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

[Qi Zheng <zhengqi.arch@bytedance.com>]

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
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:
>