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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777073679; bh=1jfIqbSPLq0qNuZBiP9OXbiRWtkR9TZEqZHMCmgXsmY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=QXAoIme3BN9MOWbMmkOHbwVnKRUFLR6VA0Nnquc++LvdF7S1NRFA0R/KtjF3T31fR svONuj93lYFCzC7uf/K9bL9As8LSUJ4ooDs+dLm5ykPwNbmO+aPeKS9XN1T4BihfV6 8qooFBLCoBDVpWEUjy1/yEYa/rgElMvoQbOEh5n9QxsAd0dbhIEJ/RPZs0rHYOM79T +t4nxZTZVaUG/WWE7s8E7hagyKZ0zd6Ro4nFwG9qimjeLy6VAWynfbBP4efTy1/ZUm WR+4oOxM1gjDxhzL6WLX534MH1AegOHN6c3AcOa811gzfzQ3ai+99KCFjCfItg8EOU evbLewer0CmVA== Date: Fri, 24 Apr 2026 19:34:37 -0400 From: Sasha Levin To: "David Hildenbrand (Arm)" Cc: Pasha Tatashin , 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" Subject: Re: [RFC 4/7] mm: add page consistency checker implementation Message-ID: References: <20260424140056.2094777-1-sashal@kernel.org> <20260424140056.2094777-5-sashal@kernel.org> <4b961a07-b72d-4c8a-ab49-23f61ed12b53@kernel.org> <12985b32-88b3-47ab-8292-2e0ec6f5fbae@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <12985b32-88b3-47ab-8292-2e0ec6f5fbae@kernel.org> X-Stat-Signature: e78yzita7ojdiamr6mpyb5g7arxbna4q X-Rspam-User: X-Rspamd-Queue-Id: 7B0D7120003 X-Rspamd-Server: rspam07 X-HE-Tag: 1777073680-76032 X-HE-Meta: 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 OjF2ZtFz S6YCHO0f4oKmiIXGSuvH2+CDsB9MRqT7GxbHCLuqY8lp3+oQf46utiscD7MK8jsZuDrec2/2n5p6MHiIPHkmpihsgLCBBx3UC4GjMjL0jOKDjfKT3AnUTTxU7+GejwYiwmBiVFEBIh9OPURkQFlQUav1Dcrfj8gQJyZDA4xLj7i12aet6ajl10GIAsqgHv4wdPiI+YaZVFINtLy0YQd3q+UDyXtbPSmcFSOAbpr3+WWZG1ChSHE8EyiyFnBu44NWkY+eP Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Apr 24, 2026 at 08:28:14PM +0200, David Hildenbrand (Arm) wrote: >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. Makes sense. Let me take a few days and see if I can find some middle ground here. -- Thanks, Sasha