linux-kernel - Re: [PATCH v4 07/13] mm: khugepaged: collapse_pte_mapped_thp() use pte_offset_map_rw

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Message-Id: <1D1872F1-7280-4F43-8213-A720C56B0646@linux.dev>
Date: Tue, 24 Sep 2024 16:52:34 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Qi Zheng <zhengqi.arch@...edance.com>
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 Sep 24, 2024, at 15:29, Qi Zheng <zhengqi.arch@...edance.com> wrote:
> 
> 
> 
> 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?

Or add a new label "out" just below the "abort". Then go to out.

> 
>>> +            }
>>> +        }
>>>    }
>>>    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