From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from stravinsky.debian.org (stravinsky.debian.org [82.195.75.108]) (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 0A59F5013D0; Fri, 18 Sep 2026 15:23:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=82.195.75.108 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789745018; cv=none; b=ZWl6zEViUDfFfXjmoYoR3XbhCXezJkNIz/AJHVABiG8bqA7NZyHRh1+HBPtG23QEczrdwPB2sgXb15cXqjdruHloi2GAYfhbYpVZcZABbTcGFQq8wPHzRq6RVTDfcF4IjPBYEG0gtxUCwIVUCZgFpC5loza3Fpba8ThxZF8INi8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789745018; c=relaxed/simple; bh=AgtCk1IR2NnW2csAy3ZG2AVpreeujXaGKrRl+JM98QE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=FFsyCMLGtKHPUYhMnQe2pYf+oiCSsKp0Sa/US5uOmf5LyLGGEsmZIIldblAWuQUCDZA2ZXWAt30WGKngyWHFwgS0zz220sHqf8lBSh6C9U6ovmFTomtX8u4YODEo+RXEzStFk98o3Wpr233+W7hbRicRUTSFV/z1oOJ3CJWT8lQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=debian.org; spf=pass smtp.mailfrom=debian.org; dkim=pass (2048-bit key) header.d=debian.org header.i=@debian.org header.b=Rd/CzOJ4; arc=none smtp.client-ip=82.195.75.108 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=debian.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=debian.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=debian.org header.i=@debian.org header.b="Rd/CzOJ4" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=debian.org; s=smtpauto.stravinsky; h=X-Debian-User:In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=wEcH5E4AEzqLkF0Ir17t22goJx7MrsfhZ+RepQ30l9U=; b=Rd/CzOJ4Wdpu0aeXB7YKPQv3Px AjPJ9UARjooUYAYq3DQxrzJq+fUfuC5JUayYjmsvt8yCxCbZj0VeTNPtxZLNFTM8YMGDnrpqBM+Yq b/WUexOikEof1KpPd+VepJJAc8Dd6Ym2ANAzqeLfjDmhnYKZmNAouR29ft6Rmnr1cjRHt/D95Tq6J ktHYO3BsruwP/wwhSUheRPB8nqU8mlkq/t99Xgierr9euxpXG/HqAPyUQeiLSPVuSeLsQYjTB5bbc rNocdUJT8Ii177MByzY5QFDdsCVelqAU2U7i1reluFG+QDcod7VoHIYunualBSHBhMS57u3Gwj1r5 yJ0+/IgQ==; Received: from authenticated-user by stravinsky.debian.org with esmtpsa (TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_256_GCM:256) (Exim 4.96) (envelope-from ) id 1x7aQN-006oxZ-1y; Fri, 18 Sep 2026 15:22:51 +0000 Date: Fri, 18 Sep 2026 08:22:42 -0700 From: Breno Leitao To: "David Hildenbrand (Arm)" Cc: Ard Biesheuvel , Ilias Apalodimas , Miaohe Lin , Naoya Horiguchi , Andrew Morton , kas@kernel.org, kexec@lists.infradead.org, Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Brendan Jackman , Johannes Weiner , Zi Yan , Oscar Salvador , Greg Kroah-Hartman , "Rafael J. Wysocki" , Danilo Krummrich , hannes@cmpxchg.or, shakeel.butt@linux.dev, linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, rmikey@meta.com, riel@surriel.com, harry@kernel.org, linux-cxl@vger.kernel.org, driver-core@lists.linux.dev, kernel-team@meta.com Subject: Re: [PATCH v5 7/9] drivers/base/memory: count inherited poisoned frames into the block Message-ID: References: <20260915-hwpoison-kho-v5-0-3bc7a57bd503@debian.org> <20260915-hwpoison-kho-v5-7-3bc7a57bd503@debian.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Debian-User: leitao On Fri, Sep 18, 2026 at 02:18:03PM +0200, David Hildenbrand (Arm) wrote: > On 9/17/26 15:01, Breno Leitao wrote: > > On Wed, Sep 16, 2026 at 04:51:40PM +0200, David Hildenbrand (Arm) wrote: > >> On 9/16/26 11:35, Breno Leitao wrote: > >>> > >>> hmmm, I am not sure I see it that way. > >>> > >>> The loop only runs for a block the bitmap marks. On a machine with nothing > >>> recorded the bitmap is all zeros, the range_contains_poisoned_memory() check > >>> right above it returns false, and the loop never executes. > >>> > >>> What every boot does pay is that check, once per block. The stub installs > >>> the table whether or not anything was ever recorded in it, so this is not a > >>> NULL test: it is two 64-bit divisions by the unit size plus a > >>> find_next_bit() over