[PATCH] erofs: don't bother with s_stack_depth increasing for now

[Amir Goldstein]

On Wed, Dec 31, 2025 at 9:42 PM Gao Xiang <hsiangkao at linux.alibaba.com> wrote:
>
> Previously, commit d53cd891f0e4 ("erofs: limit the level of fs stacking
> for file-backed mounts") bumped `s_stack_depth` by one to avoid kernel
> stack overflow, but it breaks composefs mounts, which need erofs+ovl^2
> sometimes (and such setups are already used in production for quite long
> time) since `s_stack_depth` can be 3 (i.e., FILESYSTEM_MAX_STACK_DEPTH
> needs to change from 2 to 3).
>
> After a long discussion on GitHub issues [1] about possible solutions,
> it seems there is no need to support nesting file-backed mounts as one
> conclusion (especially when increasing FILESYSTEM_MAX_STACK_DEPTH to 3).
> So let's disallow this right now, since there is always a way to use
> loopback devices as a fallback.
>
> Then, I started to wonder about an alternative EROFS quick fix to
> address the composefs mounts directly for this cycle: since EROFS is the
> only fs to support file-backed mounts and other stacked fses will just
> bump up `FILESYSTEM_MAX_STACK_DEPTH`, just check that `s_stack_depth`
> != 0 and the backing inode is not from EROFS instead.
>
> At least it works for all known file-backed mount use cases (composefs,
> containerd, and Android APEX for some Android vendors), and the fix is
> self-contained.
>
> Let's defer increasing FILESYSTEM_MAX_STACK_DEPTH for now.
>
> Fixes: d53cd891f0e4 ("erofs: limit the level of fs stacking for file-backed mounts")
> Closes: https://github.com/coreos/fedora-coreos-tracker/issues/2087 [1]
> Closes: https://lore.kernel.org/r/CAFHtUiYv4+=+JP_-JjARWjo6OwcvBj1wtYN=z0QXwCpec9sXtg@mail.gmail.com
> Cc: Amir Goldstein <amir73il at gmail.com>
> Cc: Alexander Larsson <alexl at redhat.com>
> Cc: Christian Brauner <brauner at kernel.org>
> Cc: Miklos Szeredi <mszeredi at redhat.com>
> Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
> ---

Acked-by: Amir Goldstein <amir73il at gmail.com>

But you forgot to include details of the stack usage analysis you ran
with erofs+ovl^2 setup.

I am guessing people will want to see this information before relaxing
s_stack_depth in this case.

Thanks,
Amir.

>  fs/erofs/super.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index 937a215f626c..0cf41ed7ced8 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -644,14 +644,20 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
>                  * fs contexts (including its own) due to self-controlled RO
>                  * accesses/contexts and no side-effect changes that need to
>                  * context save & restore so it can reuse the current thread
> -                * context.  However, it still needs to bump `s_stack_depth` to
> -                * avoid kernel stack overflow from nested filesystems.
> +                * context.
> +                * However, we still need to prevent kernel stack overflow due
> +                * to filesystem nesting: just ensure that s_stack_depth is 0
> +                * to disallow mounting EROFS on stacked filesystems.
> +                * Note: s_stack_depth is not incremented here for now, since
> +                * EROFS is the only fs supporting file-backed mounts for now.
> +                * It MUST change if another fs plans to support them, which
> +                * may also require adjusting FILESYSTEM_MAX_STACK_DEPTH.
>                  */
>                 if (erofs_is_fileio_mode(sbi)) {
> -                       sb->s_stack_depth =
> -                               file_inode(sbi->dif0.file)->i_sb->s_stack_depth + 1;
> -                       if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
> -                               erofs_err(sb, "maximum fs stacking depth exceeded");
> +                       inode = file_inode(sbi->dif0.file);
> +                       if (inode->i_sb->s_op == &erofs_sops ||
> +                           inode->i_sb->s_stack_depth) {
> +                               erofs_err(sb, "file-backed mounts cannot be applied to stacked fses");
>                                 return -ENOTBLK;
>                         }
>                 }
> --
> 2.43.5
>