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Subject: Re: [v2 PATCH] mm: thp: handle page cache THP correctly in
 PageTransCompoundMap
To:     Hugh Dickins <hughd@google.com>
Cc:     aarcange@redhat.com, kirill.shutemov@linux.intel.com,
        gavin.dg@linux.alibaba.com, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
References: <1571850304-82802-1-git-send-email-yang.shi@linux.alibaba.com>
 <alpine.LSU.2.11.1910231157570.1088@eggly.anvils>
 <4d3c14ef-ee86-2719-70d6-68f1a8b42c28@linux.alibaba.com>
 <alpine.LSU.2.11.1910231250260.1794@eggly.anvils>
From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <d500d98e-3577-ced1-9614-7aa5e09e6dbe@linux.alibaba.com>
Date:   Wed, 23 Oct 2019 13:50:14 -0700
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On 10/23/19 1:00 PM, Hugh Dickins wrote:
> On Wed, 23 Oct 2019, Yang Shi wrote:
>> On 10/23/19 12:28 PM, Hugh Dickins wrote:
>>>> +	return map_count >= 0 &&
>>> You have added a map_count >= 0 test there. Okay, not wrong, but not
>>> necessary, and not consistent with what's returned in the PageAnon
>>> case (if this were called for an unmapped page).
>> I was thinking about this too. I'm wondering there might be a case that the
>> PMD is split and it was the last PMD map, in this case subpage's _mapcount is
>> also equal to compound_mapcount (both is -1). So, it would return true, then
>> KVM may setup PMD map in EPT, but it might be PTE mapped later on the host.
>> But, I'm not quite sure if this is really possible or if this is really a
>> integrity problem. So, I thought it might be safer to add this check.
> The mmu_notifier_invalidate_range_start.._end() in __split_huge_pmd(),
> with KVM's locking and sequence counting, is required to protect
> against such races.

OK, it sounds safe. Thanks for confirming. Will post v4 soon.

>
> Hugh