From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-179.mta0.migadu.com (out-179.mta0.migadu.com [91.218.175.179]) (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 99C9F3EBF30 for ; Thu, 12 Feb 2026 15:25:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770909942; cv=none; b=Flkuq2pJ2YkmxhaYZfc7Y6BX5POlZmZKVrwYOXhc+7q9Iqekr/V/fUUadmxIJ7ynG05UCpzb0CCR9K9kDF8Dqj9vSqA2+5+1QyqZJkKZp9CNyenD6tuCiZc3i+Rl6967ZQN/49h5QC9lyPe6vwQkYnotAQmqPaTN+ZF+nMRCcSQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770909942; c=relaxed/simple; bh=Oy0pi5WxzWVna7P9ROz2iZn041BMfKSHQbRG3gA+doM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Kw9ZGZvBxRB6Ppr6nLpy/fXdMyIdaejXPeS+SenELbPZc81Tr2aFLqWYy2uk/y7k3l+rPGAZ6fl5n7WgV7B2GXfJyGRIxSj4+qCBl/CG1Ui6ea0ZYC9N59MwGd1PrysEcoQmY3aaCVKNjLoFQ6gB6b6zQbRFg3g4cqYjBVPj+vI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=hjkncimN; arc=none smtp.client-ip=91.218.175.179 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="hjkncimN" Message-ID: <93908945-e0a8-429c-b119-eff63ebb2479@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1770909937; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=3md81SdFS4TjjB0AXkEg5ItTPXkzpc4iibpel7Bvl6Q=; b=hjkncimNaPPExvn/emeaq/h/AlfCnw+mVVCMpaG7jfK1AU3G5MNnceGQoxpZLRm+7s8TrG SMgyuBDmGiF9cfQ+M2rMahOUxsaiyqzUKf7jsPpmGt1bnR/CpoXyBRoS/SAaXyBEptiUA9 w68x0WYS4Za0LU2R9RKu9ekkYbD3FhI= Date: Thu, 12 Feb 2026 15:25:33 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [RFC 1/2] mm: thp: allocate PTE page tables lazily at split time Content-Language: en-GB To: "Ritesh Harjani (IBM)" , "David Hildenbrand (Arm)" , 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> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: <875x82ma6q.ritesh.list@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 12/02/2026 12:13, Ritesh Harjani (IBM) wrote: > "David Hildenbrand (Arm)" writes: > >> CCing ppc folks >> > > Thanks David! > >> On 2/11/26 13:49, Usama Arif wrote: >>> When the kernel creates a PMD-level THP mapping for anonymous pages, >>> it pre-allocates a PTE page table and deposits it via >>> pgtable_trans_huge_deposit(). This deposited table is withdrawn during >>> PMD split or zap. The rationale was that split must not fail—if the >>> kernel decides to split a THP, it needs a PTE table to populate. >>> >>> However, every anon THP wastes 4KB (one page table page) that sits >>> unused in the deposit list for the lifetime of the mapping. On systems >>> with many THPs, this adds up to significant memory waste. The original >>> rationale is also not an issue. It is ok for split to fail, and if the >>> kernel can't find an order 0 allocation for split, there are much bigger >>> problems. On large servers where you can easily have 100s of GBs of THPs, >>> the memory usage for these tables is 200M per 100G. This memory could be >>> used for any other usecase, which include allocating the pagetables >>> required during split. >>> >>> This patch removes the pre-deposit for anonymous pages on architectures >>> where arch_needs_pgtable_deposit() returns false (every arch apart from >>> powerpc, and only when radix hash tables are not enabled) and allocates >>> the PTE table lazily—only when a split actually occurs. The split path >>> is modified to accept a caller-provided page table. >>> >>> PowerPC exception: >>> >>> It would have been great if we can completely remove the pagetable >>> deposit code and this commit would mostly have been a code cleanup patch, >>> unfortunately PowerPC has hash MMU, it stores hash slot information in >>> the deposited page table and pre-deposit is necessary. All deposit/ >>> withdraw paths are guarded by arch_needs_pgtable_deposit(), so PowerPC >>> behavior is unchanged with this patch. On a better note, >>> arch_needs_pgtable_deposit will always evaluate to false at compile time >>> on non PowerPC architectures and the pre-deposit code will not be >>> compiled in. >> >> 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(). > > Doing a grep search, I think that's the same for sparc and s390 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. > > Thanks for confirming! I will keep the pagetable deposit for powerpc in the next revision. I will rename pgtable_trans_huge_deposit to arch_pgtable_trans_huge_deposit and move it to arch/powerpc. It will an empty function for the rest of the architectures. >> >> 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). > > 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. > > >> If we have to keep this around, pgtable_trans_huge_deposit() should >> likely get renamed to arch_pgtable_trans_huge_deposit() etc, as there >> will not be generic support for it. >> > > Sure. That make sense since PowerPC Hash MMU will still need this. > > -ritesh