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

[Wenjie Qi]

Yes. I described the race with a calltrace-style timeline in v7. The
code is
  unchanged from v6.

https://lore.kernel.org/linux-f2fs-devel/20260618100503.2601790-1-qiwenjie@xxxxxxxxxx/T/#u

On Thu, Jun 18, 2026 at 5:10 PM Chao Yu <chao@xxxxxxxxxx> wrote:
>
> 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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>