> 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().
> 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:
