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Message-ID: <7f22c46c-2119-4de6-9d58-efcab05b5751@bytedance.com>
Date: Thu, 5 Sep 2024 14:41:43 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Muchun Song <muchun.song@...ux.dev>
Cc: David Hildenbrand <david@...hat.com>, Hugh Dickins <hughd@...gle.com>,
Matthew Wilcox <willy@...radead.org>,
"Vlastimil Babka (SUSE)" <vbabka@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>, Mike Rapoport <rppt@...nel.org>,
Vishal Moola <vishal.moola@...il.com>, Peter Xu <peterx@...hat.com>,
Ryan Roberts <ryan.roberts@....com>, christophe.leroy2@...soprasteria.com,
LKML <linux-kernel@...r.kernel.org>,
Linux Memory Management List <linux-mm@...ck.org>,
linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 07/14] mm: khugepaged: collapse_pte_mapped_thp() use
pte_offset_map_rw_nolock()
On 2024/9/5 14:32, Muchun Song wrote:
>
>
>> On Aug 30, 2024, at 14:54, Qi Zheng <zhengqi.arch@...edance.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@...edance.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:
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:
>
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