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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=DLWI4iKk; 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 837AD60772AE for ; Thu, 5 Nov 2026 19:35:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=DLWI4iKk Received: by smtp.kernel.org (Postfix) with ESMTPSA id CD4ED1F008A3; Thu, 5 Nov 2026 19:35:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793907321; bh=53L1YKr8K72nT9+wdtzmGCre0RXQPHPg0/Frt6ozyWA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=DLWI4iKkdY8yretBkpgEF0qHmeQsbFGyIGPqIQFl7cG5VQKrz7byvgxmeDoRb0vW3 2IMK2S+7K+nkJOTAPvPgmvX0sS+ZimeC4NoLjq1UcHb7hV4lZgTvE2sdr6ZTfXoZv+ bmwdJUHQjY5et6yV+mj3u3I2cgcspELGc+BTktO8by50k86xeIVKakVRqYzHEQ3lWn Daq0CW4F7SkD/p8HxXLfKmexxrfUJzgzy7JSqci5mmpCpPjEjmTX11rRlmsIKhBaDp Y3Mx4603duZfntY0isGRjwGz2yk4Sgx8yfrShgzDkUk6Atmv70+FNyIbfclpDgiYuu v7PhqHi4DXmRA== Message-ID: <4e935565-a22f-4cfe-935a-bbee3e81d7a6@kernel.org> Date: Thu, 5 Nov 2026 20:35:15 +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: Lance Yang , baolin.wang@linux.alibaba.com Cc: baohua@kernel.org, akpm@linux-foundation.org, ljs@kernel.org, riel@surriel.com, liam@infradead.org, vbabka@kernel.org, harry@kernel.org, jannh@google.com, qi.zheng@linux.dev, pfalcato@suse.de, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <8ebb182b-8a01-4d32-8c76-7e4bd9e54ddb@kernel.org> <20261105132258.99528-1-lance.yang@linux.dev> 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 11/5/26 14:22, Lance Yang wrote: > > On Thu, Nov 05, 2026 at 11:06:29AM +0100, David Hildenbrand (Arm) wrote: >> On 11/5/26 10:00, Baolin Wang wrote: >>> CC Barry, who added the TTU_SYNC in shrink_folio_list(). >>> >>> >>> Thanks Lance. But this matches the purpose of the TTU_SYNC flag, which is used >>> to avoid racy checks with PVMW_SYNC. If no TTU_SYNC flag is set, it means the >>> caller can tolerate some racy checks. >>> >>> For this case, I think we should set TTU_SYNC unconditionally in >>> shrink_folio_list(). Barry, what do you think? >> >> That makes sense. I hope this doesn't throw a big wrench into our reclaim >> performance numbers. >> >> Can someone benchmark this? >> >> But maybe we should really follow-up with the folio-size based optimization in >> sync_with_folio_pmd_zap(), where we only need a memory barrier (and sufficient >> documentation why something that cannot be PMD-mapped can get away cheaper). > > Dug into the history a bit :D > > TTU_SYNC goes back to Hugh's THP splitting fix [1]. A racing zap could > clear the PTE/PMD before updating mapcount, so the walker could skip the > lock and return before the accounting caught up. > > Barry later enabled it for large-folio reclaim [2], to avoid skipping > temporarily cleared PTEs and ending up with a partially unmapped folio. > He mentioned that small folios had the same race, but with just one PTE, > they couldn't be partially unmapped. That's where the large-folio check > in shrink_folio_list() came from. > > Then PTE table reclaim [3] added another way to end up with an empty PMD, > even for a folio that can't be PMD-mapped. The truncation fix [4] moved > the rmap flush before the PMD clear and extended synchronization in > zap_pmd_range() to cover those folios too. > > Looking back at the truncation discussion [5], you also pointed out that > page_vma_mapped_walk() needed checking, and suggested a possible > folio-size based optimization in the helper. > > [1] https://lore.kernel.org/all/c1e95853-8bcd-d8fd-55fa-e7f2488e78f@google.com/ > [2] https://lore.kernel.org/all/20240306095219.71086-1-21cnbao@gmail.com/ > [3] https://lore.kernel.org/all/92aba2b319a734913f18ba41e7d86a265f0b84e2.1733305182.git.zhengqi.arch@bytedance.com/ > [4] https://lore.kernel.org/all/14ebbd556cc4941f0ebc43da59fa6a2fdca25dc8.1793148609.git.baolin.wang@linux.alibaba.com/ > [5] https://lore.kernel.org/all/08a0907f-8e3a-4071-a2e4-fd1411c446f3@kernel.org/ > > So, how about starting with order-0 folios? > > ---8<--- > diff --git a/include/linux/rmap.h b/include/linux/rmap.h > index 74cca0e3c726..d01fe8282417 100644 > --- a/include/linux/rmap.h > +++ b/include/linux/rmap.h > @@ -872,6 +872,8 @@ struct page_vma_mapped_walk { > spinlock_t *ptl; > unsigned int flags; > bool pgoff_is_anon : 1; > + /* Set from the folio, not the number of pages being walked. */ > + bool folio_is_order0 : 1; > }; > > #define DEFINE_FOLIO_VMA_WALK(name, _folio, _vma, _address, _flags) \ > @@ -883,6 +885,7 @@ struct page_vma_mapped_walk { > .address = _address, \ > .flags = _flags, \ > .pgoff_is_anon = folio_test_anon(_folio), \ > + .folio_is_order0 = !folio_test_large(_folio), \ > } We should pass the actual folio ;) But yes, this goes into the direction I had in mind. But I do wonder whether we need a separate fix first before we improve it? -- Cheers, David