linux-ext4 - Re: possible deadlock in fs_reclaim

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Date:   Thu, 11 Feb 2021 11:50:38 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Hillf Danton <hdanton@...a.com>,
        syzbot <syzbot+a7ab8df042baaf42ae3c@...kaller.appspotmail.com>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com,
        Tahsin Erdogan <tahsin@...gle.com>, tytso@....edu
Subject: Re: possible deadlock in fs_reclaim_acquire (2)

On Thu 11-02-21 11:22:25, Jan Kara wrote:
> On Thu 11-02-21 12:07:29, Hillf Danton wrote:

I haven't received Hillf's email.

[...]
> > Fix 71b565ceff37 ("ext4: drop ext4_kvmalloc()") by restoring the
> > GFP_NOFS introduced in dec214d00e0d ("ext4: xattr inode deduplication").
> > 
> > Note this may be the fix also to possible deadlock
> >  Reported-by: syzbot+bfdded10ab7dcd7507ae@...kaller.appspotmail.com
> >  https://lore.kernel.org/linux-ext4/000000000000563a0205bafb7970@google.com/
> 
> Please no. Ext4 is using scoping API to limit allocations to GFP_NOFS
> inside transactions. In this case something didn't work which seems like a
> lockdep bug at the first sight but I'll talk to mm guys about it.
> Definitely to problem doesn't seem to be in ext4.

Agreed. kvmalloc(NOFS) is not even supported because vmalloc doesn't
support GFP_KERNEL incompatible requests.
> 
> 								Honza
> 
> > 
> > --- a/fs/ext4/xattr.c
> > +++ b/fs/ext4/xattr.c
> > @@ -1459,7 +1459,7 @@ ext4_xattr_inode_cache_find(struct inode
> >  	if (!ce)
> >  		return NULL;
> >  
> > -	ea_data = kvmalloc(value_len, GFP_KERNEL);
> > +	ea_data = kvmalloc(value_len, GFP_NOFS);
> >  	if (!ea_data) {
> >  		mb_cache_entry_put(ea_inode_cache, ce);
> >  		return NULL;
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR

-- 
Michal Hocko
SUSE Labs