &swapper_spaces[p->type];
+	if (bdi_cap_stable_pages_required(inode_to_bdi(inode)))
+		mapping_set_stable(swapper_space);
+	else
+		mapping_clear_stable(swapper_space);
+
 	if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
 		int cpu;
 

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Re: [PATCH] mm: support anonymous stable page

[Hugh Dickins]

On Fri, 18 Nov 2016, Minchan Kim wrote:
> On Thu, Nov 17, 2016 at 08:35:10PM -0800, Hugh Dickins wrote:
> > 
> > Maybe add SWP_STABLE_WRITES in include/linux/swap.h, and set that
> > in swap_info->flags according to bdi_cap_stable_pages_required(),
> > leaving mapping->host itself NULL as before?
> 
> The problem with the approach is that we need to get swap_info_struct
> in reuse_swap_page so maybe, every caller should pass swp_entry_t
> into reuse_swap_page. It would be no problem if swap slot is really
> referenced the page(IOW, pte is real swp_entry_t) but some cases
> where swap slot is already empty but the page remains in only
> swap cache, we cannot pass swp_entry_t which means that we cannot
> get swap_info_struct.

I don't see the problem: if the page is PageSwapCache (and page
lock is held), then the swp_entry_t is there in page->private:
see page_swapcount(), which reuse_swap_page() just called.

> 
> So, if I didn't miss, another option I can imagine is to move
> SWP_STABLE_WRITES to address_space->flags as AS_STABLE_WRITES.
> With that, we can always get the information without passing
> swp_entry_t. Is there any better idea?

I think what you suggest below would work fine (and be quicker
than looking up the swap_info again): but is horribly misleading
for anyone else interested in stable writes, for whom the info is
kept in the bdi, and not in this new mapping flag.

So I'd much prefer that you keep the swap special case inside the
world of swap, with a SWP_STABLE_WRITES flag.  Maybe unfold
page_swapcount() inside reuse_swap_page(), so that it only
needs a single lookup (or perhaps I'm optimizing prematurely).

Hugh

> 
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index dd15d39e1985..5397e82bfd57 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -26,6 +26,8 @@ enum mapping_flags {
>  	AS_EXITING	= 4, 	/* final truncate in progress */
>  	/* writeback related tags are not used */
>  	AS_NO_WRITEBACK_TAGS = 5,
> +	/* need stable write for swap */
> +	AS_STABLE_WRITES = 6,
>  };
>  
>  static inline void mapping_set_error(struct address_space *mapping, int error)
> @@ -55,6 +57,21 @@ static inline int mapping_unevictable(struct address_space *mapping)
>  	return !!mapping;
>  }
>  
> +static inline void mapping_set_stable(struct address_space *mapping)
> +{
> +	set_bit(AS_STABLE_WRITES, &mapping->flags);
> +}
> +
> +static inline void mapping_clear_stable(struct address_space *mapping)
> +{
> +	clear_bit(AS_STABLE_WRITES, &mapping->flags);
> +}
> +
> +static inline int mapping_stable(struct address_space *mapping)
> +{
> +	return test_bit(AS_STABLE_WRITES, &mapping->flags);
> +}
> +
>  static inline void mapping_set_exiting(struct address_space *mapping)
>  {
>  	set_bit(AS_EXITING, &mapping->flags);
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 2210de290b54..0c31fd814933 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -943,11 +943,20 @@ bool reuse_swap_page(struct page *page, int *total_mapcount)
>  	count = page_trans_huge_mapcount(page, total_mapcount);
>  	if (count <= 1 && PageSwapCache(page)) {
>  		count += page_swapcount(page);
> -		if (count == 1 && !PageWriteback(page)) {
> +		if (count != 1)
> +			goto out;
> +		if (!PageWriteback(page)) {
>  			delete_from_swap_cache(page);
>  			SetPageDirty(page);
> +		} else {
> +			struct address_space *mapping;
> +
> +			mapping = page_mapping(page);
> +			if (mapping_stable(mapping))
> +				return false;
>  		}
>  	}
> +out:
>  	return count <= 1;
>  }
>  
> @@ -2386,6 +2395,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  	unsigned long *frontswap_map = NULL;
>  	struct page *page = NULL;
>  	struct inode *inode = NULL;
> +	struct address_space *swapper_space;
>  
>  	if (swap_flags & ~SWAP_FLAGS_VALID)
>  		return -EINVAL;
> @@ -2447,6 +2457,13 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>  		error = -ENOMEM;
>  		goto bad_swap;
>  	}
> +
> +	swapper_space = &swapper_spaces[p->type];
> +	if (bdi_cap_stable_pages_required(inode_to_bdi(inode)))
> +		mapping_set_stable(swapper_space);
> +	else
> +		mapping_clear_stable(swapper_space);
> +
>  	if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) {
>  		int cpu;

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