[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

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

[Friendy.Su]

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

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

[Gao Xiang]

(+cc Amir, Jan)

On Wed, Oct 21, 2026 at 07:18:06AM +0000, Friendy.Su@sony.com wrote:
> 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
>          */

nit: single line /* Allow HSM to support fanotify pre access event */
     according to EROFS code style.

>         sb->s_iflags |= SB_I_ALLOW_HSM;
> }

I am fine with that as long as you limits SB_I_ALLOW_HSM **only if**
s_bdev is valid (IOWs, I hope we don't need to bother it with EROFS
SB_I_ALLOW_HSM + file-backed mounts; it aligns with other filesystems).

Also I think it would be better to add a testcase to check the basic
EROFS SB_I_ALLOW_HSM functionality in erofs-utils (see
experimental-tests). You don't need to add support for EROFS
SB_I_ALLOW_HSM directly in erofs-utils; but a basic functionality
testcase for this is needed.


BTW, for your use cases:

> mount must be DAX.

As for virtio-pmem, why not just use userfaultfd on the host for
virtio-pmem lazy pulling? for example,
https://github.com/firecracker-microvm/firecracker/issues/5740

Personally I still don't think monitoring FAN_PRE_ACCESS of EROFS for
this kind of usage is a good idea.

But anyway, it doesn't matter to much if it applies only to bdev/pmem.

> 
> How do you think?

Also please just adjust the patch subject to:

erofs: set SB_I_ALLOW_HSM for bdev-based mounts

Adjust the commit message accordingly to your use cases, and
explain file-backed mounts are not your cases in the commit message
as well.

And since it enables SB_I_ALLOW_HSM, it might be better to
Cc fsnotify maintainers in the following versions.

Thanks,
Gao Xiang

> 
> 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
> 
> 

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

[Friendy.Su]

Thanks for your comment. 

I will send patch v2.

________________________________________
From: Gao Xiang <xiang@kernel.org>
Sent: Wednesday, October 21, 2026 16:05
To: Su, Friendy
Cc: Gao Xiang; chao@kernel.org; linux-erofs@lists.ozlabs.org; linux-kernel@vger.kernel.org; Mo, Yuezhang; Palmer, Daniel (SGC); Amir Goldstein; Jan Kara
Subject: Re: [PATCH v1] erofs: enable SB_I_ALLOW_HSM to support fanotify pre access event

(+cc Amir, Jan) On Wed, Oct 21, 2026 at 07: 18: 06AM +0000, Friendy. Su@ sony. com wrote: > Hi Gao, > > Sorry for late reply. > > We have considered fanotify in erofs-utils. I think it must be used with > file-backend mount. It


(+cc Amir, Jan)

On Wed, Oct 21, 2026 at 07:18:06AM +0000, Friendy.Su@sony.com wrote:
> 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
>          */

nit: single line /* Allow HSM to support fanotify pre access event */
     according to EROFS code style.

>         sb->s_iflags |= SB_I_ALLOW_HSM;
> }

I am fine with that as long as you limits SB_I_ALLOW_HSM **only if**
s_bdev is valid (IOWs, I hope we don't need to bother it with EROFS
SB_I_ALLOW_HSM + file-backed mounts; it aligns with other filesystems).

Also I think it would be better to add a testcase to check the basic
EROFS SB_I_ALLOW_HSM functionality in erofs-utils (see
experimental-tests). You don't need to add support for EROFS
SB_I_ALLOW_HSM directly in erofs-utils; but a basic functionality
testcase for this is needed.


BTW, for your use cases:

> mount must be DAX.

As for virtio-pmem, why not just use userfaultfd on the host for
virtio-pmem lazy pulling? for example,
https://urldefense.com/v3/__https://github.com/firecracker-microvm/firecracker/issues/5740__;!!O7_YSHcmd9jp3hj_4dEAcyQ!1I4HPp-ti8SbtdVUzxl0B0QuNCpDMYyUh1CGGxNZEvFSg_lZnXaDitDh6Clg2me7wzbPbUl2X5E_vw$

Personally I still don't think monitoring FAN_PRE_ACCESS of EROFS for
this kind of usage is a good idea.

But anyway, it doesn't matter to much if it applies only to bdev/pmem.

>
> How do you think?

