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

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

[Friendy Su]

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;
 	if (!sbi->domain_id && test_opt(&sbi->opt, INODE_SHARE)) {
 		errorfc(fc, "domain_id is needed when inode_share is on");
 		return -EINVAL;
-- 
2.34.1

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@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://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