Re: [PATCH v3 10/13] mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()

[Qi Zheng]

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@xxxxxxxxxxxxx>

Thanks.

> Hugh