Also please just adjust the patch subject to:

erofs: set SB_I_ALLOW_HSM for bdev-based mounts

Adjust the commit message accordingly to your use cases, and
explain file-backed mounts are not your cases in the commit message
as well.

And since it enables SB_I_ALLOW_HSM, it might be better to
Cc fsnotify maintainers in the following versions.

Thanks,
Gao Xiang

>
> 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$%5Bgit%5B.%5Dkernel%5B.%5Dorg%5D
>?
>
> I am not sure why EROFS itself needs to set SB_I_ALLOW_HSM because EROFS
> is an immutable filesystem, its data should be considered as a golden
> one and it shouldn't be partially filled.
>
> Instead, SB_I_ALLOW_HSM should be set on the backing filesystems (ext4,
> xfs, ...), and if I understand correctly, combining EROFS file-backed
> mounts (pls don't use loop for this) already implements what you said
> above.
>
> And this functionality is already released in the latest erofs-utils
> and Nydus v3:
> https://urldefense.com/v3/__https://github.com/dragonflyoss/nydus/blob/v3/docs/fanotify.md__;!!O7_YSHcmd9jp3hj_4dEAcyQ!3EhQW78Lg9hjdjMscHBj-V19jmMX6nlYdkPVufSmwLJEM5yM0uTCOLuqwilIL3MGLDqMV8IXHHnVqQ$%5Bgithub%5B.%5Dcom%5D
>
> Thanks,
> Gao Xiang
>
>

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

[Amir Goldstein]

On Wed, Oct 21, 2026 at 11:43 AM Friendy.Su@sony.com
<Friendy.Su@sony.com> wrote:
>
> Thanks for your comment.
>
> I will send patch v2.
>
> ________________________________________
> From: Gao Xiang <xiang@kernel.org>
> Sent: Wednesday, October 21, 2026 16:05
> To: Su, Friendy
> Cc: Gao Xiang; chao@kernel.org; linux-erofs@lists.ozlabs.org; linux-kernel@vger.kernel.org; Mo, Yuezhang; Palmer, Daniel (SGC); Amir Goldstein; Jan Kara
> Subject: Re: [PATCH v1] erofs: enable SB_I_ALLOW_HSM to support fanotify pre access event
>
> (+cc Amir, Jan) On Wed, Oct 21, 2026 at 07: 18: 06AM +0000, Friendy. Su@ sony. com wrote: > Hi Gao, > > Sorry for late reply. > > We have considered fanotify in erofs-utils. I think it must be used with > file-backend mount. It
>
>
> (+cc Amir, Jan)
>
> On Wed, Oct 21, 2026 at 07:18:06AM +0000, Friendy.Su@sony.com wrote:
> > 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
> >          */
>
> nit: single line /* Allow HSM to support fanotify pre access event */
>      according to EROFS code style.
>
> >         sb->s_iflags |= SB_I_ALLOW_HSM;
> > }
>
> I am fine with that as long as you limits SB_I_ALLOW_HSM **only if**
> s_bdev is valid (IOWs, I hope we don't need to bother it with EROFS
> SB_I_ALLOW_HSM + file-backed mounts; it aligns with other filesystems).
>
> Also I think it would be better to add a testcase to check the basic
> EROFS SB_I_ALLOW_HSM functionality in erofs-utils (see
> experimental-tests). You don't need to add support for EROFS
> SB_I_ALLOW_HSM directly in erofs-utils; but a basic functionality
> testcase for this is needed.
>
>
> BTW, for your use cases:
>
> > mount must be DAX.
>
> As for virtio-pmem, why not just use userfaultfd on the host for
> virtio-pmem lazy pulling? for example,
> https://urldefense.com/v3/__https://github.com/firecracker-microvm/firecracker/issues/5740__;!!O7_YSHcmd9jp3hj_4dEAcyQ!1I4HPp-ti8SbtdVUzxl0B0QuNCpDMYyUh1CGGxNZEvFSg_lZnXaDitDh6Clg2me7wzbPbUl2X5E_vw$
>
> Personally I still don't think monitoring FAN_PRE_ACCESS of EROFS for
> this kind of usage is a good idea.

Personally I agree that this is odd, but practically, this allows getting
FAN_PRE_ACCESS events with extra range info on erofs file's access.

I don't see the risk in allowing that so I have no reason to object.

Thanks,
Amir.