linux-kernel - Re: [syzbot] kernel BUG in __page

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Date:   Wed, 5 Jan 2022 11:05:44 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     syzbot <syzbot+1f52b3a18d5633fa7f82@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        chinwen.chang@...iatek.com, fgheet255t@...il.com,
        Jann Horn <jannh@...gle.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller-bugs@...glegroups.com, tonymarislogistics@...dex.com,
        Vlastimil Babka <vbabka@...e.cz>, walken@...gle.com,
        Zi Yan <ziy@...dia.com>
Subject: Re: [syzbot] kernel BUG in __page_mapcount

On Tue, Dec 21, 2021 at 10:40 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Tue, Dec 21, 2021 at 10:24:27AM -0800, Yang Shi wrote:
> > It seems the THP is split during smaps walk. The reproducer does call
> > MADV_FREE on partial THP which may split the huge page.
> >
> > The below fix (untested) should be able to fix it.
>
> Did you read the rest of the thread on this?  If the page is being

I just revisited this. Now I see what you mean about "the rest of the
thread". My gmail client doesn't put them in the same thread, sigh...

Yeah, try_get_compound_head() seems like the right way.

Or we just simply treat migration entries as mapcount == 1 as Kirill
suggested or just skip migration entries since they are transient or
show migration entries separately.


> migrated, we should still account it ... also, you've changed the
> refcount, so this:
>
>         if (page_count(page) == 1) {
>                 smaps_page_accumulate(mss, page, size, size << PSS_SHIFT, dirty,
>                         locked, true);
>                 return;
>         }
>
> will never trigger.