Message-ID: <96bae224-c971-44f6-94aa-eb0328021bc2@gmail.com>
Date: Thu, 8 Jan 2026 17:30:07 +0800
From: Sheng Yong <shengyong2021@...il.com>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>, linux-erofs@...ts.ozlabs.org
Cc: shengyong2021@...il.com, shengyong1@...omi.com,
 LKML <linux-kernel@...r.kernel.org>,
 linux-fsdevel <linux-fsdevel@...r.kernel.org>,
 Dusty Mabe <dusty@...tymabe.com>, Timothée Ravier
 <tim@...sm.fr>, Alekséi Naidénov <an@...italtide.io>,
 Amir Goldstein <amir73il@...il.com>, Alexander Larsson <alexl@...hat.com>,
 Christian Brauner <brauner@...nel.org>, Miklos Szeredi
 <mszeredi@...hat.com>, Zhiguo Niu <niuzhiguo84@...il.com>
Subject: Re: [PATCH v3 RESEND] erofs: don't bother with s_stack_depth
 increasing for now

On 1/8/26 17:25, Gao Xiang wrote:
> Hi Sheng,
> 
> On 2026/1/8 17:14, Sheng Yong wrote:
>> On 1/8/26 11:07, 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@...tymabe.com>
>>> Reported-by: Timothée Ravier <tim@...sm.fr>
>>> Closes: https://github.com/coreos/fedora-coreos-tracker/issues/2087 [1]
>>> Reported-by: "Alekséi Naidénov" <an@...italtide.io>
>>> Closes: https://lore.kernel.org/r/CAFHtUiYv4+=+JP_-JjARWjo6OwcvBj1wtYN=z0QXwCpec9sXtg@mail.gmail.com
>>> Acked-by: Amir Goldstein <amir73il@...il.com>
>>> Acked-by: Alexander Larsson <alexl@...hat.com>
>>> Cc: Christian Brauner <brauner@...nel.org>
>>> Cc: Miklos Szeredi <mszeredi@...hat.com>
>>> Cc: Sheng Yong <shengyong1@...omi.com>
>>> Cc: Zhiguo Niu <niuzhiguo84@...il.com>
>>> Signed-off-by: Gao Xiang <hsiangkao@...ux.alibaba.com>
>>
>> Reviewed-and-tested-by: Sheng Yong <shengyong1@...omi.com>
>>
>> I tested the APEX scenario on an Android phone. APEX images are
>> filebacked-mounted correctly.
> 
> 
>> And for a stacked APEX testcase, it reports error as expected.
> 
Hi, Xiang,

> Just to make sure it's an invalid case (should not be used on
> Android), yes? If so, thanks for the test on the APEX side.

No, it's not a real use case, just an invalid case, and only
used to test the error handling path.

thanks,
shengyong
> 
> Thanks,
> Gao Xiang
> 
>>
>> thanks,
>> shengyong

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