[PATCH] erofs: correct FSDAX detection

[Yuezhang.Mo at sony.com]

On November 17, 2025 19:57 Gao Xiang wrote:
> The detection of the primary device is skipped incorrectly
> if the multiple or flattened feature is enabled.
> 
> It also fixes the FSDAX misdetection for non-block extra blobs.
> 
> Fixes: c6993c4cb918 ("erofs: Fallback to normal access if DAX is not supported on extra device")
> Reported-by: syzbot+31b8fb02cb8a25bd5e78 at syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/r/691af9f6.a70a0220.3124cb.0097.GAE@google.com
> Cc: Yuezhang Mo <Yuezhang.Mo at sony.com>
> Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
> ---
>  fs/erofs/super.c | 22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index f3f8d8c066e4..cd8ff98c2938 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -174,15 +174,15 @@ static int erofs_init_device(struct erofs_buf *buf, struct super_block *sb,
>                 if (!erofs_is_fileio_mode(sbi)) {
>                         dif->dax_dev = fs_dax_get_by_bdev(file_bdev(file),
>                                         &dif->dax_part_off, NULL, NULL);
> -                       if (!dif->dax_dev && test_opt(&sbi->opt, DAX_ALWAYS)) {
> -                               erofs_info(sb, "DAX unsupported by %s. Turning off DAX.",
> -                                          dif->path);
> -                               clear_opt(&sbi->opt, DAX_ALWAYS);
> -                       }
>                 } else if (!S_ISREG(file_inode(file)->i_mode)) {
>                         fput(file);
>                         return -EINVAL;
>                 }
> +               if (!dif->dax_dev && test_opt(&sbi->opt, DAX_ALWAYS)) {
> +                       erofs_info(sb, "DAX unsupported by %s. Turning off DAX.",
> +                                  dif->path);
> +                       clear_opt(&sbi->opt, DAX_ALWAYS);
> +               }
>                 dif->file = file;
>         }
> 
> @@ -215,13 +215,13 @@ static int erofs_scan_devices(struct super_block *sb,
>                           ondisk_extradevs, sbi->devs->extra_devices);
>                 return -EINVAL;
>         }
> -       if (!ondisk_extradevs) {
> -               if (test_opt(&sbi->opt, DAX_ALWAYS) && !sbi->dif0.dax_dev) {
> -                       erofs_info(sb, "DAX unsupported by block device. Turning off DAX.");
> -                       clear_opt(&sbi->opt, DAX_ALWAYS);
> -               }
> -               return 0;
> +
> +       if (test_opt(&sbi->opt, DAX_ALWAYS) && !sbi->dif0.dax_dev) {
> +               erofs_info(sb, "DAX unsupported by block device. Turning off DAX.");
> +               clear_opt(&sbi->opt, DAX_ALWAYS);
>         }
> +       if (!ondisk_extradevs)
> +               return 0;

Hi Gao Xiang,

If using multiple devices, is there still file data on the primary device?
If the primary device only contains metadata, the primary device does not need
to support DAX.

> 
>         if (!sbi->devs->extra_devices && !erofs_is_fscache_mode(sb))
>                 sbi->devs->flatdev = true;
> --
> 2.43.5