> On Sep 5, 2024, at 14:41, Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:
>
>
>
> On 2024/9/5 14:32, Muchun Song wrote:
>>> On Aug 30, 2024, at 14:54, Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>>>>> ---
>>>>> 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:
