Re: [PATCH] btrfs: save i_size in compress_file_range

[Filipe Manana <fdmanana@gmail.com>]

On Fri, Oct 11, 2019 at 2:05 PM Josef Bacik <josef@toxicpanda.com> wrote:
>
> We hit a regression while rolling out 5.2 internally where we were
> hitting the following panic
>
> kernel BUG at mm/page-writeback.c:2659!
> RIP: 0010:clear_page_dirty_for_io+0xe6/0x1f0
> Call Trace:
>  __process_pages_contig+0x25a/0x350
>  ? extent_clear_unlock_delalloc+0x43/0x70
>  submit_compressed_extents+0x359/0x4d0
>  normal_work_helper+0x15a/0x330
>  process_one_work+0x1f5/0x3f0
>  worker_thread+0x2d/0x3d0
>  ? rescuer_thread+0x340/0x340
>  kthread+0x111/0x130
>  ? kthread_create_on_node+0x60/0x60
>  ret_from_fork+0x1f/0x30
>
> this is happening because the page is not locked when doing
> clear_page_dirty_for_io.  Looking at the core dump it was because our
> async_extent had a ram_size of 24576 but our async_chunk range only
> spanned 20480, so we had a whole extra page in our ram_size for our
> async_extent.
>
> This happened because we try not to compress pages outside of our
> i_size, however a cleanup patch changed us to do
>
> actual_end = min_t(u64, i_size_read(inode), end + 1);
>
> which is problematic because i_size_read() can evaluate to different
> values in between checking and assigning.  So either a expanding
> truncate or a fallocate could increase our i_size while we're doing

Well, not just those operations but a write starting at or beyond eof
could also do it,
since at writeback time we don't hold the inode's lock and the
buffered write path doesn't lock the range if it starts at or past
current i_size.

> writeout and actual_end would end up being past the range we have
> locked.
>
> I confirmed this was what was happening by installing a debug kernel
> that had
>
> actual_end = min_t(u64, i_size_read(inode), end + 1);
> if (actual_end > end + 1) {
>         printk(KERN_ERR "WE GOT FUCKED\n");

I think we coud be more polite on changelogs and mailing lists, or we
could add a tag in the subjects warning about improper content like
video games and music records :)

>         actual_end = end + 1;
> }
>
> and installing it onto 500 boxes of the tier that had been seeing the
> problem regularly.  Last night I got my debug message and no panic,
> confirming what I expected.
>
> Fixes: 62b37622718c ("btrfs: Remove isize local variable in compress_file_range")
> cc: stable@vger.kernel.org
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Anyway, looks good to me.

Reviewed-by: Filipe Manana <fdmanana@suse.com>

> ---
>  fs/btrfs/inode.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 2eb1d7249f83..9a483d1f61f8 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -474,6 +474,7 @@ static noinline int compress_file_range(struct async_chunk *async_chunk)
>         u64 start = async_chunk->start;
>         u64 end = async_chunk->end;
>         u64 actual_end;
> +       loff_t i_size = i_size_read(inode);
>         int ret = 0;
>         struct page **pages = NULL;
>         unsigned long nr_pages;
> @@ -488,7 +489,13 @@ static noinline int compress_file_range(struct async_chunk *async_chunk)
>         inode_should_defrag(BTRFS_I(inode), start, end, end - start + 1,
>                         SZ_16K);
>
> -       actual_end = min_t(u64, i_size_read(inode), end + 1);
> +       /*
> +        * We need to save i_size before now because it could change in between
> +        * us evaluating the size and assigning it.  This is because we lock and
> +        * unlock the page in truncate and fallocate, and then modify the i_size
> +        * later on.
> +        */
> +       actual_end = min_t(u64, i_size, end + 1);
>  again:
>         will_compress = 0;
>         nr_pages = (end >> PAGE_SHIFT) - (start >> PAGE_SHIFT) + 1;
> --
> 2.21.0
>


-- 
Filipe David Manana,

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