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b=cg4pvRYN/PneO0ZuQ2YEt6S07AB1iVEyGwdOwjUG12NppCtClxN/K5TI5eU/g/XlU wl3U14eFnstmcFS+P30wJUNka+2y7ntRS/aolCXbTmV2hqgiCQm6kzA3FtGjAM+Afm C3e9fBA8VHZGjSJ5LTURTZC297efhP7BqeBEASOElZW1ngkY4kM6AXSXxXbuDNBv8E ekS/QIcNgQKCxuCz9xQ/b9l8gZ7LiIO9Rh+NK9M1PzxI07tDuHG3/Z2PYCXFvutful c3xTzrNUIjID/q/w9Gm6RfxYPTgI3Q/4zui7oVQ2HcCPFFztXblatEv6W5AqjWsCw9 TTV1uyJV8nrLQ== Message-ID: Date: Mon, 23 Feb 2026 14:01:53 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] Per-process page size To: Pedro Falcato , Dev Jain Cc: lsf-pc@lists.linux-foundation.org, ryan.roberts@arm.com, catalin.marinas@arm.com, will@kernel.org, ardb@kernel.org, willy@infradead.org, hughd@google.com, baolin.wang@linux.alibaba.com, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20260217145026.3880286-1-dev.jain@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Stat-Signature: qt3a7xwsdf43w5g6pyxwxmdri9rwffqh X-Rspamd-Queue-Id: 9349680009 X-Rspam-User: X-HE-Tag: 1771851723-484385 X-HE-Meta: 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 5BUIsHaw nK7+RqHNIyOfEfPfyJ4RmSBGdJTszPiXnjuKz9jzao1vaLm6hBd9/rUvt9frKB9gsr8eza9cp64VrbMoPrKGwaQhTYe6J4EFrwdCW/jP358r1vSUsDS7Owsam+nxezRtkK5f1WsRG7fT3ZcMWcx+MSWYRf2qukWJtpIZisibd+Rzdgvy2wWnU3vaeSfXxipzNyJ8SmIh+BIVhkqZzXYap0/HQf5D10eLi0za/IQ5XtCVT4awFOkulC7HTkHQiJl4CzqHh X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/23/26 13:49, Pedro Falcato wrote: > On Mon, Feb 23, 2026 at 10:37:55AM +0530, Dev Jain wrote: >>> I don't understand. What exactly are you trying to do here? Maintain 2 >>> different paging structures, one for core mm and the other for the arch? As >>> done in architectures with no radix tree paging structures? >> >> The mm->pgd will be the software pagetable. So suppose that do_anonymous_page is >> doing set_ptes on the PTE table belonging to the software pagetable. We will >> hook a "native_set_ptes" into set_ptes, which will set the ptes on a different >> pagetable maintained by arm64 code (probably mm_context_t->native_pgd). > > Traditionally, you do this kind of funky manipulation in update_mmu_cache. > > But this is still an extremely complex and invasive change (that I assume most > people would not like to see) with dubious benefit. > >> >>> >>> If so, that's wildly inefficient, unless you're willing to go into reclaimable >>> page tables on the arm64 side. And that brings extra problems and extra fun :) >> >> I didn't understand the reclaimable reference, but yes we need to make this efficient. > > I'm not talking about CPU runtime efficiency, but memory efficiency. Doing > this makes you essentially duplicate page tables - not exactly ideal. This is > a Known Problem in classic UNIX systems which do something similar > (but not the same): anonymous memory pointers are stored in some intermediary > structure (SunOS and UVM call it "amap"), and paging structures are entirely > redundant there. They can freely tear down a page table because they can freely > put it together from the amap and file mappings (what they call vm_object and > we call address_space). > > Anyway, I'm boring you with these funny historical details so you can understand > the similarities: the Linux page table format generally matches hardware, and > we store anonymous memory "state" there, so you can't ever tear-down a pgtable > without losing state of whatever was mapped there before. However, if you go > down the "arm64 now has a separate pgtable structure", the roles switch: > arm64's internal page table format makes for the real page tables, and linux's > pgtable structure is nothing more than an "amap". So you could (and perhaps > should) freely reclaim arm64 MMU page tables once memory pressure hits, because > they are freely discardable. > > Does this make sense? I've been thinking about building the 64k page tables similar to how HMM/KVM handles it, invalidating them through mmu notifiers etc and building them on demand. Considering the 64k MMU of a process just like a special device that builds its own page tables. This way, they could get reclaimed more easily and most of the core + arm64 page able manipulation code could be kept as is. However, I don't know how much the performance impact of that approach would be. -- Cheers, David