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b=dBGzJzgzSO+9iMjTj2UNcR0sC5r8JHSYQ3uy2B6LUzNIgTfbrP8Agr6brEbN4jaw/ HhJ8JDU2JxGY8BLCuqt/vK04qyCcfpdhULqy6yG5QGdt46x8yPnvn3zRNQQazplva5 7PB6l6YMhVa1KkIWwnfuDhz9rIPfHH2xdmifCv8i6E4xXJiMAfyDkmZM/gQxXekbT4 wgFsMwWHmOCv0C/6By+Ou1mZyYTSgUp7hF617TVZTysJv8Wkq82MBKcVW4uURngvpA 3VLAwJ1kpslP/ML4/IlrdIvHlCu98UbNzd8AT2+wblgqmgQnj7GreZfIDAmHi5X1vY CblqO/OqoNwHw== Message-ID: <22d65938-4569-4e5e-8f30-bfe9b38b6493@kernel.org> Date: Thu, 12 Feb 2026 16:39:41 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 1/2] mm: thp: allocate PTE page tables lazily at split time To: "Ritesh Harjani (IBM)" , Usama Arif , Andrew Morton , lorenzo.stoakes@oracle.com, willy@infradead.org, linux-mm@kvack.org Cc: fvdl@google.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, npache@redhat.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, vbabka@suse.cz, lance.yang@linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com, Madhavan Srinivasan , Michael Ellerman , linuxppc-dev@lists.ozlabs.org References: <20260211125507.4175026-1-usama.arif@linux.dev> <20260211125507.4175026-2-usama.arif@linux.dev> <13ab56cb-7fdb-4ee4-9170-f9f4fa4b6e37@kernel.org> <875x82ma6q.ritesh.list@gmail.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-Rspamd-Queue-Id: 5FF80160012 X-Stat-Signature: dz3u4h7rhq7xz7ek96dotm91im1qfqnu X-Rspam-User: X-HE-Tag: 1770910791-676408 X-HE-Meta: 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 Dil8CM0V 8I12dNEE95xkt/EFYD4IyA/+Va9vNKpcHUVsw9muNENvvzGjFA6tF70d0JcHsVIlG1O3S7PQ8TlSrAfTa5xLqQHZhtC/52PkNBrmaU9rSOGH+1fOp/6h90cDrxGEP5HzjKxe8r3TN7BBc5EF8gI8SvxkDvK197TyojWqtoj+HyZpP3xVXiFMERf2mYY4bKbzyYauCZS4HUNa0yEolozk8tza8uD6LQfVTeAEhGjPoYFCDT40jWvJRTYuFAXUsxZSQjbZAuPMsp+FsWhStT/z1963HZzO59ZCMAWIrd/m2iZGyRx2Xad95E2BmbnqdGyPqZFOrS5EbjSw46Ho= 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: >> >> Is there a way to remove this? It's always been a confusing hack, now >> it's unpleasant to have around :) >> > > Hash MMU on PowerPC works fundamentally different than other MMUs > (unlike Radix MMU on PowerPC). So yes, it requires few tricks to fit > into the Linux's multi-level SW page table model. ;) :) > >> In particular, seeing that radix__pgtable_trans_huge_deposit() just 1:1 >> copied generic pgtable_trans_huge_deposit() hurts my belly. >> > > On PowerPC, pgtable_t can be a pte fragment. > > typedef pte_t *pgtable_t; > > That means a single page can be shared among other PTE page tables. So, we > cannot use page->lru which the generic implementation uses. I guess due > to this, there is a slight change in implementation of > radix__pgtable_trans_huge_deposit(). Ah, did not spot this difference, but makes sense. Still ugly, but make sense. Fortunately it would go away with this RFC. > > Doing a grep search, I think that's the same for sparc and s390 as well. ... and I also did not realize that s390x+sparc have separate implementations we can now get rid of as well. > >> >> IIUC, hash is mostly used on legacy power systems, radix on newer ones. >> >> So one obvious solution: remove PMD THP support for hash MMUs along with >> all this hacky deposit code. >> > > Unfortunately, please no. There are real customers using Hash MMU on > Power9 and even on older generations and this would mean breaking Hash > PMD THP support for them. > I was expecting this answer :) > >> >> the "vma_is_anonymous(vma) && !arch_needs_pgtable_deposit()" and similar >> checks need to be wrapped in a reasonable helper and likely this all >> needs to get cleaned up further. >> >> The implementation if the generic pgtable_trans_huge_deposit and the >> radix handlers etc must be removed. If any code would trigger them it >> would be a bug. >> > > Sure, I think after this patch series, the radix__pgtable_trans_huge_deposit() > will mostly be a dead code anyways. I will spend some time going > through this series and will also give it a test on powerpc HW (with > both Hash and Radix MMU). Thanks! The series will grow quite a bit I think, so retesting new revisions will be very appreciated! > > I guess, we should also look at removing pgtable_trans_huge_deposit() and > pgtable_trans_huge_withdraw() implementations from s390 and sparc, since > those too will be dead code after this. Exactly. -- Cheers, David