Date:   Tue, 14 Dec 2021 10:56:41 -0800
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Chao Yu <chao@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: skip f2fs_preallocate_blocks() for
 overwrite case
Message-ID: <Ybjo6Zqpc7Wnev/r@google.com>
References: <20210928151911.11189-1-chao@kernel.org>
 <YVNoHudG5c65X85G@google.com>
 <65f6c366-9e5b-fe7f-7c38-061996d1882b@kernel.org>
 <dec765de-407b-07c3-75f6-ec7f71c618b7@kernel.org>
 <YXwyvllUOm6jLiF5@google.com>
 <3e653a3d-ddb9-e115-d871-3659a1ba5530@kernel.org>
 <96959788-73b2-6e9b-3aa7-b1e23e9da417@kernel.org>
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On 12/12, Chao Yu wrote:
> Ping,
> 
> On 2021/10/30 11:02, Chao Yu wrote:
> > On 2021/10/30 1:43, Jaegeuk Kim wrote:
> > > On 10/29, Chao Yu wrote:
> > > > Ping,
> > > > 
> > > > On 2021/9/29 8:05, Chao Yu wrote:
> > > > > On 2021/9/29 3:08, Jaegeuk Kim wrote:
> > > > > > On 09/28, Chao Yu wrote:
> > > > > > > In f2fs_file_write_iter(), let's use f2fs_overwrite_io() to
> > > > > > > check whethere it is overwrite case, for such case, we can skip
> > > > > > > f2fs_preallocate_blocks() in order to avoid f2fs_do_map_lock(),
> > > > > > > which may be blocked by checkpoint() potentially.
> > > > > > > 
> > > > > > > Signed-off-by: Chao Yu <chao@kernel.org>
> > > > > > > ---
> > > > > > > ?? fs/f2fs/file.c | 4 ++++
> > > > > > > ?? 1 file changed, 4 insertions(+)
> > > > > > > 
> > > > > > > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > > > > > > index 13deae03df06..51fecb2f4db5 100644
> > > > > > > --- a/fs/f2fs/file.c
> > > > > > > +++ b/fs/f2fs/file.c
> > > > > > > @@ -4321,6 +4321,10 @@ static ssize_t f2fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
> > > > > > > ?????????? preallocated = true;
> > > > > > > ?????????? target_size = iocb->ki_pos + iov_iter_count(from);
> > > > > > > +??????? if (f2fs_overwrite_io(inode, iocb->ki_pos,
> > > > > > > +??????????????????????? iov_iter_count(from)))
> > > > > > > +??????????? goto write;
> > > > > > 
> > > > > > This calls f2fs_map_blocks() which can be duplicate, if it's not the overwirte
> > > > > > case. Do we have other benefit?
> > > > > 
> > > > > f2fs_overwrite_io() will break for append write case w/ below check:
> > > > > 
> > > > > ? ????if (pos + len > i_size_read(inode))
> > > > > ? ??????? return false;
> > > > > 
> > > > > I guess we may only suffer double f2fs_map_blocks() for write hole
> > > > > case, e.g. truncate to large size & write inside the filesize. For
> > > > > this case, how about adding a condition to allow double f2fs_map_blocks()
> > > > > only if write size is smaller than a threshold?
> > > 
> > > I still don't see the benefit much to do double f2fs_map_blocks. What is the
> > > problem here?
> > 
> > There is potential hangtask happened during swapfile's writeback:
> > 
> > - loop_kthread_worker_fn
> >  ?- kthread_worker_fn
> >  ? - loop_queue_work
> >  ?? - lo_rw_aio
> >  ??? - f2fs_file_write_iter
> >  ???? - f2fs_preallocate_blocks
> >  ????? - f2fs_map_blocks
> >  ?????? - down_read
> >  ??????? - rwsem_down_read_slowpath
> >  ???????? - schedule
> > 
> > I try to mitigate such issue by preallocating swapfile's block address and
> > avoid f2fs_do_map_lock() as much as possible in swapfile's writeback path...

How about checking i_blocks and i_size instead of checking the entire map?

> > 
> > Thanks,
> > 
> > > 
> > > > > 
> > > > > Thanks,
> > > > > 
> > > > > > 
> > > > > > > +
> > > > > > > ?????????? err = f2fs_preallocate_blocks(iocb, from);
> > > > > > > ?????????? if (err) {
> > > > > > > ?? out_err:
> > > > > > > -- 
> > > > > > > 2.32.0
> > > > > 
> > > > > 
> > > > > _______________________________________________
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