Re: [f2fs-dev] [PATCH v5] f2fs: use post-decrement count for cp_wait wakeup

[Chao Yu]

On 6/18/26 11:38, Wenjie Qi wrote:
>    The race is between dec_page_count() and the later get_pages() check:
>    another CP-data writeback can be submitted after the counter reaches zero
>    but before get_pages() observes it, so the zero transition may miss the
>    cp_wait wakeup.

Can you describe race condition like below calltrace? which will be easier to
understand?

    loop device                             umount
    - worker_thread
     - loop_process_work
      - do_req_filebacked
       - lo_rw_aio
        - lo_rw_aio_complete
         - blk_mq_end_request
          - blk_update_request
           - f2fs_write_end_io
            - dec_page_count
            - folio_end_writeback
                                            - kill_f2fs_super
                                             - kill_block_super
                                              - f2fs_put_super
                                             : free(sbi)
           : get_pages(, F2FS_WB_CP_DATA)
             accessed sbi which is freed

Thanks,

>    v6 also adds dec_page_count_return() and uses it instead of accessing
>    nr_pages directly.  The wakeup logic is unchanged from v5.
>
> https://lore.kernel.org/linux-f2fs-devel/20260618031008.2447279-1-qiwenjie@xxxxxxxxxx/T/#u
>
> On Thu, Jun 18, 2026 at 2:09 AM Jaegeuk Kim <jaegeuk@xxxxxxxxxx> wrote:
> > On 06/16, Wenjie Qi wrote:
> > > f2fs_write_end_io() decrements the writeback page counter and then
> > > reads it again with get_pages() to decide whether the last
> > > F2FS_WB_CP_DATA completion should wake cp_wait.
> > >
> > > Use atomic_dec_return() for F2FS_WB_CP_DATA completions so the wakeup
> > > decision is made from the value produced by the decrement itself. Keep
> > > the existing dec_page_count() path for other writeback counters.
> >
> > Is there a race condition to do this? If so, can you describe? And, I think
> > we need a wrapper function instead of calling nr_pages directly.
> >
> > > Fixes: e234088758fc ("f2fs: avoid wait if IO end up when do_checkpoint for better performance")
> > > Fixes: ce2739e482bc ("f2fs: fix to avoid UAF in f2fs_write_end_io()")
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Wenjie Qi <qiwenjie@xxxxxxxxxx>
> > > ---
> > >   fs/f2fs/data.c | 12 +++++++-----
> > >   1 file changed, 7 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > > index d83a21998ec2..58d23eb74ec2 100644
> > > --- a/fs/f2fs/data.c
> > > +++ b/fs/f2fs/data.c
> > > @@ -392,15 +392,17 @@ static void f2fs_write_end_io(struct bio *bio)
> > >                if (f2fs_in_warm_node_list(folio))
> > >                        f2fs_del_fsync_node_entry(sbi, folio);
> > >
> > > -             dec_page_count(sbi, type);
> > > -
> > >                /*
> > >                 * we should access sbi before folio_end_writeback() to
> > >                 * avoid racing w/ kill_f2fs_super()
> > >                 */
> > > -             if (type == F2FS_WB_CP_DATA && !get_pages(sbi, type) &&
> > > -                             wq_has_sleeper(&sbi->cp_wait))
> > > -                     wake_up(&sbi->cp_wait);
> > > +             if (type == F2FS_WB_CP_DATA) {
> > > +                     if (!atomic_dec_return(&sbi->nr_pages[type]) &&
> > > +                         wq_has_sleeper(&sbi->cp_wait))
> > > +                             wake_up(&sbi->cp_wait);
> > > +             } else {
> > > +                     dec_page_count(sbi, type);
> > > +             }
> > >
> > >                folio_clear_f2fs_gcing(folio);
> > >                folio_end_writeback(folio);
> > >
> > > base-commit: c0b65f6129c7fbb526e921dd60261650f1b2bef9
> > > --
> > > 2.43.0
> > >
> > >
> > >
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