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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=XEHHg9Ol; 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 099DA606A31D for ; Tue, 20 Oct 2026 08:37:01 +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=XEHHg9Ol Received: by smtp.kernel.org (Postfix) with ESMTPSA id A9E8A1F000C5; Tue, 20 Oct 2026 08:36:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792485421; bh=WcS40+UCUrjVpcC++oCr3nHQbVKlVGoZMIffrsu1aX4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=XEHHg9Olk8StMgpraNq127BzFNi/fUtG6knLn7JfC/DIIYj418LHGLGaldG/k5Cox Udr65vKEX5iR/OlUG+u6a+t/7JdsbjIpal8l1bfauGXlIkkzMTRvj4iX7B6VmupVX+ vH55Bc8apWD8dw7+S4ufMPNujjoBbNYjAV1THtlb+eqsLoQ0/hQFam4webHHUvT0TV WgdPEJvMaFedzZtA/35anUHrSww7oJ//dvAwF1CtpHi4OgshSHp7O7jXhlXTTGP+2F WrViVsQuS1IWxdbJq234SGfdSPvuW2Y1KbJQrWnLFpz8DLrawVjzlkXsELS0Ta4kxK mLgMf5TVBTFaw== Message-ID: <938e215b-a2fd-4f1a-b4c5-e46b8af0f00b@kernel.org> Date: Tue, 20 Oct 2026 10:36:56 +0200 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 v2] mm: memory: fix truncation removing mapped folios To: Baolin Wang , akpm@linux-foundation.org Cc: ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, qi.zheng@linux.dev, jack@suse.cz, pfalcato@suse.de, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <542c97482eae84f86704bcee56482633d1b32321.1791616645.git.baolin.wang@linux.alibaba.com> <8245424a-f068-4be5-9e58-713cbff1f8ae@kernel.org> <405811f5-ee36-4b3c-80d0-7dc6827ab5d4@linux.alibaba.com> <6660d3bb-3485-4c34-8734-3ea99df920f1@kernel.org> <02c9a968-92e2-49e2-bffc-74d264a5c911@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: 8bit On 10/16/26 03:47, Baolin Wang wrote: > > > On 10/16/26 3:00 AM, David Hildenbrand (Arm) wrote: >> On 10/15/26 08:41, Baolin Wang wrote: >>> >>> >>> >>> Yes, I've considered that. However, implicit memory barriers give me a headache, >>> so I ended up going with explicit memory barriers (suggested by Pedro). >>> >>> If you don't like memory barriers, let me re-check whether the implicit memory >>> barriers here can ensure race safety with sync_with_folio_pmd_zap(). A >>> simplified logic is as follows: >>> >>> CPU 0 (truncation)                        CPU 1 (zapping) >>> down_read() ->Acquire A1.                pte spin_lock() ->Acquire A4 >>>                                           clear mapcount >>> >>>                                           pmd spin_lock() ->Acquire A5 >>>                                           clear pmd >>>                                           pmd spin_unlock() ->Release R5 >>> >>> if (pmd_none()) { //read pmd >>>                                           pte spin_unlock() ->Release R4 >>>      pmd spin_lock() ->Acquire A2 >>>      pmd spin_unlock() ->Release R2 >>> } >>> up_read() -> Release R1 >>> >>> spin_lock(y) ->Acquire A3 >>> folio_mapped() // read mapcount >>> spin_unlock(y) ->Release R3 >>> >>> CPU 0 constraints: >>> 1. R2 is a release, so "read pmd" completes before R2. >>> 2. A2 completes before A3 (ACQUIRE vs ACQUIRE). >>> 3. "read mapcount" completes after A3, hence after A2. >>> >>> CPU 1 constraints: >>> 1. R5 is a release. Both "clear mapcount" and "clear pmd" complete before R5. >>> >>> According to above constraints, if CPU0 sees pmd_none() caused by CPU1's clear >>> pmd, and CPU0 then acquires the same pmd lock released by CPU1 at R5, then >>> CPU1's clear mapcount happens before CPU0's later folio_mapped(). >>> >>> Therefore folio_mapped() is guaranteed not to read the old mapcount in this >>> case. So I think sync_with_folio_pmd_zap() can work, but needs a small change to >>> avoid the double pmd_none() check. >> >> >> I'd be okay with some (follow-up?) memory barrier handling inside >> sync_with_folio_pmd_zap(), for example, based on the folio size and with proper >> documentation why we need the PT lock in one case and why the memory barrier is >> ok in the other case. > > I'm afraid that we can't fit all the memory barriers into > sync_with_folio_pmd_zap(). As the current patch does, we need memory barriers on > both the writer and reader sides. Obviously only the reader-side one would go in there. [...] >>>                  if (pmd_none(*pmd)) { >>> +                       if (details && details->single_folio) >>> +                               sync_with_folio_pmd_zap(tlb->mm, pmd); >>>                          addr = next; >>>                          continue; >>>                  } >> >> We do have another sync_with_folio_pmd_zap() user in page_vma_mapped_walk(), >> does that one need care as well? > > Good point. But I don't want to add page_vma_mapped_walk() changes into this fix > patch. I need to analyze all callers of page_vma_mapped_walk() and the potential > races. I'll look into whether it should be changed in a follow-up patch. Yes, please have a look. I am 99.9% sure that we face the exact same issue there as well. -- Cheers, David