[PATCH] btrfs: Do not check for PagePrivate twice

[PATCH] btrfs: Do not check for PagePrivate twice

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

We are checking PagePrivate twice, once with lock and once without.
Perform the check only once.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 fs/btrfs/extent_io.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index cceaf05aada2..425ba359178c 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3959,9 +3959,6 @@ int btree_write_cache_pages(struct address_space *mapping,
 		for (i = 0; i < nr_pages; i++) {
 			struct page *page = pvec.pages[i];
 
-			if (!PagePrivate(page))
-				continue;
-
 			spin_lock(&mapping->private_lock);
 			if (!PagePrivate(page)) {
 				spin_unlock(&mapping->private_lock);
-- 
2.16.4

[PATCH] btrfs: Do not check for writeback in btrfs_releasepage()

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

We check for PageWriteback in try_to_release_page(), the
sole caller of address_space->releasepage(). We don't need
to check it again in btrfs_releasepage()

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 fs/btrfs/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 0f2754eaa05b..10303c2379a9 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -8793,7 +8793,7 @@ static int __btrfs_releasepage(struct page *page, gfp_t gfp_flags)
 
 static int btrfs_releasepage(struct page *page, gfp_t gfp_flags)
 {
-	if (PageWriteback(page) || PageDirty(page))
+	if (PageDirty(page))
 		return 0;
 	return __btrfs_releasepage(page, gfp_flags);
 }
-- 
2.16.4

Re: [PATCH] btrfs: Do not check for writeback in btrfs_releasepage()

[Goldwyn Rodrigues]

Scratch this. btrfs_releasepage() is called from btrfs_invalidatepage()
as well. Sorry for the noise.


On 13:15 18/10, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> We check for PageWriteback in try_to_release_page(), the
> sole caller of address_space->releasepage(). We don't need
> to check it again in btrfs_releasepage()
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>  fs/btrfs/inode.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 0f2754eaa05b..10303c2379a9 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -8793,7 +8793,7 @@ static int __btrfs_releasepage(struct page *page, gfp_t gfp_flags)
>  
>  static int btrfs_releasepage(struct page *page, gfp_t gfp_flags)
>  {
> -	if (PageWriteback(page) || PageDirty(page))
> +	if (PageDirty(page))
>  		return 0;
>  	return __btrfs_releasepage(page, gfp_flags);
>  }
> -- 
> 2.16.4
> 

-- 
Goldwyn

Re: [PATCH] btrfs: Do not check for PagePrivate twice

[Filipe Manana]

On Sat, Oct 19, 2019 at 10:05 AM Goldwyn Rodrigues <rgoldwyn@suse.de> wrote:
>
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
>
> We are checking PagePrivate twice, once with lock and once without.
> Perform the check only once.

Have you checked if there's some performance degradation after
removing the check?
My guess is it's there to avoid taking the lock, as the lock can be
heavily used on a system under heavy load (maybe even if it's not too
heavy, since we generate a lot of dirty metadata due to cow).
The page may have been released after locking the mapping, that's why
we check it twice, and after unlocking we are sure it can not be
released due to taking a reference on the extent buffer.

Thanks.

>
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>  fs/btrfs/extent_io.c | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index cceaf05aada2..425ba359178c 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -3959,9 +3959,6 @@ int btree_write_cache_pages(struct address_space *mapping,
>                 for (i = 0; i < nr_pages; i++) {
>                         struct page *page = pvec.pages[i];
>
> -                       if (!PagePrivate(page))
> -                               continue;
> -
>                         spin_lock(&mapping->private_lock);
>                         if (!PagePrivate(page)) {
>                                 spin_unlock(&mapping->private_lock);
> --
> 2.16.4
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”

Re: [PATCH] btrfs: Do not check for PagePrivate twice

[David Sterba]

On Mon, Oct 21, 2019 at 10:52:06AM +0100, Filipe Manana wrote:
> On Sat, Oct 19, 2019 at 10:05 AM Goldwyn Rodrigues <rgoldwyn@suse.de> wrote:
> > We are checking PagePrivate twice, once with lock and once without.
> > Perform the check only once.
> 
> Have you checked if there's some performance degradation after
> removing the check?
> My guess is it's there to avoid taking the lock, as the lock can be
> heavily used on a system under heavy load (maybe even if it's not too
> heavy, since we generate a lot of dirty metadata due to cow).
> The page may have been released after locking the mapping, that's why
> we check it twice, and after unlocking we are sure it can not be
> released due to taking a reference on the extent buffer.

That's my understanding as well, so the duplicate unlocked check should
stay.