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[2003:cb:c72f:c700:a76f:473d:d5cf:25a8]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43017465f3bsm15971755e9.0.2024.10.08.00.52.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 08 Oct 2024 00:52:38 -0700 (PDT) Message-ID: <75fac79a-0ff2-4ba0-bdd7-954efe2d9f41@redhat.com> Date: Tue, 8 Oct 2024 09:52:37 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/mremap: Fix move_normal_pmd/retract_page_tables race To: Qi Zheng , Jann Horn Cc: akpm@linux-foundation.org, linux-mm@kvack.org, willy@infradead.org, hughd@google.com, lorenzo.stoakes@oracle.com, joel@joelfernandes.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20241007-move_normal_pmd-vs-collapse-fix-2-v1-1-5ead9631f2ea@google.com> <1c114925-9206-42b1-b24b-bb123853a359@bytedance.com> Content-Language: en-US From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 08.10.24 05:53, Qi Zheng wrote: > Hi Jann, > > On 2024/10/8 05:42, Jann Horn wrote: > > [...] > >> >> diff --git a/mm/mremap.c b/mm/mremap.c >> index 24712f8dbb6b..dda09e957a5d 100644 >> --- a/mm/mremap.c >> +++ b/mm/mremap.c >> @@ -238,6 +238,7 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr, >> { >> spinlock_t *old_ptl, *new_ptl; >> struct mm_struct *mm = vma->vm_mm; >> + bool res = false; >> pmd_t pmd; >> >> if (!arch_supports_page_table_move()) >> @@ -277,19 +278,25 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr, >> if (new_ptl != old_ptl) >> spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); >> >> - /* Clear the pmd */ >> pmd = *old_pmd; >> + >> + /* Racing with collapse? */ >> + if (unlikely(!pmd_present(pmd) || pmd_leaf(pmd))) > > Since we already hold the exclusive mmap lock, after a racing > with collapse occurs, the pmd entry cannot be refilled with > new content by page fault. So maybe we only need to recheck > pmd_none(pmd) here? My thinking was that it is cheap and more future proof to check that we really still have a page table here. For example, what if collapse code is ever changed to replace the page table by the collapsed PMD? So unless there is a good reason not to have this check here, I would keep it like that. Thanks! -- Cheers, David / dhildenb