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b=BfcDfTu5kMhfOFwtq2aussdMKxyjF4VYbIYn9mu6LXi4xciiTyoj8dkh0g83w9bDm b4fM2hILOrIZLAnBRF4eYfWBrp6ZVLRGHbbZ9j2xjrwQVaK6hPqpLKzGbeAkpEe+xw nfXM4TBg1aI91r+OT2cwUdWoaJvMRTOnHutn8xwMMtgB0CSReXlf5HCaOUCfD7/PSF Zg5fA7pWT0ekUlTLAQ5zW3uP2+nM9Ikn1iixVJoatyfDUAXfKhhhsN5MDk1NpX/VEG wU6cXDvcEumt6f92G3Zt7PJMo7rIwEeWAG9Z+QbijFCAAgQnzHOnRheZl8QcxhGq2W /vEGRSEgZS2ug== Message-ID: <608c87ce-10d9-4012-b6e9-298d5a356801@kernel.org> Date: Fri, 13 Mar 2026 14:20:26 +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> 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-Rspamd-Queue-Id: B91A540016 X-Stat-Signature: x3qmpnejd8t6qx5h818dmfjjwy4zz9bf X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1773408039-645930 X-HE-Meta: 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 uBq5Fl2X nX302nV9BOPV+cf6nPaB63bNGAGR65zxQD2c5wshqxCTFoWqvtHrfjonHZPA5xG1gpotU9D1QFecZ84gFYaLvvdDfuzSvVoL0ObrDkWfBYJqqv+OlNxcA6CBIdalZd87rn6jWmQ0lBJuYMxO6AH0oVl3bbKkQxjYEnnT2+/fpAkrv8gkExqCyQpE65Nzvi308t9x4txu63AKFzi4U1szK6ezWcvKVgsnmfTWX/Xt1nNTf5mpQCXrFzOFH42kyEp9TySlN Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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? -- Cheers, David