linux-kernel - Re: [PATCH v2] erofs: don't bother with s_stack

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Message-ID: <20260108102613.33bbc6d4@pumpkin>
Date: Thu, 8 Jan 2026 10:26:13 +0000
From: David Laight <david.laight.linux@...il.com>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: Amir Goldstein <amir73il@...il.com>, Sheng Yong
 <shengyong2021@...il.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>, Alexander Larsson
 <alexl@...hat.com>, Christian Brauner <brauner@...nel.org>, Miklos Szeredi
 <mszeredi@...hat.com>, Zhiguo Niu <niuzhiguo84@...il.com>,
 shengyong1@...omi.com, linux-erofs mailing list
 <linux-erofs@...ts.ozlabs.org>
Subject: Re: [PATCH v2] erofs: don't bother with s_stack_depth increasing
 for now

On Thu, 8 Jan 2026 16:05:03 +0800
Gao Xiang <hsiangkao@...ux.alibaba.com> wrote:

> Hi Amir,
> 
> On 2026/1/8 16:02, Amir Goldstein wrote:
> > On Thu, Jan 8, 2026 at 4:10 AM Gao Xiang <hsiangkao@...ux.alibaba.com> wrote:  
> 
> ...
> 
> >>>>
> >>>> 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) ||  
> >>
> >> Sorry it should be `!inode->i_sb->s_bdev`, I've
> >> fixed it in v3 RESEND:  
> > 
> > A RESEND implies no changes since v3, so this is bad practice.
> >   
> >> https://lore.kernel.org/r/20260108030709.3305545-1-hsiangkao@linux.alibaba.com
> >>  
> > 
> > Ouch! If the erofs maintainer got this condition wrong... twice...
> > Maybe better using the helper instead of open coding this non trivial check?
> > 
> > if ((inode->i_sb->s_op == &erofs_sops &&
> >        erofs_is_fileio_mode(EROFS_I_SB(inode)))  
> 
> I was thought to use that, but it excludes fscache as the
> backing fs.. so I suggest to use !s_bdev directly to
> cover both file-backed mounts and fscache cases directly.

Is it worth just allocating each fs a 'stack needed' value and then
allowing the mount if the total is low enough.
This is equivalent to counting the recursion depth, but lets erofs only
add (say) 0.5.
Ideally you'd want to do static analysis to find the value to add,
but 'inspired guesswork' is probably good enough.

Isn't there also a big difference between recursive mounts (which need
to do read/write on the underlying file) and overlay mounts (which just
pass the request onto the lower filesystem).

	David

> 
> Thanks,
> Gao Xiang
> 
> > 
> > Thanks,
> > Amir.  
> 
>