[PATCH v3 RESEND] erofs: don't bother with s_stack_depth increasing for now

[Christian Brauner]

On Thu, Jan 08, 2026 at 11:07:09AM +0800, Gao Xiang 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 when stacking an unlimited number of EROFS on top of
> each other.
> 
> This fix breaks composefs mounts, which need EROFS+ovl^2 sometimes
> (and such setups are already used in production for quite a long time).
> 
> One way to fix this regression is to bump FILESYSTEM_MAX_STACK_DEPTH
> from 2 to 3, but proving that this is safe in general is a high bar.
> 
> After a long discussion on GitHub issues [1] about possible solutions,
> one conclusion is that there is no need to support nesting file-backed
> EROFS mounts on stacked filesystems, because there is always the option
> to use loopback devices as a fallback.
> 
> As a quick fix for the composefs regression for this cycle, instead of
> bumping `s_stack_depth` for file backed EROFS mounts, we disallow
> nesting file-backed EROFS over EROFS and over filesystems with
> `s_stack_depth` > 0.
> 
> This works for all known file-backed mount use cases (composefs,
> containerd, and Android APEX for some Android vendors), and the fix is
> self-contained.
> 
> Essentially, we are allowing one extra unaccounted fs stacking level of
> EROFS below stacking filesystems, but EROFS can only be used in the read
> path (i.e. overlayfs lower layers), which typically has much lower stack
> usage than the write path.
> 
> We can consider increasing FILESYSTEM_MAX_STACK_DEPTH later, after more
> stack usage analysis or using alternative approaches, such as splitting
> the `s_stack_depth` limitation according to different combinations of
> stacking.
> 
> Fixes: d53cd891f0e4 ("erofs: limit the level of fs stacking for file-backed mounts")
> Reported-and-tested-by: Dusty Mabe <dusty at dustymabe.com>
> Reported-by: Timothée Ravier <tim at siosm.fr>
> Closes: https://github.com/coreos/fedora-coreos-tracker/issues/2087 [1]
> Reported-by: "Alekséi Naidénov" <an at digitaltide.io>
> Closes: https://lore.kernel.org/r/CAFHtUiYv4+=+JP_-JjARWjo6OwcvBj1wtYN=z0QXwCpec9sXtg@mail.gmail.com
> Acked-by: Amir Goldstein <amir73il at gmail.com>
> Acked-by: Alexander Larsson <alexl at redhat.com>
> Cc: Christian Brauner <brauner at kernel.org>
> Cc: Miklos Szeredi <mszeredi at redhat.com>
> Cc: Sheng Yong <shengyong1 at xiaomi.com>
> Cc: Zhiguo Niu <niuzhiguo84 at gmail.com>
> Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
> ---

Acked-by: Christian Brauner <brauner at kernel.org>