Re: [PATCH v2 2/3] btrfs: use bio::remaining for async checksumming synchronization

[Qu Wenruo]

在 2026/9/5 08:11, Qu Wenruo 写道:
> 在 2026/9/3 15:53, Daniel Vacek 写道:
> > We can use bio::remaining counter to sync the offloaded checksuming.
> > As a result we can slim down the btrfs_bio structure by 24 bytes
> > and simplify the code a bit.
> >
> > $ pahole | diff
> > -    /* size: 328, cachelines: 6, members: 15 */
> > +    /* size: 304, cachelines: 5, members: 14 */
> >
> > Moreover this will allow us enabling async checksumming with encryption
> > where we need to checksum the bounce bio instead of our regular one
> > embedded in btrfs_bio. And so we need to extend it's lifetime. This is
> > the preffered way to do so.
> >
> > Signed-off-by: Daniel Vacek <neelx@xxxxxxxx>
> > Reviewed-by: Qu Wenruo <wqu@xxxxxxxx>
> > ---
> >
> > No change since v1
> > ---
> >   fs/btrfs/bio.c       | 4 ----
> >   fs/btrfs/bio.h       | 4 ----
> >   fs/btrfs/file-item.c | 6 ++----
> >   3 files changed, 2 insertions(+), 12 deletions(-)
> >
> > diff --git a/fs/btrfs/bio.c b/fs/btrfs/bio.c
> > index 19b4855969f5..771b7d598aee 100644
> > --- a/fs/btrfs/bio.c
> > +++ b/fs/btrfs/bio.c
> > @@ -103,7 +103,6 @@ static struct btrfs_bio *btrfs_split_bio(struct 
> > btrfs_fs_info *fs_info,
> >       bbio->can_use_append = orig_bbio->can_use_append;
> >       bbio->is_scrub = orig_bbio->is_scrub;
> >       bbio->is_remap = orig_bbio->is_remap;
> > -    bbio->async_csum = orig_bbio->async_csum;
> >       atomic_inc(&orig_bbio->pending_ios);
> >       return bbio;
> > @@ -114,9 +113,6 @@ void btrfs_bio_end_io(struct btrfs_bio *bbio, 
> > blk_status_t status)
> >       /* Make sure we're already in task context. */
> >       ASSERT(in_task());
> > -    if (bbio->async_csum)
> > -        wait_for_completion(&bbio->csum_done);
> > -
> >       bbio->bio.bi_status = status;
> >       if (bbio->bio.bi_pool == &btrfs_clone_bioset) {
> >           struct btrfs_bio *orig_bbio = bbio->private;
> > diff --git a/fs/btrfs/bio.h b/fs/btrfs/bio.h
> > index b7bd377a0162..bbf362b8668b 100644
> > --- a/fs/btrfs/bio.h
> > +++ b/fs/btrfs/bio.h
> > @@ -58,7 +58,6 @@ struct btrfs_bio {
> >               struct btrfs_ordered_extent *ordered;
> >               struct btrfs_ordered_sum *sums;
> >               struct work_struct csum_work;
> > -            struct completion csum_done;
> >               struct bvec_iter csum_saved_iter;
> >               u64 orig_physical;
> >               u64 orig_logical;
> > @@ -93,9 +92,6 @@ struct btrfs_bio {
> >       /* Whether the bio is coming from copy_remapped_data_io(). */
> >       bool is_remap:1;
> > -    /* Whether the csum generation for data write is async. */
> > -    bool async_csum:1;
> > -
> >       /* Whether the bio is written using zone append. */
> >       bool can_use_append:1;
> > diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
> > index 4a7681557ec1..0fed4e0d32d5 100644
> > --- a/fs/btrfs/file-item.c
> > +++ b/fs/btrfs/file-item.c
> > @@ -818,9 +818,8 @@ static void csum_one_bio_work(struct work_struct 
> > *work)
> >       struct btrfs_bio *bbio = container_of(work, struct btrfs_bio, 
> > csum_work);
> >       ASSERT(btrfs_op(&bbio->bio) == BTRFS_MAP_WRITE);
> > -    ASSERT(bbio->async_csum == true);
> >       csum_one_bio(bbio);
> > -    complete(&bbio->csum_done);
> > +    bio_endio(&bbio->bio);
> >   }
> >   /*
> > @@ -854,8 +853,7 @@ int btrfs_csum_one_bio(struct btrfs_bio *bbio, 
> > bool async)
> >           csum_one_bio(bbio);
> >           return 0;
> >       }
> > -    init_completion(&bbio->csum_done);
> > -    bbio->async_csum = true;
> > +    bio_inc_remaining(bio);
>
> There seems to be a window where the bio can be finished before submission.
>
> Firstly at btrfs_csum_one_bio() time, bio->bi_endio is not yet 
> initialized, it's only properly assigned at btrfs_submit_bio().
>
> Then we queue the csum generation work.
>
> But by some bad timing, the bio submission is delayed, we can have the 
> following sequence:
>
>          Submission               |       Csum generation
> ---------------------------------+------------------------------------
> btrfs_submit_chunk()             |
> |- btrfs_csum_one_bio()          |
> |  |- bio_inc_remaining()        |
> |  |- schedule_work()            | csum_one_bio_work()
> |                                | |- bio_endio()
> |                                | Now the bio is finished, although
> |                                | bi_end_io is NULL, nothing real
> |                                | happened.

Damn it, I really need some tea before reviewing patches in the morning.

A bio has bi_remaining initialized to 1, so bio_inc_remaining() will 
change it to 2.

After bio_inc_remaining(), the next bio_endio() will not call 
bi_end_io(), but only decrease the bi_remaining back to 1.

So it won't call bi_end_io() in this case.

Please discard the above analysis.
> |- btrfs_submit_bio()
>
>
> This can be even worse, if the bio_endio() is called when 
> btrfs_submit_bio() has only partially setup the bio (e.g. bi_end_io() is 
> set, but bi_bdev is not set)
>
> Not mention now it changed the context where the endio function is called.
>
> Previously csum_one_bio_work() will never call bi_end_io() function, but 
> now it can.
>
> The change has a much larger impact than I initially thought.
> >       INIT_WORK(&bbio->csum_work, csum_one_bio_work);
> >       schedule_work(&bbio->csum_work);
> >       return 0;