erofs: don't bother with s_stack_depth increasing for now
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@gmail.com>
Cc: Alexander Larsson <alexl@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
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