Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and
 PMD migration entry
To:     Will Deacon <will.deacon@arm.com>
Cc:     Andrea Arcangeli <aarcange@redhat.com>,
        Zi Yan <zi.yan@cs.rutgers.edu>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kirill.shutemov@linux.intel.com,
        akpm@linux-foundation.org, mhocko@suse.com,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
References: <1539057538-27446-1-git-send-email-anshuman.khandual@arm.com>
 <7E8E6B14-D5C4-4A30-840D-A7AB046517FB@cs.rutgers.edu>
 <84509db4-13ce-fd53-e924-cc4288d493f7@arm.com>
 <1968F276-5D96-426B-823F-38F6A51FB465@cs.rutgers.edu>
 <5e0e772c-7eef-e75c-2921-e80d4fbe8324@arm.com>
 <2398C491-E1DA-4B3C-B60A-377A09A02F1A@cs.rutgers.edu>
 <20181017020930.GN30832@redhat.com>
 <9d9aaf03-617a-d383-7d59-8b98fdd3c1e7@arm.com>
 <20181106003509.GA27283@brain-police>
Message-ID: <9370f2b9-0fcd-6bbb-fa29-568bbd9aba59@arm.com>
Date:   Tue, 6 Nov 2018 15:21:46 +0530
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.9.1
MIME-Version: 1.0
In-Reply-To: <20181106003509.GA27283@brain-police>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 11/06/2018 06:05 AM, Will Deacon wrote:
> On Fri, Nov 02, 2018 at 11:45:00AM +0530, Anshuman Khandual wrote:
>> On 10/17/2018 07:39 AM, Andrea Arcangeli wrote:
>>> What we need to do during split is an invalidate of the huge TLB.
>>> There's no pmd_trans_splitting anymore, so we only clear the present
>>> bit in the PTE despite pmd_present still returns true (just like
>>> PROT_NONE, nothing new in this respect). pmd_present never meant the
>>
>> On arm64, the problem is that pmd_present() is tied with pte_present() which
>> checks for PTE_VALID (also PTE_PROT_NONE) but which gets cleared during PTE
>> invalidation. pmd_present() returns false just after the first step of PMD
>> splitting. So pmd_present() needs to be decoupled from PTE_VALID which is
>> same as PMD_SECT_VALID and instead should depend upon a pte bit which sticks
>> around like PAGE_PSE as in case of x86. I am working towards a solution.
> 
> Could we not just go via a PROT_NONE mapping during the split, instead of
> having to allocate a new software bit to treat these invalid ptes as
> present?

The problem might occur during page fault (i.e __handle_mm_fault). As discussed
previously on this thread any potential PTE sticky bit would be used for both
pmd_trans_huge() and pmd_present() wrappers to maintain existing semantics. At
present, PMD state analysis during page fault has conditional block like this.

                if (pmd_trans_huge(orig_pmd) || pmd_devmap(orig_pmd)) {
                        if (pmd_protnone(orig_pmd) && vma_is_accessible(vma))
                                return do_huge_pmd_numa_page(&vmf, orig_pmd);

Using PROT_NONE for pmd_trans_huge() might force PMD page fault to go through
NUMA fault handling all the time as both pmd_trans_huge() and pmd_protnone()
will return true in that situation.