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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=h9xTXffN; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D52CB606A326 for ; Sat, 7 Nov 2026 19:58:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=h9xTXffN Received: by smtp.kernel.org (Postfix) with ESMTPSA id 45CB51F0089F; Sat, 7 Nov 2026 19:58:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794081497; bh=5UTQ1ZEAazioDw1y7y/k+rDXclQDwetkhM2B+ZU2ofE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=h9xTXffNqwZQ2qHfRSoA53rzLxQP4/X8HsgHPusSRZWPug3STKIsz0vkRk76+ytnB mKdYiVTK8/qRG3H2YaVEi7bn2LOv981Q/HxqyOMx/nlMmKMwpBaliSiMQrX0LWyfvg FQ+OiCmsRR8EwpkGWSR/ztD5S/ihW1aKdc5sQfjsy7Qq2N6I9CMEHGiLc4cyF2vNxa fk7upvCZ1E6cL/CtsbRIYtI2aLVhNNY2fgKwVvy6E9zgoSVG3dpP1Y1VsUkABmSbJE BFuNf9o3OQt/IeMbuCzQHqHDRwe1XhsC/QkBJnZ42rUMd7I+/Sreb1IiBpwvjwDDWI SqOfDahzRRyRw== Message-ID: <4a64ea17-6ac2-406e-a432-b82c481a4f84@kernel.org> Date: Sat, 7 Nov 2026 20:58:09 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes To: Baolin Wang , Pedro Falcato Cc: akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, lance.yang@linux.dev, qi.zheng@linux.dev, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <44740e38-77ae-457e-97db-d6e0a0149dc2@linux.alibaba.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/6/26 03:12, Baolin Wang wrote: > > > On 11/5/26 7:37 PM, Pedro Falcato wrote: >> On Thu, Nov 05, 2026 at 10:56:28AM +0800, Baolin Wang wrote: >>> Similar to commit 243cf1f67b36 ("mm: memory: fix truncation removing mapped >>> folios"), the '!pmd_present()' check in page_vma_mapped_walk() can also race >>> with concurrent page table reclamation via zap_empty_pte_table(). >>> >>> page_vma_mapped_walk() is used by many functions including try_to_unmap(), >>> try_to_migrate(), folio_mkclean(), pfn_mkclean_range(), remove_migration_ptes(), >>> page_mapped_in_vma(), and mapping_wrprotect_range(). Except for try_to_unmap() >>> and try_to_migrate(), the other callers do not perform post-walk accounting >>> checks, so the race has no observable impact on them. >>> >>> For migration, the race can cause unnecessary retries. For reclaim via >>> try_to_unmap(), the race can cause failure to reclaim a folio that is in >>> fact already unmapped (as shown below), which may lead to unexpected OOM >>> or increased refaults. >> >> This is very handwavy. Do you have numbers? Does this actually make sense? > > This is hard to measure in practice. But in theory, if we can't reclaim > cold large folios that are actually unmapped due to the racy checks, it > could cause cold/hot inversion (e.g., in MGLRU) and increase refaults. > In extreme cases, failing to reclaim unmapped large folios could even > cause OOM. Yeah, but the question is if it really matters in practice. I agree that page_mapped_in_vma() should sync properly, but the current impact might not be as severe. -- Cheers, David