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Date:   Sat, 5 Aug 2023 20:55:25 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Qi Zheng <zhengqi.arch@bytedance.com>
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Subject: Re: [PATCH v3 10/13] mm/khugepaged: collapse_pte_mapped_thp() with
 mmap_read_lock()
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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.

Hugh