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AJvYcCXHMFDhoUSpNH986aqe0eQX9ra/g36KN+d8qu35EXbRNUpE2os93fL5DFLgtFat2M9lBKFH4rRJbc62eC4=@vger.kernel.org X-Gm-Message-State: AOJu0YxSzWj1+GhMb35JTsiuxiTlq48Gxb6Jzl5WXVdYDW720liAXef8 fYr015Z/99lQCYcEbh+hiY+gXSLYyebZBccXQ0WgNTvcmTp4T3OOvnsyj75Us2MekxERVn9MFIf Y X-Google-Smtp-Source: AGHT+IHQyky+Kk9qEKi7IWE1UWr/UmQJSB+2gVrWMjTiY9kash16TX5475rtXBjRZ94+FApWIS8qXA== X-Received: by 2002:a17:902:e5c3:b0:202:35cb:b0c6 with SMTP id d9443c01a7336-2050c3bf6e5mr52764945ad.34.1725000866519; Thu, 29 Aug 2024 23:54:26 -0700 (PDT) Received: from [10.4.59.158] ([139.177.225.242]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-205155452ecsm20825575ad.219.2024.08.29.23.54.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 29 Aug 2024 23:54:26 -0700 (PDT) Message-ID: Date: Fri, 30 Aug 2024 14:54:16 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 07/14] mm: khugepaged: collapse_pte_mapped_thp() use pte_offset_map_rw_nolock() Content-Language: en-US To: Muchun Song Cc: david@redhat.com, hughd@google.com, willy@infradead.org, vbabka@kernel.org, akpm@linux-foundation.org, rppt@kernel.org, vishal.moola@gmail.com, peterx@redhat.com, ryan.roberts@arm.com, christophe.leroy2@cs-soprasteria.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org References: <24be821f-a95f-47f1-879a-c392a79072cc@linux.dev> From: Qi Zheng In-Reply-To: <24be821f-a95f-47f1-879a-c392a79072cc@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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 >> --- >>   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: >