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

[Eric Biggers]

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.

> 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:

    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.

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

- Eric