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From:   Lorenzo Stoakes <lstoakes@gmail.com>
Date:   Tue, 24 Nov 2020 17:59:52 +0000
Message-ID: <CAA5enKZ=6=AoknavW4RJ+T+aiPBFSf8uEjJ+ODcc+nMTD2k5kQ@mail.gmail.com>
Subject: Re: linux-next boot error: WARNING in prepare_kswapd_sleep
To:     syzbot <syzbot+ce635500093181f39c1c@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Linux-Next Mailing List <linux-next@vger.kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Alex Shi <alex.shi@linux.alibaba.com>, Hui Su <sh_def@163.com>
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On Tue, 24 Nov 2020 at 07:54, syzbot
<syzbot+ce635500093181f39c1c@syzkaller.appspotmail.com> wrote:
> syzbot found the following issue on:
>
> HEAD commit:    d9137320 Add linux-next specific files for 20201124

This appears to be a product of 4b2904f3 ("mm/memcg: add missed
warning in mem_cgroup_lruvec") adding a VM_WARN_ON_ONCE() to
mem_cgroup_lruvec, which when invoked from a function other than
mem_cgroup_page_lruvec() can in fact be called with the condition
false.
If we move the check back into mem_cgroup_page_lruvec() it resolves
the issue. I enclose a simple version of this below, happy to submit
as a proper patch if this is the right approach:


diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 87ed56dc75f9..27cc40a490b2 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -618,7 +618,6 @@ static inline struct lruvec
*mem_cgroup_lruvec(struct mem_cgroup *memcg,
                goto out;
        }

-       VM_WARN_ON_ONCE(!memcg);
        if (!memcg)
                memcg = root_mem_cgroup;

@@ -645,6 +644,7 @@ static inline struct lruvec
*mem_cgroup_lruvec(struct mem_cgroup *memcg,
 static inline struct lruvec *mem_cgroup_page_lruvec(struct page *page,
                                                struct pglist_data *pgdat)
 {
+       VM_WARN_ON_ONCE_PAGE(!page_memcg(page), page);
        return mem_cgroup_lruvec(page_memcg(page), pgdat);
 }