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Thu, 08 Jan 2026 10:32:40 +0800 Message-ID: <41b8a0bb-96d3-4eba-a5b8-77b0b0ed4730@linux.alibaba.com> Date: Thu, 8 Jan 2026 10:32:37 +0800 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] erofs: don't bother with s_stack_depth increasing for now To: Sheng Yong , linux-erofs@lists.ozlabs.org Cc: LKML , linux-fsdevel , Dusty Mabe , =?UTF-8?Q?Timoth=C3=A9e_Ravier?= , =?UTF-8?B?QWxla3PDqWkgTmFpZMOpbm92?= , Amir Goldstein , Alexander Larsson , Christian Brauner , Miklos Szeredi , Zhiguo Niu , shengyong1@xiaomi.com References: <0c34f3fa-c573-4343-b8ea-6832530f0069@linux.alibaba.com> <20260106170504.674070-1-hsiangkao@linux.alibaba.com> <3acec686-4020-4609-aee4-5dae7b9b0093@gmail.com> From: Gao Xiang In-Reply-To: <3acec686-4020-4609-aee4-5dae7b9b0093@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Hi Sheng, On 2026/1/8 10:26, Sheng Yong wrote: > On 1/7/26 01:05, 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-by: Dusty Mabe >> Reported-by: Timothée Ravier >> Closes: https://github.com/coreos/fedora-coreos-tracker/issues/2087 [1] >> Reported-by: "Alekséi Naidénov" >> Closes: https://lore.kernel.org/r/CAFHtUiYv4+=+JP_-JjARWjo6OwcvBj1wtYN=z0QXwCpec9sXtg@mail.gmail.com >> Acked-by: Amir Goldstein >> Cc: Alexander Larsson >> Cc: Christian Brauner >> Cc: Miklos Szeredi >> Cc: Sheng Yong >> Cc: Zhiguo Niu >> Signed-off-by: Gao Xiang >> --- >> v2: >>   - Update commit message (suggested by Amir in 1-on-1 talk); >>   - Add proper `Reported-by:`. >> >>   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 || > > Hi, Xiang > > In Android APEX scenario, apex images formatted as EROFS are packed in > system.img which is also EROFS format. As a result, it will always fail > to do APEX-file-backed mount since `inode->i_sb->s_op == &erofs_sops' > is true. > Any thoughts to handle such scenario? Sorry, I forgot this popular case, I think it can be simply resolved by the following diff: diff --git a/fs/erofs/super.c b/fs/erofs/super.c index 0cf41ed7ced8..e93264034b5d 100644 --- a/fs/erofs/super.c +++ b/fs/erofs/super.c @@ -655,7 +655,7 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc) */ if (erofs_is_fileio_mode(sbi)) { inode = file_inode(sbi->dif0.file); - if (inode->i_sb->s_op == &erofs_sops || + if ((inode->i_sb->s_op == &erofs_sops && !sb->s_bdev) || inode->i_sb->s_stack_depth) { erofs_err(sb, "file-backed mounts cannot be applied to stacked fses"); return -ENOTBLK; "!sb->s_bdev" covers file-backed EROFS mounts and (deprecated) fscache EROFS mounts, I will send v3 soon. Thanks, Gao Xiang > > thanks, > shengyong > >> +                inode->i_sb->s_stack_depth) { >> +                erofs_err(sb, "file-backed mounts cannot be applied to stacked fses"); >>                   return -ENOTBLK; >>               } >>           }