RE: [External Mail] Re: [PATCH] f2fs: optimize f2fs DIO overwrites

["정요한(JOUNG YOHAN) Mobile AE" <yohan.joung@sk.com>]

> From: Daeho Jeong <daeho43@gmail.com>
> Sent: Wednesday, March 12, 2025 6:14 AM
> To: Chao Yu <chao@kernel.org>
> Cc: Yohan Joung <jyh429@gmail.com>; jaegeuk@kernel.org; linux-f2fs-
> devel@lists.sourceforge.net; linux-kernel@vger.kernel.org; 정요한(JOUNG
> YOHAN) Mobile AE <yohan.joung@sk.com>
> Subject: [External Mail] Re: [PATCH] f2fs: optimize f2fs DIO overwrites
> 
> On Tue, Mar 11, 2025 at 5:00 AM Chao Yu <chao@kernel.org> wrote:
> >
> > On 3/7/25 22:56, Yohan Joung wrote:
> > > this is unnecessary when we know we are overwriting already
> > > allocated blocks and the overhead of starting a transaction can be
> > > significant especially for multithreaded workloads doing small writes.
> >
> > Hi Yohan,
> >
> > So you're trying to avoid f2fs_map_lock() in dio write path, right?
> >
> > Thanks,
> >
> > >
> > > Signed-off-by: Yohan Joung <yohan.joung@sk.com>
> > > ---
> > >  fs/f2fs/data.c | 20 ++++++++++++++++++++  fs/f2fs/f2fs.h |  1 +
> > > fs/f2fs/file.c |  5 ++++-
> > >  3 files changed, 25 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c index
> > > 09437dbd1b42..728630037b74 100644
> > > --- a/fs/f2fs/data.c
> > > +++ b/fs/f2fs/data.c
> > > @@ -4267,6 +4267,26 @@ static int f2fs_iomap_begin(struct inode *inode,
> loff_t offset, loff_t length,
> > >       return 0;
> > >  }
> > >
> > > +static int f2fs_iomap_overwrite_begin(struct inode *inode, loff_t
> offset,
> > > +             loff_t length, unsigned flags, struct iomap *iomap,
> > > +             struct iomap *srcmap)
> > > +{
> > > +     int ret;
> > > +
> > > +     /*
> > > +      * Even for writes we don't need to allocate blocks, so just
> pretend
> > > +      * we are reading to save overhead of starting a transaction.
> > > +      */
> > > +     flags &= ~IOMAP_WRITE;
> > > +     ret = f2fs_iomap_begin(inode, offset, length, flags, iomap,
> srcmap);
> > > +     WARN_ON_ONCE(!ret && iomap->type != IOMAP_MAPPED);
> > > +     return ret;
> > > +}
> > > +
> > >  const struct iomap_ops f2fs_iomap_ops = {
> > >       .iomap_begin    = f2fs_iomap_begin,
> > >  };
> > > +
> > > +const struct iomap_ops f2fs_iomap_overwrite_ops = {
> > > +     .iomap_begin    = f2fs_iomap_overwrite_begin,
> > > +};
> > > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h index
> > > c6cc2694f9ac..0511ab5ed42a 100644
> > > --- a/fs/f2fs/f2fs.h
> > > +++ b/fs/f2fs/f2fs.h
> > > @@ -3936,6 +3936,7 @@ void f2fs_destroy_post_read_processing(void);
> > >  int f2fs_init_post_read_wq(struct f2fs_sb_info *sbi);  void
> > > f2fs_destroy_post_read_wq(struct f2fs_sb_info *sbi);  extern const
> > > struct iomap_ops f2fs_iomap_ops;
> > > +extern const struct iomap_ops f2fs_iomap_overwrite_ops;
> > >
> > >  static inline struct page *f2fs_find_data_page(struct inode *inode,
> > >               pgoff_t index, pgoff_t *next_pgofs) diff --git
> > > a/fs/f2fs/file.c b/fs/f2fs/file.c index 82b21baf5628..bb2fe6dac9b6
> > > 100644
> > > --- a/fs/f2fs/file.c
> > > +++ b/fs/f2fs/file.c
> > > @@ -4985,6 +4985,7 @@ static ssize_t f2fs_dio_write_iter(struct kiocb
> *iocb, struct iov_iter *from,
> > >       const ssize_t count = iov_iter_count(from);
> > >       unsigned int dio_flags;
> > >       struct iomap_dio *dio;
> > > +     const struct iomap_ops *iomap_ops = &f2fs_iomap_ops;
> > >       ssize_t ret;
> > >
> > >       trace_f2fs_direct_IO_enter(inode, iocb, count, WRITE); @@
> > > -5025,7 +5026,9 @@ static ssize_t f2fs_dio_write_iter(struct kiocb
> *iocb, struct iov_iter *from,
> > >       dio_flags = 0;
> > >       if (pos + count > inode->i_size)
> > >               dio_flags |= IOMAP_DIO_FORCE_WAIT;
> > > -     dio = __iomap_dio_rw(iocb, from, &f2fs_iomap_ops,
> > > +     else if (f2fs_overwrite_io(inode, pos, count))
> > > +             iomap_ops = &f2fs_iomap_overwrite_ops;
> > > +     dio = __iomap_dio_rw(iocb, from, iomap_ops,
> > >                            &f2fs_iomap_dio_write_ops, dio_flags, NULL, 0);
> > >       if (IS_ERR_OR_NULL(dio)) {
> > >               ret = PTR_ERR_OR_ZERO(dio);
> 
> I think we can add the overwrite check in f2fs_iomap_begin() before
> setting the map.m_may_create, instead of adding a new function
> f2fs_iomap_overwrite_begin().
> What do you think?
Daeho, Is this the way you want it changed? If so, I'll upload it like this 
static int f2fs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
			    unsigned int flags, struct iomap *iomap,
			    struct iomap *srcmap)
{
	struct f2fs_map_blocks map = {};
	pgoff_t next_pgofs = 0;
	int err;

	map.m_lblk = F2FS_BYTES_TO_BLK(offset);
	map.m_len = F2FS_BYTES_TO_BLK(offset + length - 1) - map.m_lblk + 1;
	map.m_next_pgofs = &next_pgofs;
	map.m_seg_type = f2fs_rw_hint_to_seg_type(F2FS_I_SB(inode),
						inode->i_write_hint);
	if ((flags & IOMAP_WRITE) && !f2fs_overwrite_io(inode, offset, length))
		map.m_may_create = true;

> 
> >