linux-kernel - Re: [PATCH v3] mm,thp: recheck each page before collapsing file THP

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Message-ID: <1C277765-F16E-4818-855C-FB0D1EC23772@fb.com>
Date:   Wed, 6 Nov 2019 05:45:41 +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 v3] mm,thp: recheck each page before collapsing file THP



> On Nov 5, 2019, at 8:58 PM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> On Sat, 19 Oct 2019 05:24:00 +0000 Song Liu <songliubraving@...com> wrote:
> 
>>> We don't have a ref on that page.  After we've released the xarray lock
>>> we have no business playing with *page at all, correct?
>> 
>> Yeah, this piece is not just redundant, but also buggy. I am also 
>> including some information about it. 
>> 
>> Updated commit log:
>> 
>> ============================= 8< =============================
>> 
>> 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. Add a warning with the PageDirty() check as it should 
>> not happen. This warning is added after page_mapping() check, because 
>> if the page is truncated, it might be dirty.
> 
> I've lost the plot on this patch.  I have the v3 patch plus these fixes:
> 
> http://lkml.kernel.org/r/20191028221414.3685035-1-songliubraving@fb.com
> http://lkml.kernel.org/r/20191022191006.411277-1-songliubraving@fb.com
> http://lkml.kernel.org/r/20191030200736.3455046-1-songliubraving@fb.com
> 
> and there's a v4 which I can't correlate with the above.  And there has
> been discussion about deferring some of the filemap_flush() changes
> until later.
> 
> So I think it's best if we just start again.  Can you please prepare
> and send out a v5 (which might be a 2-patch series)?

Sounds good. Sending v5 soon. 

Thanks,
Song