linux-kernel - Re: [syzbot] general protection fault in list_lru

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Message-ID: <CAHk-=wii1peDbW+eZipUnLmU_STXx6Vm30PiQnjhfUmgYrSd+Q@mail.gmail.com>
Date:   Fri, 25 Mar 2022 13:29:02 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     syzbot <syzbot+f8c45ccc7d5d45fc5965@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [syzbot] general protection fault in list_lru_add

On Fri, Mar 25, 2022 at 2:52 AM Muchun Song <songmuchun@...edance.com> wrote:
>
> We can see that we put the dentry (ffff88807ebda0f8) into
> the list_lru (ffff888011bd47f0). But we do not allocate struct
> list_lru_one for the memcg (ffff88801c530000).  Then it panics.

Hmm.

Looking at memcg_slab_pre_alloc_hook(), I note that it will return
success without doing the LRU checking for several cases.

So since you can reproduce the problem, I would suggest you add some
debug code to __d_alloc() that prints out something big if it gets a
dentry but you can't look up the list_lru_one() for that dentry.

Hmm?

The only other situation I can think of is if dentry->d_sb were to
change during the dentry lifetime, but I don't think that can happen.
The only assignment I can find with "git grep" is that

        dentry->d_sb = sb;

in __d_alloc(), and while it's possible my grep pattern was bogus, it
sounds unlikely.

               Linus