From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 816AB46DFFF; Tue, 21 Jul 2026 16:23:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784651029; cv=none; b=QfnAmEsvAGw/Nciwe1fkCMMXRysTlVOGoPgbPTU5VmrOuApt9kmJ2x/+7cXpUTIsixUuAnjtF2nSkehs+JwpKQpsVlMX4N08URYznZ7zeWdYROJeuYpmB6OY5Jg15VxrOfAtHnfyr9RlYsT+m49KDalfKExadAdTwOt2px0fQOw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784651029; c=relaxed/simple; bh=tjJtgiuDSo149zRX4v7F6PKpZ2RqGzP7eQmqNffrFgw=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=IreLI87wwHHyY67ZxMLnUV2yFBMpXob5OMZCqQjICC7rawlsMTOE8igcFu03F8WIMCCp1UvjWNnW3FM1lUpSUmPehz3HOyXT9SDFKId85s3i9nabaN0Rh5D5wMzXuCTLNY54UgeSgtcTIFkZWe8DxDs7OLYiurFWplxdbeMd0F4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mKMTfTfZ; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mKMTfTfZ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D7C571F00A3A; Tue, 21 Jul 2026 16:23:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784651028; bh=XB4al3rOoBg4JSOO19Uh9eYt2qBCEAxlPfiPknctj/I=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=mKMTfTfZUJdKGwWB88q3MmHstD1zIbNqfuQyZ+BRaAhhajf7LR9JlLvHXO02AACeZ FTlA0E0QLniF/9UduU1JcK1Zt/0PJvKV+ojBQKZCr+3D8Ut/wKtGHCRw5BkHT5EPSj GQD7zZIHCIv9D7iMSQPw7l76zQxEOd5wMZ+hwVjw9NzgIyfjEJrr0pK3P8do0X2/YS z3gNy8oRMf0+fn8Cqp1vwF8N5ezh4wRzEbz8gk/+aeM2Qdnu7V800g9/NYGbN1OOIj Rwp6d5daE20K+Y5iJgr+4zLFeNl5BXFmfKqOxS7DGq5Y/CxU4aIDzlJE2qmeCE88N7 pQhTF9DI3MSNA== From: "Mike Rapoport (Microsoft)" Date: Tue, 21 Jul 2026 19:23:24 +0300 Subject: [PATCH RFC 01/18] x86/mm/pat: Don't gate cpa_lock on debug_pagealloc_enabled() Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260721-generic-set-memory-v0-1-v1-1-2c1fc62306b3@kernel.org> References: <20260721-generic-set-memory-v0-1-v1-0-2c1fc62306b3@kernel.org> In-Reply-To: <20260721-generic-set-memory-v0-1-v1-0-2c1fc62306b3@kernel.org> To: linux-mm@kvack.org Cc: Albert Ou , Andrew Morton , Andy Lutomirski , Borislav Petkov , Brendan Jackman , Catalin Marinas , Dave Hansen , David Hildenbrand , Huacai Chen , Ingo Molnar , Ingo Molnar , Kevin Brodsky , "Liam R. Howlett" , Lorenzo Stoakes , Michal Hocko , Mike Rapoport , Palmer Dabbelt , Paul Walmsley , Peter Zijlstra , "H. Peter Anvin" , Suren Baghdasaryan , Thomas Gleixner , Vlastimil Babka , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, loongarch@lists.linux.dev, x86@kernel.org, Dave Hansen X-Mailer: b4 0.16-dev The splitting and merging of kernel page table mappings between small and large is protected by cpa_lock. The merging is relatively new but the splitting is ancient. The splitting has a locking optimization: since DEBUG_PAGEALLOC forces all mappings to 4k, there are no large pages to split. So the code that *might* cause a split can just skip the locking (and a few other things). This is entertaining, but it adds complexity and makes for weird locking rules. Plus it's all for a debugging feature which makes the kernel super slow in the first place. Optimizing something which is already super slow and not used in production is not the best way to spend our complexity budget. Stop gating cpa_lock on debug_pagealloc_enabled() to simplify the code and the locking rules. [ dhansen: flesh out changelog ] Suggested-by: Dave Hansen Signed-off-by: Mike Rapoport (Microsoft) Signed-off-by: Dave Hansen Link: https://patch.msgid.link/20260715144519.934289-1-rppt@kernel.org Link: https://lore.kernel.org/all/aab44f08-89f8-47fe-bee4-0ab6b25968c6@intel.com/ --- arch/x86/mm/pat/set_memory.c | 19 +++++++------------ 1 file changed, 7 insertions(+), 12 deletions(-) diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c index d023a40a1e03..e8316f5ffa8a 100644 --- a/arch/x86/mm/pat/set_memory.c +++ b/arch/x86/mm/pat/set_memory.c @@ -62,10 +62,9 @@ enum cpa_warn { static const int cpa_warn_level = CPA_PROTECT; /* - * Serialize cpa() (for !DEBUG_PAGEALLOC which uses large identity mappings) - * using cpa_lock. So that we don't allow any other cpu, with stale large tlb - * entries change the page attribute in parallel to some other cpu - * splitting a large page entry along with changing the attribute. + * Serialize cpa() using cpa_lock so that we don't allow any other cpu, with + * stale large tlb entries, to change the page attribute in parallel to some + * other cpu splitting a large page entry along with changing the attribute. */ static DEFINE_SPINLOCK(cpa_lock); @@ -1235,11 +1234,9 @@ static int split_large_page(struct cpa_data *cpa, pte_t *kpte, { struct ptdesc *ptdesc; - if (!debug_pagealloc_enabled()) - spin_unlock(&cpa_lock); + spin_unlock(&cpa_lock); ptdesc = pagetable_alloc(GFP_KERNEL, 0); - if (!debug_pagealloc_enabled()) - spin_lock(&cpa_lock); + spin_lock(&cpa_lock); if (!ptdesc) return -ENOMEM; @@ -2023,11 +2020,9 @@ static int __change_page_attr_set_clr(struct cpa_data *cpa, int primary) if (cpa->flags & (CPA_ARRAY | CPA_PAGES_ARRAY)) cpa->numpages = 1; - if (!debug_pagealloc_enabled()) - spin_lock(&cpa_lock); + spin_lock(&cpa_lock); ret = __change_page_attr(cpa, primary); - if (!debug_pagealloc_enabled()) - spin_unlock(&cpa_lock); + spin_unlock(&cpa_lock); if (ret) goto out; -- 2.53.0