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

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

> 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().

> 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: