Re: [PATCH v1] erofs: enable SB_I_ALLOW_HSM to support fanotify pre access event

[Gao Xiang]

Hi Friendy,

On Sat, Oct 10, 2026 at 05:40:00PM +0800, Friendy Su wrote:
> This patch makes erofs support a use case below:
> 
> erofs file data are not available at local, but need be
> filled at fanotify pre access event handler.
> 
> E.g. when format erofs to blob mode, meta data image
> is store locally, blob image size is big so it is stored remotely.
> At erofs mount, a loop device based on a sparse image file is
> used as blob device. The sparse image file size is equal to blob
> image size. Like:
> 
> $ sudo mount -o device=/dev/loop1 /dev/sdb1 /mnt/erofs/
> 
> Then at file access, need retrieve data from remote, then fill
> into sparse image at FAN_PRE_ACCESS event handler.
> 
> Signed-off-by: Friendy Su <friendy.su@xxxxxxxx>
> Reviewed-by: Yuezhang Mo <Yuezhang.Mo@xxxxxxxx>
> Reviewed-by: Daniel Palmer <daniel.palmer@xxxxxxxx>
> ---
>  fs/erofs/super.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index 0967aace9f7f..b92d3353319d 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -659,6 +659,10 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
>  	sb->s_maxbytes = MAX_LFS_FILESIZE;
>  	sb->s_op = &erofs_sops;
>  
> +	/*
> +	 * Allow HSM to support fanotify pre access event
> +	 */
> +	sb->s_iflags |= SB_I_ALLOW_HSM;

Did you notice
https://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git/tree/lib/backends/fanotify.c
?

I am not sure why EROFS itself needs to set SB_I_ALLOW_HSM because EROFS
is an immutable filesystem, its data should be considered as a golden
one and it shouldn't be partially filled.

Instead, SB_I_ALLOW_HSM should be set on the backing filesystems (ext4,
xfs, ...), and if I understand correctly, combining EROFS file-backed
mounts (pls don't use loop for this) already implements what you said
above.

And this functionality is already released in the latest erofs-utils
and Nydus v3:
https://github.com/dragonflyoss/nydus/blob/v3/docs/fanotify.md

Thanks,
Gao Xiang