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Thu, 12 Feb 2026 15:39:50 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2372FC4CEF7; Thu, 12 Feb 2026 15:39:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770910790; bh=uDJdvdtdIL8nTfRDZsIpuEy7of+XLqGVIjb1L7/IFKw=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; 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 X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list 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 >> >> 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