From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E132A348463 for ; Mon, 29 Jun 2026 07:35:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782718543; cv=none; b=VquLw5n8KAT9yejl79CUqknGARHYMp8EYV/tiPQjAO3wqfSlK3v9S1M9U0LelwVaSSdpPE0f0A+OnT4M46D6CwLswTJJGpDov/Lnv/i50jX4egBTwkpV04uGL6XIbJ4m0WY7vL/NzIR0QeQ+l2YXi6MzaekTtdxK6SeaTh5ESIY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782718543; c=relaxed/simple; bh=klIDKNbhnlXm4+EZXLzePYjjEuLW5jpNa1qqh72tJFc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: In-Reply-To:Content-Type:Content-Disposition; b=pBBSXSOhbB7d8sXEHly9gLLIz4aDUJx/NjnyD3tEJY5Rk0jQ4yM/ObeRc0HTf7/tGVOtXc3iRK+wIhFp7+JxELmmI2lLP4HU1bBn9f5kmUMZAxPli37nbr7ZA/0LGZ2912vpFi4QaFusfM5sAqbPU8hvB6wcsE2u5/e1uDcos3A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=SZ195P8I; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="SZ195P8I" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1782718534; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=YhwD5e0TAt0qKmZwhk19yeLkiMwZuJytbMfHyxkkjWg=; b=SZ195P8IXiaCWp6NVf6wUsA3qc5eTkZ7xd/kb1uqe3iF73/H/VKEqN4WrMHM93UWuJv8Jw AVbBhrXJO1PHV3XQL/tq0NOhqX1TtlUJA6dW6T/oPQ/DAoF97X8K++SiJ6WvfDXtAGHbKx jVrFdZo9PpQz1n0PKGCubUcQ4lxJfow= Received: from mail-pj1-f71.google.com (mail-pj1-f71.google.com [209.85.216.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-452-8rhJwU5BPRyJHvJIOEcRkg-1; Mon, 29 Jun 2026 03:35:28 -0400 X-MC-Unique: 8rhJwU5BPRyJHvJIOEcRkg-1 X-Mimecast-MFC-AGG-ID: 8rhJwU5BPRyJHvJIOEcRkg_1782718526 Received: by mail-pj1-f71.google.com with SMTP id 98e67ed59e1d1-36bbcd40642so1774096a91.0 for ; Mon, 29 Jun 2026 00:35:27 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1782718526; x=1783323326; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-gg:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=YhwD5e0TAt0qKmZwhk19yeLkiMwZuJytbMfHyxkkjWg=; b=I1C6c3f04TYR/bX7UGw5uoNMLp6yFOhsrBs7AXIlYb1GHflUPlbiUXK1BADC6HW9GR VwCbLzqCtLm9GU7PgF5PFKc/HLWQlyAMxCb1eT0s8Ah9na+SaTxFf3PmSrpjS1d1g8Ug zgSHhiDinw9N4aThZ7XcL9dS0nhYgrYz9iSOM21r+4VIZR/A4C0fs6JLi8lwFPL2s7Ej cwDrNPGTKHf0Yn9IybYbde90BS1Ewy0ZefAz5Q70LUc0nqj4fDkALKKUWHp+vUin1BrV SF1l5igWZZ3zYSyqRC8NivU4BYIy4jqHY5Ivvi7sX5X0Gs0/b53h8ZYALJMhXTinCY+c /DmA== X-Forwarded-Encrypted: i=1; AFNElJ/M6EXqR1iIZdZV2Fijfvh37BpYL338D+dlurGsGXJe4qv4ZR7UMFZC0KSnREsLoX83QBPZf1iDMTU=@vger.kernel.org X-Gm-Message-State: AOJu0Yw5rkF01OqqkaB8GRAYZAbKElsNdk1cwUVmIY1r/Np6W3KyWufk +MhfN9E0ajm4uNuppdLu9cqvX9rqztLZ6KZpGv8DBPzU4WSAowNQcUjsveRU8e+WrI6sxsulTpf pSA7Wuu2VW2oOq56fKSwfoIQFf+qUbrLF+eFM1gH1CYx3umw3lebX+sS/8ZU7MQ== X-Gm-Gg: AfdE7cnUpppF3ltPyRyuh9ipPkoqu/FEwygHayhvo2NzEfcqtixcbvh1KN7G5qU+ejo uNXNihpvMqtF4MVEXYVIjcXYBxhFp+ZAh1sky+PkEZaIcjt6hIEIveiSU86YHmLGGwQd64EG/7P mgIT+xhehRbALATvHrTm4k9IEcpZ3swiLvI58vNBArRQpcJpcbDRIw1hugZDeKYXXTQ+V3p77Zn XHnE1gNNquum0E+P111g51WbDp4gjQzOh0tmLZp2OztGHrl4fH5On6YwDEoJXkUO8JqfslfR/OS N/9Sp/3yA9PGXyow7qiqWhM7P6hgSJWhZEL1BWmSTQvmSUz5Kt41nBCPneCVyhlJRy9JNDDes2T tNGmN9PDSTUdDin5WD1swp7m/uhx8hk3s X-Received: by 2002:a05:6a21:a08:b0:3bf:79d6:f063 with SMTP id adf61e73a8af0-3bf79d740bdmr5424833637.43.1782718525441; Mon, 29 Jun 2026 00:35:25 -0700 (PDT) X-Received: by 2002:a05:6a21:a08:b0:3bf:79d6:f063 with SMTP id adf61e73a8af0-3bf79d740bdmr5424801637.43.1782718524686; Mon, 29 Jun 2026 00:35:24 -0700 (PDT) Received: from redhat.com (IGLD-80-230-85-71.inter.net.il. [80.230.85.71]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-c92bd55985asm7323848a12.31.2026.06.29.00.35.