linux-kernel - Re: [PATCH v3 10/13] mm/khugepaged: collapse_pte_mapped_thp() with mmap_read

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Message-ID: <3403e5d3-e840-cea7-1863-b6b9ab7ecae0@bytedance.com>
Date:   Mon, 7 Aug 2023 10:21:20 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
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        Anshuman Khandual <anshuman.khandual@....com>,
        Miaohe Lin <linmiaohe@...wei.com>,
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        Michael Ellerman <mpe@...erman.id.au>,
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        linux-arm-kernel@...ts.infradead.org, sparclinux@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 10/13] mm/khugepaged: collapse_pte_mapped_thp() with
 mmap_read_lock()



On 2023/8/6 11:55, Hugh Dickins wrote:
> On Thu, 3 Aug 2023, Qi Zheng wrote:
>> On 2023/7/12 12:42, Hugh Dickins wrote:
>>> Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp().
>>> It does need mmap_read_lock(), but it does not need mmap_write_lock(),
>>> nor vma_start_write() nor i_mmap lock nor anon_vma lock.  All racing
>>> paths are relying on pte_offset_map_lock() and pmd_lock(), so use those.
> ...
>>> @@ -1681,47 +1634,76 @@ int collapse_pte_mapped_thp(struct mm_struct *mm,
>>> unsigned long addr,
>>>    
>>>      if (pte_none(ptent))
>>>    			continue;
>>> -		page = vm_normal_page(vma, addr, ptent);
>>> -		if (WARN_ON_ONCE(page && is_zone_device_page(page)))
>>> +		/*
>>> +		 * We dropped ptl after the first scan, to do the
>>> mmu_notifier:
>>> +		 * page lock stops more PTEs of the hpage being faulted in,
>>> but
>>> +		 * does not stop write faults COWing anon copies from existing
>>> +		 * PTEs; and does not stop those being swapped out or
>>> migrated.
>>> +		 */
>>> +		if (!pte_present(ptent)) {
>>> +			result = SCAN_PTE_NON_PRESENT;
>>>    			goto abort;
>>> +		}
>>> +		page = vm_normal_page(vma, addr, ptent);
>>> +		if (hpage + i != page)
>>> +			goto abort;
>>> +
>>> +		/*
>>> +		 * Must clear entry, or a racing truncate may re-remove it.
>>> +		 * TLB flush can be left until pmdp_collapse_flush() does it.
>>> +		 * PTE dirty? Shmem page is already dirty; file is read-only.
>>> +		 */
>>> +		pte_clear(mm, addr, pte);
>>
>> This is not non-present PTE entry, so we should call ptep_clear() to let
>> page_table_check track the PTE clearing operation, right? Otherwise it
>> may lead to false positives?
> 
> You are right: thanks a lot for catching that: fix patch follows.

With fix patch:

Reviewed-by: Qi Zheng <zhengqi.arch@...edance.com>

Thanks.

> 
> Hugh