Re: [PATCH 2/3] btrfs: Wait for extent bits to release page

[Filipe Manana <fdmanana@gmail.com>]

On Fri, Jun 5, 2020 at 9:48 PM Goldwyn Rodrigues <rgoldwyn@suse.de> wrote:
>
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
>
> While trying to release a page, the extent containing the page may be locked
> which would stop the page from being released. Wait for the
> extent lock to be cleared, if blocking is allowed and then clear
> the bits.
>
> While we are at it, clean the code of try_release_extent_state() to make
> it simpler.
>
> Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
> Reviewed-by: Nikolay Borisov <nborisov@suse.com>
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>

I'm confused Goldwyn.

Previously in another thread [1] you mentioned you dropped this patch
from a previous patchset because
it was causing locking issues (iirc you mentioned a deadlock in
another different thread).

Now you send exactly the same patch (unless I missed some very subtle
change, in which case keeping the reviewed-by tags is not correct).
Are the locking issues gone? What fixed them?
And how did you trigger those issues, some specific fstest (which?),
some other test (which/how)?

And if this patch is now working for some reason, then why are patches
1/3 and 3/3 needed?
Wasn't patch 1/3 motivated exactly because this patch (2/3) was
causing the locking issues.

Thanks.

[1] https://lore.kernel.org/linux-btrfs/20200526164428.sirhx6yjsghxpnqt@fiona/

> ---
>  fs/btrfs/extent_io.c | 37 ++++++++++++++++---------------------
>  fs/btrfs/extent_io.h |  2 +-
>  fs/btrfs/inode.c     |  4 ++--
>  3 files changed, 19 insertions(+), 24 deletions(-)
>
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index c59e07360083..0ab444d2028d 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -4466,33 +4466,28 @@ int extent_invalidatepage(struct extent_io_tree *tree,
>   * are locked or under IO and drops the related state bits if it is safe
>   * to drop the page.
>   */
> -static int try_release_extent_state(struct extent_io_tree *tree,
> +static bool try_release_extent_state(struct extent_io_tree *tree,
>                                     struct page *page, gfp_t mask)
>  {
>         u64 start = page_offset(page);
>         u64 end = start + PAGE_SIZE - 1;
> -       int ret = 1;
>
>         if (test_range_bit(tree, start, end, EXTENT_LOCKED, 0, NULL)) {
> -               ret = 0;
> -       } else {
> -               /*
> -                * at this point we can safely clear everything except the
> -                * locked bit and the nodatasum bit
> -                */
> -               ret = __clear_extent_bit(tree, start, end,
> -                                ~(EXTENT_LOCKED | EXTENT_NODATASUM),
> -                                0, 0, NULL, mask, NULL);
> -
> -               /* if clear_extent_bit failed for enomem reasons,
> -                * we can't allow the release to continue.
> -                */
> -               if (ret < 0)
> -                       ret = 0;
> -               else
> -                       ret = 1;
> +               if (!gfpflags_allow_blocking(mask))
> +                       return false;
> +               wait_extent_bit(tree, start, end, EXTENT_LOCKED);
>         }
> -       return ret;
> +       /*
> +        * At this point we can safely clear everything except the locked and
> +        * nodatasum bits. If clear_extent_bit failed due to -ENOMEM,
> +        * don't allow release.
> +        */
> +       if (__clear_extent_bit(tree, start, end,
> +                               ~(EXTENT_LOCKED | EXTENT_NODATASUM), 0, 0,
> +                               NULL, mask, NULL) < 0)
> +               return false;
> +
> +       return true;
>  }
>
>  /*
> @@ -4500,7 +4495,7 @@ static int try_release_extent_state(struct extent_io_tree *tree,
>   * in the range corresponding to the page, both state records and extent
>   * map records are removed
>   */
> -int try_release_extent_mapping(struct page *page, gfp_t mask)
> +bool try_release_extent_mapping(struct page *page, gfp_t mask)
>  {
>         struct extent_map *em;
>         u64 start = page_offset(page);
> diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
> index 9a10681b12bf..6cba4ad6ebc1 100644
> --- a/fs/btrfs/extent_io.h
> +++ b/fs/btrfs/extent_io.h
> @@ -189,7 +189,7 @@ typedef struct extent_map *(get_extent_t)(struct btrfs_inode *inode,
>                                           struct page *page, size_t pg_offset,
>                                           u64 start, u64 len);
>
> -int try_release_extent_mapping(struct page *page, gfp_t mask);
> +bool try_release_extent_mapping(struct page *page, gfp_t mask);
>  int try_release_extent_buffer(struct page *page);
>
>  int extent_read_full_page(struct page *page, get_extent_t *get_extent,
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 1242d0aa108d..8cb44c49c1d2 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -7887,8 +7887,8 @@ btrfs_readpages(struct file *file, struct address_space *mapping,
>
>  static int __btrfs_releasepage(struct page *page, gfp_t gfp_flags)
>  {
> -       int ret = try_release_extent_mapping(page, gfp_flags);
> -       if (ret == 1) {
> +       bool ret = try_release_extent_mapping(page, gfp_flags);
> +       if (ret) {
>                 ClearPagePrivate(page);
>                 set_page_private(page, 0);
>                 put_page(page);
> --
> 2.25.0
>


-- 
Filipe David Manana,

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