From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 F226A33D4F0; Fri, 24 Apr 2026 18:28:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777055304; cv=none; b=lQboMblvYjtIrJsk0yhlqKmENu4/zIKFHUDQl6YA075X5e+mrOdL7w3hTa1nx72Z3ZvMLk+zDsS+pRgfFY84DSoOoln6PbqjBI2W5o/eFscBy6YEfw4l41dwz46rEDGBb21ddQSm1jsSMtLqcIpSLagWqrTpsbiLatTXtdM1cLU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777055304; c=relaxed/simple; bh=nsCK8lA2BzQi4jFTXuDqLHYn5FbisryKK3hHm2rfaCQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WYlRlDYtz9QcfJ4T0ilrStLvhNPSjpG0ZAOGxWfbpONzL1zaE2Z2IdVJMNKpXx3tfV+dpEOZ/qD6YzNeg2IgdB6ioYbAHyD93O8yxjaQ4cDyuoRMK1qSGokuERW8STQOaUWD9zNjnaAo1MFhau0woHsbvoGxQc8UpmUPtmVd9Wg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=n1BTff07; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="n1BTff07" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F30E6C2BCB0; Fri, 24 Apr 2026 18:28:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777055303; bh=nsCK8lA2BzQi4jFTXuDqLHYn5FbisryKK3hHm2rfaCQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=n1BTff07OGkd3Ad8zf+5Bg+/axUDSkNbg4CLYAhI1ceEUwVlSFzuGBu8va3AnWbfm R9MDSh1MWXNVKGAfKQZhvO7Xh601sjrX7CiCWftSrNhbp76H9xrctIXgNkXfdqc+5J p9vNHjPyrVzsjO/5J6nMqkebRtSgewFkQKOUJJTzCVOhEfTkBEU/hSNYD4gsQbxsb5 OGZjVoSabsi/F7bgQcZ1cFU1UoghdExt9DoSMe7U1D+VwL4tC2GqdEzpeQLHFmPH4u yzPQh1OYyts/vthGnzJmF2OYhJ8dkwv8ep3mF9KXPaIz2SI/wc7zRj7t1iMUbQXcoj hyT5th/hP8pYA== Message-ID: <12985b32-88b3-47ab-8292-2e0ec6f5fbae@kernel.org> Date: Fri, 24 Apr 2026 20:28:14 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 4/7] mm: add page consistency checker implementation To: Pasha Tatashin , Sasha Levin Cc: akpm@linux-foundation.org, corbet@lwn.net, ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, skhan@linuxfoundation.org, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Sasha Levin , Sanif Veeras , "Claude:claude-opus-4-7" References: <20260424140056.2094777-1-sashal@kernel.org> <20260424140056.2094777-5-sashal@kernel.org> <4b961a07-b72d-4c8a-ab49-23f61ed12b53@kernel.org> 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 4/24/26 17:06, Pasha Tatashin wrote: > On 04-24 10:49, Sasha Levin wrote: >> On Fri, Apr 24, 2026 at 04:25:41PM +0200, David Hildenbrand (Arm) wrote: >>> >>> One bitmap that covers all sparse memory available at boot. >>> >>> Conclusion: Just horrible. >> >> Depends on who's looking at the code :) >> >> I picked it for auditability: covering the whole range with two >> memblock_alloc'd arrays means the only thing on the lookup path is the bitmap >> words themselves, which is what the dual-bitmap invariant already checks. > > The issue is that we are going back in time to a flat memory, > without NUMA or hotplug support. We need an abstraction that avoids > allocating this memory in enormous contiguous chunks, as thit approach > will not work on modern hardware. > >> >> We could go with per-section bitmaps which will fix the waste but pull >> mem_section[] into the trust boundary, so we'd have to start validating it too. > > Page-ext provides all of these capabilities, but as you described in the > cover letter, it does not meet your requirements. Therefore, I believe > a new abstraction layer is needed. If we decided that we want this (and I am not convinced), we definitely want something that supports sparsity and, in particular, something that support memory hotplug. -- Cheers, David