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Jun 2026 00:35:24 -0700 (PDT) Date: Mon, 29 Jun 2026 03:34:59 -0400 From: "Michael S. Tsirkin" To: "David Hildenbrand (Arm)" Cc: linux-kernel@vger.kernel.org, Miaohe Lin , Naoya Horiguchi , Andrew Morton , Oscar Salvador , Andi Kleen , Hidehiro Kawai , Rik van Riel , Vlastimil Babka , Lorenzo Stoakes , "Liam R. Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Hao Li , Kiryl Shutsemau , Byungchul Park , linux-mm@kvack.org, linux-cxl@vger.kernel.org Subject: Re: [PATCH 0/2] mm: memory-failure: fix HWPoison flag race with non-atomic page flag ops Message-ID: <20260629030657-mutt-send-email-mst@kernel.org> References: <0b5f8b4b-d7dc-4b79-9555-a5b36265f3a9@kernel.org> Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <0b5f8b4b-d7dc-4b79-9555-a5b36265f3a9@kernel.org> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: fYeTYq8tC2MytdZ_wz5JhlkmKXCgQMtXu-5hE0frNyg_1782718526 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Mon, Jun 29, 2026 at 08:49:37AM +0200, David Hildenbrand (Arm) wrote: > On 6/28/26 23:45, Michael S. Tsirkin wrote: > > I don't like it that we are adding overhead to the good path for > > the benefit of memory failure, which never triggers on many systems, > > but I don't have a better idea. Pls take a look. > > As I said on Friday. > > "It's also doesn't address the mf_mutex implications and the x86 thingies I > mentioned. Well I did attempt addressing this. These would be these two: (a) We don't hold the mf_mutex on all call paths, but we really need it so a page_test_set_hwpoison() cannot race in weird ways with the other primitives I think. page_test_set_hwpoison was this code you wrote: +static void page_set_hwpoison(struct page *page) +{ + lockdep_assert_held(&mf_mutex); + + while (!PageHWPoison(page)) { + SetPageHWPoison(page); + + /* Make sure concurrent non-atomic writers completed. */ + synchronize_rcu(); + } +} and indeed the test+set combination seems racy. But consider the version I posted, for example: +/* + * Drain any in-flight non-atomic page flag operations that could + * clobber a concurrently set HWPoison bit. Retries until the bit sticks. + */ +static void set_hwpoison_drain_rcu(struct page *p) +{ + do { + synchronize_rcu(); + } while (!TestSetPageHWPoison(p)); +} + ... +static bool test_and_set_hwpoison_drain_rcu(struct page *p) +{ + bool was_set = TestSetPageHWPoison(p); + + set_hwpoison_drain_rcu(p); + return was_set; +} does not seem racy without a lock. But maybe I don't get it. (b) There are some leftover SetPageHWPoison etc. instances. The ones in arch/x86/kernel/cpu/mce/core.c likely cannot grab the mutex, but maybe they are corner cases either way and we can document the situation. Well, I