Re: [PATCH] f2fs: skip f2fs_preallocate_blocks() for overwrite case

[Chao Yu]

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@xxxxxxxxxx>
> > ---
> >   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?

Thanks,

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