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Date:   Wed, 19 Jul 2023 10:54:42 +0530
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Subject: Re: [PATCH v3 04/13] powerpc: assert_pte_locked() use
 pte_offset_map_nolock()
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To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Yang Shi <shy828301@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Alistair Popple <apopple@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Steven Price <steven.price@arm.com>,
        SeongJae Park <sj@kernel.org>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Huang Ying <ying.huang@intel.com>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Zack Rusin <zackr@vmware.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        Russell King <linux@armlinux.org.uk>,
        "David S. Miller" <davem@davemloft.net>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Heiko Carstens <hca@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Jann Horn <jannh@google.com>,
        Vishal Moola <vishal.moola@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>, Zi Yan <ziy@nvidia.com>,
        linux-arm-kernel@lists.infradead.org, sparclinux@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <7cd843a9-aa80-14f-5eb2-33427363c20@google.com>
 <e8d56c95-c132-a82e-5f5f-7bb1b738b057@google.com>
 <87msztbiy8.fsf@linux.ibm.com>
 <392f311f-83ac-a5a2-d16e-2c7736d1b577@google.com>
From:   Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>
In-Reply-To: <392f311f-83ac-a5a2-d16e-2c7736d1b577@google.com>
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On 7/19/23 10:34 AM, Hugh Dickins wrote:
> On Tue, 18 Jul 2023, Aneesh Kumar K.V wrote:
>> Hugh Dickins <hughd@google.com> writes:
>>
>>> Instead of pte_lockptr(), use the recently added pte_offset_map_nolock()
>>> in assert_pte_locked().  BUG if pte_offset_map_nolock() fails: this is
>>> stricter than the previous implementation, which skipped when pmd_none()
>>> (with a comment on khugepaged collapse transitions): but wouldn't we want
>>> to know, if an assert_pte_locked() caller can be racing such transitions?
>>>
>>
>> The reason we had that pmd_none check there was to handle khugpaged. In
>> case of khugepaged we do pmdp_collapse_flush and then do a ptep_clear.
>> ppc64 had the assert_pte_locked check inside that ptep_clear.
>>
>> _pmd = pmdp_collapse_flush(vma, address, pmd);
>> ..
>> ptep_clear()
>> -> asset_ptep_locked()
>> ---> pmd_none
>> -----> BUG
>>
>>
>> The problem is how assert_pte_locked() verify whether we are holding
>> ptl. It does that by walking the page table again and in this specific
>> case by the time we call the function we already had cleared pmd .
> 
> Aneesh, please clarify, I've spent hours on this.
> 
> From all your use of past tense ("had"), I thought you were Acking my
> patch; but now, after looking again at v3.11 source and today's,
> I think you are NAKing my patch in its present form.
> 

Sorry for the confusion my reply created. 

> You are pointing out that anon THP's __collapse_huge_page_copy_succeeded()
> uses ptep_clear() at a point after pmdp_collapse_flush() already cleared
> *pmd, so my patch now leads that one use of assert_pte_locked() to BUG.
> Is that your point?
> 

Yes. I haven't tested this yet to verify that it is indeed hitting that BUG.
But a code inspection tells me we will hit that BUG on powerpc because of
the above details.

> I can easily restore that khugepaged comment (which had appeared to me
> out of date at the time, but now looks still relevant) and pmd_none(*pmd)
> check: but please clarify.
> 

That is correct. if we add that pmd_none check back we should be good here.


-aneesh