Re: [PATCH] btrfs: fix a potential hole-punching failure

[bingjing chang]

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Filipe Manana <fdmanana@xxxxxxxxx> 於 2021年3月24日 週三 下午8:16寫道：
>
> On Wed, Mar 24, 2021 at 11:15 AM bingjingc <bingjingc@xxxxxxxxxxxx> wrote:
> >
> > From: BingJing Chang <bingjingc@xxxxxxxxxxxx>
> >
> > In commit d77815461f04 ("btrfs: Avoid trucating page or punching hole in
> > a already existed hole."), existed holes can be skipped by calling
> > find_first_non_hole() to adjust *start and *len. However, if the given
> > len is invalid and large, when an EXTENT_MAP_HOLE extent is found, the
> > *len will not be set to zero because (em->start + em->len) is less than
> > (*start + *len). Then the ret will be 1 but the *len will not be set to
> > 0. The propagated non-zero ret will result in fallocate failure.
> >
> > In the while-loop of btrfs_replace_file_extents(), len is not updated
> > every time before it calls find_first_non_hole(). That is, if the last
> > file extent in the given hole-punching range has been dropped but
> > btrfs_drop_extents() fails with -ENOSPC (btrfs_drop_extents() runs out
> > of reserved space of the given transaction), the problem can happen.
>
> This is not entirely clear. Dropping the last extent and still
> returning ENOSPC is confusing.
> I think you mean that it drops the last file extent item that does not
> represent hole (disk_bytenr > 0), and after it there's only one file
> extent item representing a hole (disk_bytenr == 0).
> It fails with -ENOSPC when attempting to drop the file extent item
> representing the hole, after successfully dropping the non-hole file
> extent item.
> Is that it?
>

Thank you for your comments. You're right.
Saying the last file extent is not correct and confusing.
I revised and send the v2 patch for fixing the commit message. Thank you.

> > After it calls find_first_non_hole(), the cur_offset will be adjusted
> > to be larger than or equal to end. However, since the len is not set to
> > zero. The break-loop condition (ret && !len) will not meet. After it
> > leaves the while-loop, uncleared ret will result in fallocate failure.
>
> Ok, fallocate will return 1, an unexpected return value.
>
> >
> > We're not able to construct a reproducible way to let
> > btrfs_drop_extents() fails with -ENOSPC after it drops the last file
> > extent but with remaining holes. However, it's quite easy to fix. We
> > just need to update and check the len every time before we call
> > find_first_non_hole(). To make the while loop more readable, we also
> > pull the variable updates to the bottom of loop like this:
> > while (cur_offset < end) {
> >         ...
> >         // update cur_offset & len
> >         // advance cur_offset & len in hole-punching case if needed
> > }
> >
> > Reported-by: Robbie Ko <robbieko@xxxxxxxxxxxx>
> > Fixes: d77815461f04 ("btrfs: Avoid trucating page or punching hole in a
> > already existed hole.")
> > Reviewed-by: Robbie Ko <robbieko@xxxxxxxxxxxx>
> > Reviewed-by: Chung-Chiang Cheng <cccheng@xxxxxxxxxxxx>
> > Signed-off-by: BingJing Chang <bingjingc@xxxxxxxxxxxx>
>
> Looks good.
> Please just update that paragraph to be more clear about what is going on.
>
> Thanks.
>
> > ---
> >  fs/btrfs/file.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> > index 0e155f0..dccb017 100644
> > --- a/fs/btrfs/file.c
> > +++ b/fs/btrfs/file.c
> > @@ -2735,8 +2735,6 @@ int btrfs_replace_file_extents(struct inode *inode, struct btrfs_path *path,
> >                         extent_info->file_offset += replace_len;
> >                 }
> >
> > -               cur_offset = drop_args.drop_end;
> > -
> >                 ret = btrfs_update_inode(trans, root, BTRFS_I(inode));
> >                 if (ret)
> >                         break;
> > @@ -2756,7 +2754,9 @@ int btrfs_replace_file_extents(struct inode *inode, struct btrfs_path *path,
> >                 BUG_ON(ret);    /* shouldn't happen */
> >                 trans->block_rsv = rsv;
> >
> > -               if (!extent_info) {
> > +               cur_offset = drop_args.drop_end;
> > +               len = end - cur_offset;
> > +               if (!extent_info && len) {
> >                         ret = find_first_non_hole(BTRFS_I(inode), &cur_offset,
> >                                                   &len);
> >                         if (unlikely(ret < 0))
> > --
> > 2.7.4
> >
>
>
> --
> Filipe David Manana,
>
> “Whether you think you can, or you think you can't — you're right.”

Thanks,
BingJing Chang