Currently, .fadvise() doesn't work well if page cache sharing is on
since shared inodes belong to a pseudo fs generated with init_pseudo(),
and sb->s_bdi is the default one &noop_backing_dev_info.

Then, generic_fadvise() will just behave as a no-op if sb->s_bdi is
&noop_backing_dev_info, but as the bdev fs (the bdev fs changes
inode_to_bdi() instead), it's actually NOT a pure memfs.

Let's generate a real bdi for erofs_ishare_mnt instead.

Fixes: d86d7817c042 ("erofs: implement .fadvise for page cache share")
Cc: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
---
v2:
 - handle super_setup_bdi() failure properly.

 fs/erofs/ishare.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/fs/erofs/ishare.c b/fs/erofs/ishare.c
index 829d50d5c717..ec433bacc592 100644
--- a/fs/erofs/ishare.c
+++ b/fs/erofs/ishare.c
@@ -200,8 +200,19 @@ struct inode *erofs_real_inode(struct inode *inode, bool *need_iput)
 
 int __init erofs_init_ishare(void)
 {
-	erofs_ishare_mnt = kern_mount(&erofs_anon_fs_type);
-	return PTR_ERR_OR_ZERO(erofs_ishare_mnt);
+	struct vfsmount *mnt;
+	int ret;
+
+	mnt = kern_mount(&erofs_anon_fs_type);
+	if (IS_ERR(mnt))
+		return PTR_ERR(mnt);
+	/* generic_fadvise() doesn't work if s_bdi == &noop_backing_dev_info */
+	ret = super_setup_bdi(mnt->mnt_sb);
+	if (ret)
+		kern_unmount(mnt);
+	else
+		erofs_ishare_mnt = mnt;
+	return ret;
 }
 
 void erofs_exit_ishare(void)
-- 
2.43.5

On 2026/3/24 23:54, Gao Xiang wrote:
> Currently, .fadvise() doesn't work well if page cache sharing is on
> since shared inodes belong to a pseudo fs generated with init_pseudo(),
> and sb->s_bdi is the default one &noop_backing_dev_info.
> 
> Then, generic_fadvise() will just behave as a no-op if sb->s_bdi is
> &noop_backing_dev_info, but as the bdev fs (the bdev fs changes
> inode_to_bdi() instead), it's actually NOT a pure memfs.
> 
> Let's generate a real bdi for erofs_ishare_mnt instead.
> 
> Fixes: d86d7817c042 ("erofs: implement .fadvise for page cache share")
> Cc: Hongbo Li <lihongbo22@huawei.com>
> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> ---
> v2:
>   - handle super_setup_bdi() failure properly.
> 
>   fs/erofs/ishare.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/erofs/ishare.c b/fs/erofs/ishare.c
> index 829d50d5c717..ec433bacc592 100644
> --- a/fs/erofs/ishare.c
> +++ b/fs/erofs/ishare.c
> @@ -200,8 +200,19 @@ struct inode *erofs_real_inode(struct inode *inode, bool *need_iput)
>   
>   int __init erofs_init_ishare(void)
>   {
> -	erofs_ishare_mnt = kern_mount(&erofs_anon_fs_type);
> -	return PTR_ERR_OR_ZERO(erofs_ishare_mnt);
> +	struct vfsmount *mnt;
> +	int ret;
> +
> +	mnt = kern_mount(&erofs_anon_fs_type);
> +	if (IS_ERR(mnt))
> +		return PTR_ERR(mnt);
> +	/* generic_fadvise() doesn't work if s_bdi == &noop_backing_dev_info */
> +	ret = super_setup_bdi(mnt->mnt_sb);
> +	if (ret)
> +		kern_unmount(mnt);
> +	else
> +		erofs_ishare_mnt = mnt;
> +	return ret;
>   }

ok, looks good.

Reviewed-by: Hongbo Li <lihongbo22@huawei.com>

Thanks,
Hongbo

>   
>   void erofs_exit_ishare(void)