the single word a 128M block covers at one bit per 2M. > >>> > >>> The real cost (that "for loop above"), comes when you kexec (not on cold > >>> boot -- given the bitmap is empty), and you are trying to init > >>> a memory block that has poisoned pages into it. Which seems the right > >>> trade-off, no? > >>> > >>> That said, can we do better? Yes. The silly win is to let the table say > >>> whether anything was ever recorded in it, something like a > >>> linux_efi_poisoned_memory->empty that the first recorded frame clears, > >>> and return on that before the bitmap is reached at all. > >>> > >>> Is this what you are looking for, or something more drastic? > >> > >> Ah, that magical "range_contains_poisoned_memory" does a bitmap scan? > >> > >> I'm sorry, but that is absolutely confusing. > >> > >> There is no way someone will figure out that range_contains_poisoned_memory() > >> queries some efi specific bitmap that won't even be able to represent any memory > >> outside of it's range. > >> > >> I don't really have time to give a better solution, but starting with the > >> naming, range_contains_poisoned_memory() is just absolutely misleading. > > > > Fair point, I'll clean up the naming in the next revision. > > > > I'll also add that ->empty field, which should help locate this bit > > faster and may let us skip the bitmap query entirely on the happy path. > > > > Anything else you'd like addressed? > > > > Good to know this moved the needle from "David hates this feature" to > > "David only hates the naming" -- I'll take that as progress. :-P > > ;) > > I think the crucial part is to find a way to cleanly distinguish our source of > information, and also how the source does only apply to some memory. Right, we have two source for poisoned page information, today. 1) LINUX_EFI_POISONED_MEMORY: Used to track memory block that got poisioned, and will be passed around during kexec. 2) PG_hwpoison on struct page: Used by the memory subsystem to avoid touching it. And I understand that this design is fine, and we want to be easy to identify what we are querying on function name. For instance, I understand you confusion in range_contains_poisoned_memory() came from: range_contains_poisoned_memory(): * What the caller reads: * "is any memory in this range hardware poisoned?" * What actually runs: * "is any bit set in an EFI table that a PREVIOUS kernel wrote, at 2 MiB granularity, for this memory region?" So, I think think this is a naming issue, and I need to think more about it. Maybe appending efiposioned (on data that is coming from EFI config table). Let me think more about it. > Regarding this patch here, I'd assume it's sufficient. > > But I do wonder why we are walking pages when we have a bitmap to walk/process > at hand? Because the counter has to agree with the page flag, and the page flag is not the bitmap. PG_hwpoison in a block is the union of every source that poisoned a frame; the inherited EFI table is one of them, and it is the coarse and partial one. You might ask why I do not just count the bits set in the bitmape and multiply by the frames a unit covers. That was my first try. It over-counts: a bit stands for a whole 2M unit, but only the frames that reach __free_pages_core() get flagged -- CMA comes back through __free_pages(), the initrd and __init memory through free_reserved_pages(), and KHO-preserved frames never get an initialised struct page at all. And the over-count cannot be undone later. If the walk itself is what bothers you, the way out is not the bitmap but counting as we flag: hwpoison_boot_page() already knows the pfn, so it can bump a per-block-id counter in a small memblock array that memblk_nr_poison_init() then just reads. Exact by construction, no walk. Happy to go that way instead if you prefer it.