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

["Friendy.Su@sony.com" <Friendy.Su@sony.com>]

Hi Gao,

Sorry for late reply.

We have considered fanotify in erofs-utils. I think it must be used with 
file-backend mount. It actually utilizes FAN_PRE_ACCESS on ext4 or xfs. 
Also file-backend mount can not support DAX mount.

But for our case, file-backend mount can not be used. Firstly, there is no 
ext4 or xfs file system in actual environment to put a sparse erofs image 
file. We have to mount a normal block device. Secondly, mount must be DAX.

The basic flow is file content is fetched from remote then filled into 
local erofs file on the DAX mounted block device in FAN_PRE_ACCESS handler.

"/dev/loop0" is just an example, not our actual use case. The actual block 
device is something like /dev/pmem0.

In order to make SB_I_ALLOW_HSM only set under block device backend, 
we can do like this:

if (!erofs_is_fileio_mode(sbi)) {
        /*
         * Allow HSM to support fanotify pre access event
         */
        sb->s_iflags |= SB_I_ALLOW_HSM;
}

How do you think?

Best Regards
Friendy

________________________________________
From: Gao Xiang <xiang@kernel.org>
Sent: Sunday, October 11, 2026 5:20
To: Su, Friendy
Cc: xiang@kernel.org; chao@kernel.org; linux-erofs@lists.ozlabs.org; linux-kernel@vger.kernel.org; Mo, Yuezhang; Palmer, Daniel (SGC)
Subject: Re: [PATCH v1] erofs: enable SB_I_ALLOW_HSM to support fanotify pre access event

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. 


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@sony.com>
> Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
> Reviewed-by: Daniel Palmer <daniel.palmer@sony.com>
> ---
>  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://urldefense.com/v3/__https://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git/tree/lib/backends/fanotify.c__;!!O7_YSHcmd9jp3hj_4dEAcyQ!3EhQW78Lg9hjdjMscHBj-V19jmMX6nlYdkPVufSmwLJEM5yM0uTCOLuqwilIL3MGLDqMV8LkNo1Chg$[git[.]kernel[.]org]
?

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://urldefense.com/v3/__https://github.com/dragonflyoss/nydus/blob/v3/docs/fanotify.md__;!!O7_YSHcmd9jp3hj_4dEAcyQ!3EhQW78Lg9hjdjMscHBj-V19jmMX6nlYdkPVufSmwLJEM5yM0uTCOLuqwilIL3MGLDqMV8IXHHnVqQ$[github[.]com]

Thanks,
Gao Xiang