Re: [PATCH v2 07/14] mm: khugepaged: collapse_pte_mapped_thp() use pte_offset_map_rw_nolock()

[Muchun Song <muchun.song@linux.dev>]

> On Sep 5, 2024, at 14:41, Qi Zheng <zhengqi.arch@bytedance.com> wrote:
> 
> 
> 
> On 2024/9/5 14:32, Muchun Song wrote:
>>> On Aug 30, 2024, at 14:54, Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>>> 
>>> 
>>> 
>>> On 2024/8/29 16:10, Muchun Song wrote:
>>>> On 2024/8/22 15:13, Qi Zheng 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 write lock of mmap_lock is not held, and 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.
>>>>> 
>>>>> For the case where the ptl is released first and then the pml is acquired,
>>>>> the PTE page may have been freed, so we must do pmd_same() check before
>>>>> reacquiring the ptl.
>>>>> 
>>>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>>>> ---
>>>>>  mm/khugepaged.c | 16 +++++++++++++++-
>>>>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>>>> 
>>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>>> index 53bfa7f4b7f82..15d3f7f3c65f2 100644
>>>>> --- a/mm/khugepaged.c
>>>>> +++ b/mm/khugepaged.c
>>>>> @@ -1604,7 +1604,7 @@ 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)
>>>>> @@ -1612,6 +1612,9 @@ 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++) {
>>>>> @@ -1657,6 +1660,16 @@ 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);
>>>>> +        /*
>>>>> +         * We called pte_unmap() and release the ptl before acquiring
>>>>> +         * the pml, which means we left the RCU critical section, so the
>>>>> +         * PTE page may have been freed, so we must do pmd_same() check
>>>>> +         * before reacquiring the ptl.
>>>>> +         */
>>>>> +        if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) {
>>>>> +            spin_unlock(pml);
>>>>> +            goto pmd_change;
>>>> Seems we forget to flush TLB since we've cleared some pte entry?
>>> 
>>> See comment above the ptep_clear():
>>> 
>>> /*
>>> * 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.
>>> */
>>> 
>>> The TLB flush was handed over to pmdp_collapse_flush(). If a
>> But you skipped pmdp_collapse_flush().
> 
> I skip it only in !pmd_same() case, at which time it must be cleared
> by other thread, which will be responsible for flushing TLB:

WOW! AMAZING! You are right.

> 
> CPU 0				CPU 1
> 				pmd_clear
> 				spin_unlock
> 				flushing tlb
> spin_lock
> if (!pmd_same) 
> 	goto pmd_change;
> pmdp_collapse_flush
> 
> Did I miss something?
> 
>>> concurrent thread free the PTE page at this time, the TLB will
>>> also be flushed after pmd_clear().
>>> 
>>>>> +        }
>>>>>          if (ptl != pml)
>>>>>              spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
>>>>>      }
>>>>> @@ -1688,6 +1701,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>>>>>          pte_unmap_unlock(start_pte, ptl);
>>>>>      if (pml && pml != ptl)
>>>>>          spin_unlock(pml);
>>>>> +pmd_change:
>>>>>      if (notified)
>>>>>          mmu_notifier_invalidate_range_end(&range);
>>>>>  drop_folio: