linux-kernel - Re: [PATCH v5 1/2] mm,thp: recheck each page before collapsing file THP

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Message-ID: <47A1A0C0-8E38-4F4E-B7D6-7B390E7F7813@fb.com>
Date:   Fri, 8 Nov 2019 03:58:44 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     open list <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        Kernel Team <Kernel-team@...com>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Johannes Weiner" <hannes@...xchg.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v5 1/2] mm,thp: recheck each page before collapsing file
 THP



> On Nov 7, 2019, at 7:53 PM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> On Tue, 5 Nov 2019 22:09:29 -0800 Song Liu <songliubraving@...com> wrote:
> 
>> In collapse_file(), for !is_shmem case, current check cannot guarantee
>> the locked page is up-to-date.  Specifically, xas_unlock_irq() should
>> not be called before lock_page() and get_page(); and it is necessary to
>> recheck PageUptodate() after locking the page.
>> 
>> With this bug and CONFIG_READ_ONLY_THP_FOR_FS=y, madvise(HUGE)'ed .text
>> may contain corrupted data.  This is because khugepaged mistakenly
>> collapses some not up-to-date sub pages into a huge page, and assumes
>> the huge page is up-to-date.  This will NOT corrupt data in the disk,
>> because the page is read-only and never written back.  Fix this by
>> properly checking PageUptodate() after locking the page.  This check
>> replaces "VM_BUG_ON_PAGE(!PageUptodate(page), page);".
>> 
>> Also, move PageDirty() check after locking the page.  Current
>> khugepaged should not try to collapse dirty file THP, because it is
>> limited to read-only .text. The only case we hit a dirty page here is
>> when the page hasn't been written since write. Bail out and retry when
>> this happens.
> 
> Incorrect data is pretty serious.  Should we backport this into -stable
> kernels?
> 
> (I suspect I already asked this in response to earier versions, sorry ;))

This is new feature (and new bug :( ) in 5.4. So no need to back port.

Thanks,
Song