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AJvYcCVvS+av9O7mD9QXsgX8QaGeBIigxJ++b2dZiZ9McuG7knQLUsl3VJDiJ6vK+fJC+lXjB0A9Kk0syNlARl0=@vger.kernel.org X-Gm-Message-State: AOJu0YxTNOMKQY3wfM0GDtGMYxYAWd57v+KVD2K6vVBuwdOme4F9XGVn o1+P01OETIzmEU7Lc871RqNyufi7LttvymDoJg0YkOurysMDeRba/2UWvlDsKrU= X-Google-Smtp-Source: AGHT+IHN1g2uzj5AhSKq4svLeyrNe2KgneEIeIGEQbArzd+een94HcwN+eGFkVCRRabnx0ftu/fetg== X-Received: by 2002:a05:6a20:e18a:b0:1cf:658e:5107 with SMTP id adf61e73a8af0-1d6dfa44dd3mr22323407637.21.1728374286725; Tue, 08 Oct 2024 00:58:06 -0700 (PDT) Received: from [10.68.125.128] ([203.208.167.150]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-71df0d67bc8sm5764221b3a.177.2024.10.08.00.58.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 08 Oct 2024 00:58:06 -0700 (PDT) Message-ID: Date: Tue, 8 Oct 2024 15:58:00 +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] mm/mremap: Fix move_normal_pmd/retract_page_tables race Content-Language: en-US To: David Hildenbrand Cc: Jann Horn , 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> <75fac79a-0ff2-4ba0-bdd7-954efe2d9f41@redhat.com> From: Qi Zheng In-Reply-To: <75fac79a-0ff2-4ba0-bdd7-954efe2d9f41@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2024/10/8 15:52, David Hildenbrand wrote: > 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? Ah, make sense. Acked-by: Qi Zheng > > So unless there is a good reason not to have this check here, I would > keep it like that. > > Thanks! >