Re: [f2fs-dev] [PATCH] f2fs: fix to tag STATX_DIOALIGN only if inode support dio

[Chao Yu]

On 2024/9/11 1:06, Eric Biggers wrote:
> On Tue, Sep 10, 2024 at 08:57:53PM +0800, Chao Yu via Linux-f2fs-devel wrote:
> > After commit 5c8764f8679e ("f2fs: fix to force buffered IO on inline_data
> > inode"), f2fs starts to force using buffered IO on inline_data inode.
> >
> > And also, it will cause f2fs_getattr() returning invalid zeroed value on
> > .dio_mem_align and .dio_offset_align fields, however, STATX_DIOALIGN flag
> > was been tagged. User may use zeroed .stx_dio_offset_align value
> > since STATX_DIOALIGN was been tagged, then it causes a deadloop during
> > generic/465 test due to below logic:
> >
> > align=stx_dio_offset_align(it equals to zero)
> > page_size=4096
> > while [ $align -le $page_size ]; do
> > 	echo "$AIO_TEST -a $align -d $testfile.$align" >> $seqres.full
> > 	$AIO_TEST -a $align -d $testfile.$align 2>&1 | tee -a $seqres.full
> > 	align=$((align * 2))
> > done
> >
> > Quoted from description of statx manual:
> >
> > " If  a  filesystem  does  not support a field or if it has an
> >    unrepresentable value (for instance, a file with an exotic type),
> >    then the mask bit corresponding to that field will be cleared in
> >    stx_mask even if the user asked for it and a dummy value will be
> >    filled in for compatibility purposes if one is available (e.g.,
> >    a dummy UID and GID may be specified to mount under some
> >    circumstances)."
> >
> > We should not set STATX_DIOALIGN flag in kstat.stx_mask if inode
> > does not support DIO, so that it can indicate related fields contain
> > dummy value, and avoid following incorrect use of them.
> >
> > Fixes: c8c02272a9f7 ("f2fs: support STATX_DIOALIGN")
>
> When claiming to be Fixing a commit, please make sure to Cc the author of that
> commit.

No problem, will make sure they were Cced.

> > Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
> > ---
> >   fs/f2fs/file.c | 11 ++++-------
> >   1 file changed, 4 insertions(+), 7 deletions(-)
> >
> > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > index 99903eafa7fe..f0b8b77e93ba 100644
> > --- a/fs/f2fs/file.c
> > +++ b/fs/f2fs/file.c
> > @@ -906,14 +906,11 @@ int f2fs_getattr(struct mnt_idmap *idmap, const struct path *path,
> >   	 * f2fs sometimes supports DIO reads but not DIO writes.  STATX_DIOALIGN
> >   	 * cannot represent that, so in that case we report no DIO support.
> >   	 */
> > -	if ((request_mask & STATX_DIOALIGN) && S_ISREG(inode->i_mode)) {
> > -		unsigned int bsize = i_blocksize(inode);
> > -
> > +	if ((request_mask & STATX_DIOALIGN) && S_ISREG(inode->i_mode) &&
> > +				!f2fs_force_buffered_io(inode, WRITE)) {
> > +		stat->dio_mem_align = F2FS_BLKSIZE;
> > +		stat->dio_offset_align = F2FS_BLKSIZE;
> >   		stat->result_mask |= STATX_DIOALIGN;
> > -		if (!f2fs_force_buffered_io(inode, WRITE)) {
> > -			stat->dio_mem_align = bsize;
> > -			stat->dio_offset_align = bsize;
> > -		}
> >   	}
> >   
> >   	flags = fi->i_flags;
>
> No, this patch is wrong and the existing code is correct.  The cases are:

Yes, you're right, thanks for pointing out this!

>      STATX_DIOALIGN set and stx_dio_*_align nonzero:
>          File supports DIO.
>
>      STATX_DIOALIGN set and stx_dio_*_align zero:
>          File doesn't support DIO.
>
>      STATX_DIOALIGN unset:
>          Filesystem doesn't support STATX_DIOALIGN, so it's unknown whether the
>          file supports DIO or not.

Above description is clear to me.

> Please see the statx(2) manual page which explains this too.

However, below manual seems not very clear about explaining what does it
mean about STATX_DIOALIGN is set or not? At least not so clear like above
description.

       stx_dio_mem_align
              The alignment (in bytes) required for user memory buffers for direct I/O (O_DIRECT) on this file, or 0 if direct I/O is not supported on this file.

              STATX_DIOALIGN (stx_dio_mem_align and stx_dio_offset_align) is supported on block devices since Linux 6.1.  The support on regular files varies by filesystem; it is supported by ext4, f2fs, and xfs since  Linux
              6.1.

       stx_dio_offset_align
              The  alignment  (in  bytes)  required  for  file  offsets  and  I/O  segment  lengths  for  direct  I/O  (O_DIRECT)  on this file, or 0 if direct I/O is not supported on this file.  This will only be nonzero if
              stx_dio_mem_align is nonzero, and vice versa.

Thanks,

> - Eric