Re: [PATCH v2 5/8] fs: add a filemap_fdatawrite_wbc helper

[Nikolay Borisov <nborisov@suse.com>]

On 29.06.21 г. 16:59, Josef Bacik wrote:
> Btrfs sometimes needs to flush dirty pages on a bunch of dirty inodes in
> order to reclaim metadata reservations.  Unfortunately most helpers in
> this area are too smart for us
> 
> 1) The normal filemap_fdata* helpers only take range and sync modes, and
>    don't give any indication of how much was written, so we can only
>    flush full inodes, which isn't what we want in most cases.
> 2) The normal writeback path requires us to have the s_umount sem held,
>    but we can't unconditionally take it in this path because we could
>    deadlock.
> 3) The normal writeback path also skips inodes with I_SYNC set if we
>    write with WB_SYNC_NONE.  This isn't the behavior we want under heavy
>    ENOSPC pressure, we want to actually make sure the pages are under
>    writeback before returning, and if another thread is in the middle of
>    writing the file we may return before they're under writeback and
>    miss our ordered extents and not properly wait for completion.
> 4) sync_inode() uses the normal writeback path and has the same problem
>    as #3.
> 
> What we really want is to call do_writepages() with our wbc.  This way
> we can make sure that writeback is actually started on the pages, and we
> can control how many pages are written as a whole as we write many
> inodes using the same wbc.  Accomplish this with a new helper that does
> just that so we can use it for our ENOSPC flushing infrastructure.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Reviewed-by: Nikolay Borisov <nborisov@suse.com>