From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5D91CF53D6E for ; Mon, 16 Mar 2026 16:06:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=PIn2O1UO9o+NNd6ndwmGUY3VFuQhtoVSGRTxlJ6QDXU=; b=nplb/+FNyPoVmy5JID4L1EEmP1 gv4ycNurH4KO6L6bcdMHevCH+9JrFeYOIpSSzAMvBlA03uzvMgqQiUHv5vvoeJd2wCkqB9/Oq+i54 wZ13oKeL2TUBwg3FlYoHyviWNb6ZfTpPEAal3v2qEgt0dI7ZJfhTOf5hnSodvrC1A1Vd2pBxcTemv /zKJu1IcSluh7USNiJbDA1G+v7kKrIz62b977MiEeKvfq2ReMG98MmENco7cTwjPxuoK4fHf4FcK+ i7Qjhw4H8UVY1+thnn4aQOgBgD9OqFYU+XrMWfljQwmeyyHlwwvVzwPRcWZM/17atdUW+SnjM5rX5 MIf3KgQA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1w2ASj-00000004PkD-1UTQ; Mon, 16 Mar 2026 16:06:37 +0000 Received: from tor.source.kernel.org ([172.105.4.254]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1w2ASi-00000004Pk2-0gDq for linux-arm-kernel@lists.infradead.org; Mon, 16 Mar 2026 16:06:36 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 2F66E60018; Mon, 16 Mar 2026 16:06:35 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id DE505C19421; Mon, 16 Mar 2026 16:06:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773677194; bh=cj/5YGwp5+c5NaW2gvBI49y4YZXx3uALNGH84PsRfnQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=DeWlppA5IL2ampk8lC/lAlXdW2xozEGYLFwBl9YZIQKe4hPZ8uN3N+03S+mDPUWX0 NZfR+s/+8ijYrFJBOD93xN6vQFoodaOwoTTJT8NOO/LMuWCum/HGpLZSeQIvbWY8na tm//MejtWYxEpkak9IZVMJjAImEElE2xU8OkHOepcSDBgGC8mGOY3D3+TS0EKDTVn7 vHQkbomZs7tmq4EQt6Z5n4Hr82QnmcfGcwoBcpfXeNNHKTx3ya8+E8PkhGIL2sJ0kV X3iTy2r6NkYCRh4UtPUd+1GxoRBm6PCvHG85M7rfxDWXv1ozPyy4hGGQPEWbW5hmhz EL2ka3FW6YkPQ== Message-ID: Date: Mon, 16 Mar 2026 17:06:24 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/4] arm64/mm: contpte-sized exec folios for 16K and 64K pages To: Usama Arif , Andrew Morton , ryan.roberts@arm.com Cc: ajd@linux.ibm.com, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, brauner@kernel.org, catalin.marinas@arm.com, dev.jain@arm.com, jack@suse.cz, kees@kernel.org, kevin.brodsky@arm.com, lance.yang@linux.dev, Liam.Howlett@oracle.com, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, npache@redhat.com, rmclure@linux.ibm.com, Al Viro , will@kernel.org, willy@infradead.org, ziy@nvidia.com, hannes@cmpxchg.org, kas@kernel.org, shakeel.butt@linux.dev, kernel-team@meta.com References: <20260310145406.3073394-1-usama.arif@linux.dev> <608c87ce-10d9-4012-b6e9-298d5a356801@kernel.org> <9e9edebb-3953-4bcd-80e2-614dcec5b402@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 3/13/26 20:59, Usama Arif wrote: > > > On 13/03/2026 16:20, David Hildenbrand (Arm) wrote: >> On 3/10/26 15:51, Usama Arif wrote: >>> On arm64, the contpte hardware feature coalesces multiple contiguous PTEs >>> into a single iTLB entry, reducing iTLB pressure for large executable >>> mappings. >>> >>> exec_folio_order() was introduced [1] to request readahead at an >>> arch-preferred folio order for executable memory, enabling contpte >>> mapping on the fault path. >>> >>> However, several things prevent this from working optimally on 16K and >>> 64K page configurations: >>> >>> 1. exec_folio_order() returns ilog2(SZ_64K >> PAGE_SHIFT), which only >>> produces the optimal contpte order for 4K pages. For 16K pages it >>> returns order 2 (64K) instead of order 7 (2M), and for 64K pages it >>> returns order 0 (64K) instead of order 5 (2M). Patch 1 fixes this by >>> using ilog2(CONT_PTES) which evaluates to the optimal order for all >>> page sizes. >>> >>> 2. Even with the optimal order, the mmap_miss heuristic in >>> do_sync_mmap_readahead() silently disables exec readahead after 100 >>> page faults. The mmap_miss counter tracks whether readahead is useful >>> for mmap'd file access: >>> >>> - Incremented by 1 in do_sync_mmap_readahead() on every page cache >>> miss (page needed IO). >>> >>> - Decremented by N in filemap_map_pages() for N pages successfully >>> mapped via fault-around (pages found in cache without faulting, >>> evidence that readahead was useful). Only non-workingset pages >>> count and recently evicted and re-read pages don't count as hits. >>> >>> - Decremented by 1 in do_async_mmap_readahead() when