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Message-ID: <2343da2e-f91f-4861-bb22-28f77db98c52@bytedance.com>
Date: Tue, 24 Sep 2024 15:29:14 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Muchun Song <muchun.song@...ux.dev>
Cc: david@...hat.com, hughd@...gle.com, willy@...radead.org,
vbabka@...nel.org, akpm@...ux-foundation.org, rppt@...nel.org,
vishal.moola@...il.com, peterx@...hat.com, ryan.roberts@....com,
christophe.leroy2@...soprasteria.com, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 07/13] mm: khugepaged: collapse_pte_mapped_thp() use
pte_offset_map_rw_nolock()
On 2024/9/24 15:14, Muchun Song wrote:
>
>
>> On Sep 24, 2024, at 14:11, Qi Zheng <zhengqi.arch@...edance.com> wrote:
>> In collapse_pte_mapped_thp(), we may modify the pte and pmd entry after
>> acquring the ptl, so convert it to using pte_offset_map_rw_nolock(). At
>> this time, the pte_same() check is not performed after the PTL held. So we
>> should get pgt_pmd and do pmd_same() check after the ptl held.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> ---
>> mm/khugepaged.c | 14 +++++++++++---
>> 1 file changed, 11 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 6498721d4783a..8ab79c13d077f 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1605,7 +1605,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>> if (userfaultfd_armed(vma) && !(vma->vm_flags & VM_SHARED))
>> pml = pmd_lock(mm, pmd);
>>
>> - start_pte = pte_offset_map_nolock(mm, pmd, haddr, &ptl);
>> + start_pte = pte_offset_map_rw_nolock(mm, pmd, haddr, &pgt_pmd, &ptl);
>> if (!start_pte) /* mmap_lock + page lock should prevent this */
>> goto abort;
>> if (!pml)
>> @@ -1613,6 +1613,9 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>> else if (ptl != pml)
>> spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
>>
>> + if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd))))
>> + goto abort;
>> +
>> /* step 2: clear page table and adjust rmap */
>> for (i = 0, addr = haddr, pte = start_pte;
>> i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE, pte++) {
>> @@ -1645,7 +1648,6 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>> nr_ptes++;
>> }
>>
>> - pte_unmap(start_pte);
>> if (!pml)
>> spin_unlock(ptl);
>>
>> @@ -1658,13 +1660,19 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>> /* step 4: remove empty page table */
>> if (!pml) {
>> pml = pmd_lock(mm, pmd);
>> - if (ptl != pml)
>> + if (ptl != pml) {
>> spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
>> + if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
>> + spin_unlock(pml);
>> + goto abort;
>
> Drop the reference of folio and the mm counter twice at the label of abort and the step 3.
My bad, should set nr_ptes to 0 and call flush_tlb_mm() here, right?
>
>> + }
>> + }
>> }
>> pgt_pmd = pmdp_collapse_flush(vma, haddr, pmd);
>> pmdp_get_lockless_sync();
>> if (ptl != pml)
>> spin_unlock(ptl);
>> + pte_unmap(start_pte);
>> spin_unlock(pml);
>
> Why not?
>
> pte_unmap_unlock(start_pte, ptl);
> if (pml != ptl)
> spin_unlock(pml);
Both are fine, will do.
Thanks,
Qi
>
>>
>> mmu_notifier_invalidate_range_end(&range);
>> --
>> 2.20.1
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