did try to document the situation - it's in the commit log for patch 1: Note: the MCE handler in arch/x86/kernel/cpu/mce/core.c also calls SetPageHWPoison() and is subject to the same race. It cannot use the drain helpers (MCE context cannot call synchronize_rcu()). For recoverable MCE errors, memory_failure() is queued via work items (kill_me_maybe/kill_me_never) and will re-set the bit via test_and_set_hwpoison_drain_rcu() if it was clobbered. The mce_panic() path sets HWPoison for kdump right before panic() so the race is irrelevant there. The MCG_STATUS_SEAM_NR path does not queue memory_failure(), but the affected page belongs to a TDX guest whose CPU core has already been marked dead - the page is not subject to concurrent non-atomic flag operations in the buddy allocator, so the race does not apply. > ... > > I'll either take care of that myself or find someone that can work on this with > attention to all details. > " > > This is nothing to vibe-code. This needs a real expert. Well I had this sitting on the disk anyway, so I thought I'd post. I wouldn't call this vibe-code - a bunch of manual work went into this, llms mostly as a grep/sed replacement. But hey. I don't object to someone taking over, for sure. Was fun, and maybe these patches will be helpful as a starting point. In particular, maybe I should have been more explicit about how your points from Friday are addressed. If you want to add a bit more to explain the exact concerns here, for whoever works on this next, feel free to do so. > > > > Non-atomic page flag operations (page->flags.f &= ~mask, __set_bit, > > __clear_bit) can race with atomic TestSetPageHWPoison() in > > memory_failure(). The non-atomic RMW reads flags, memory_failure() > > atomically sets HWPoison, then the RMW writes back the old value > > without HWPoison, clobbering the bit. > > > > The race was confirmed by injecting a cpu_relax() delay between the > > load and store of the non-atomic RMW in __free_pages_prepare, then > > running concurrent MADV_HWPOISON injection. The clobbered HWPoison > > bit was observed repeatedly. > > > > This series fixes the race by: > > > > 1. Having memory_failure() call synchronize_rcu() + retry after > > setting HWPoison, so that any in-flight non-atomic RMW that > > read the old flags value completes before we proceed. > > > > 2. Wrapping all non-atomic page flag operations in > > rcu_read_lock/rcu_read_unlock (CONFIG_MEMORY_FAILURE only), > > so that synchronize_rcu() actually drains them. > > > > Performance impact (page alloc+free microbenchmark, 200K iterations, > > 20 runs, KVM guest, error bars are 3-sigma): > > > > !PREEMPT_RCU (x86): > > insns/iter cycles/iter > > base: 12237 +/- 1 17954 +/- 136 > > patched: +22 +/- 1 -124 +/- 122 > > (+0.18%) (within noise) > > > > PREEMPT_RCU: > > insns/iter cycles/iter > > base: 12512 +/- 3 18541 +/- 214 > > patched: +95 +/- 3 -12 +/- 161 > > (+0.76%) (within noise) > > > > When !CONFIG_MEMORY_FAILURE, all wrappers compile away completely. > > > > Suggested-by: David Hildenbrand > > No ;) > > -- > Cheers, > > David