a PG_readahead >>> marker page is found (indicates sequential consumption of readahead >>> pages). >>> >>> When mmap_miss exceeds MMAP_LOTSAMISS (100), all readahead is >>> disabled. On 64K pages, both decrement paths are inactive: >>> >>> - filemap_map_pages() is never called because fault_around_pages >>> (65536 >> PAGE_SHIFT = 1) disables should_fault_around(), which >>> requires fault_around_pages > 1. With only 1 page in the >>> fault-around window, there is nothing "around" to map. >>> >>> - do_async_mmap_readahead() never fires for exec mappings because >>> exec readahead sets async_size = 0, so no PG_readahead markers >>> are placed. >>> >>> With no decrements, mmap_miss monotonically increases past >>> MMAP_LOTSAMISS after 100 faults, disabling exec readahead >>> for the remainder of the mapping. >>> Patch 2 fixes this by moving the VM_EXEC readahead block >>> above the mmap_miss check, since exec readahead is targeted (one >>> folio at the fault location, async_size=0) not speculative prefetch. >>> >>> 3. Even with correct folio order and readahead, contpte mapping requires >>> the virtual address to be aligned to CONT_PTE_SIZE (2M on 64K pages). >>> The readahead path aligns file offsets and the buddy allocator aligns >>> physical memory, but the virtual address depends on the VMA start. >>> For PIE binaries, ASLR randomizes the load address at PAGE_SIZE (64K) >>> granularity, giving only a 1/32 chance of 2M alignment. When >>> misaligned, contpte_set_ptes() never sets the contiguous PTE bit for >>> any folio in the VMA, resulting in zero iTLB coalescing benefit. >>> >>> Patch 3 fixes this for the main binary by bumping the ELF loader's >>> alignment to PAGE_SIZE << exec_folio_order() for ET_DYN binaries. >>> >>> Patch 4 fixes this for shared libraries by adding a contpte-size >>> alignment fallback in thp_get_unmapped_area_vmflags(). The existing >>> PMD_SIZE alignment (512M on 64K pages) is too large for typical shared >>> libraries, so this smaller fallback (2M) succeeds where PMD fails. >>> >>> I created a benchmark that mmaps a large executable file and calls >>> RET-stub functions at PAGE_SIZE offsets across it. "Cold" measures >>> fault + readahead cost. "Random" first faults in all pages with a >>> sequential sweep (not measured), then measures time for calling random >>> offsets, isolating iTLB miss cost for scattered execution. >>> >>> The benchmark results on Neoverse V2 (Grace), arm64 with 64K base pages, >>> 512MB executable file on ext4, averaged over 3 runs: >>> >>> Phase | Baseline | Patched | Improvement >>> -----------|--------------|--------------|------------------ >>> Cold fault | 83.4 ms | 41.3 ms | 50% faster >>> Random | 76.0 ms | 58.3 ms | 23% faster >> >> I'm curious: is a single order really what we want? >> >> I'd instead assume that we might want to make decisions based on the >> mapping size. >> >> Assume you have a 128M mapping, wouldn't we want to use a different >> alignment than, say, for a 1M mapping, a 128K mapping or a 8k mapping? >> > > So I see 2 benefits from this. Page fault and iTLB coverage. IMHO page > faults are not that big of a deal? If the text section is hot, it wont > get flushed after faulting in. So the real benefit comes from improved > iTLB coverage. > > For a 128M mapping, 2M alignment gives 64 contpte entries. Aligning > to something larger (say 128M) wouldn't give any additional TLB > coalescing, each 2M-aligned region independently qualifies for contpte. > > Mappings smaller than 2M can't benefit from contpte regardless of > alignment, so falling back to PAGE_SIZE would be the optimal behaviour. > Adding intermediate sizes (e.g. 512K, 128K) wouldn't map to any > hardware boundary and adds complexity without TLB benefit? I might be wrong, but I think you are mixing two things here: (1) "Minimum" folio size (exec_folio_order()) (2) VMA alignment. (2) should certainly be as large as (1), but assume we can get a 2M folio on arm64 4k, why shouldn't we align it to 2M if the region is reasonably sized, and use a PMD? -- Cheers, David