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

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Date:   Tue, 21 Dec 2021 18:40:15 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Yang Shi <shy828301@...il.com>
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 10:24:27AM -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
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.