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b=YctH8PgTgpS6wXB6v3K+szH6x9HccNE1bv+uHpSJyoU2sXQx0hQ5OgOGBSa3Pf2nm ong8+NvmImY/tAM2kRCyTGGfP9xIFQ067cR07TTgslPzQwhz7cJN0TJID/rq/C+ddG pvw3n6bp5QOvVL8V4ekLUrzUn8wkUAA6KhRWEXocepB2YmxqaJ+qYp0zbeJI2DeiF3 PZLcxwNnvYV77PxZfKY6EYlzJkDxqR9R/s5AUGL3uDtdioKcfy+GUhmwc6VGBURkLE R9Y3RGplEaLwOKHy3HboLnM6CoKDSQPIJxrjC7r/eLTZSrcEmmZuyYPSzYlPpb4GeC OcTqoGnrs6mYA== Message-ID: <6625614e-457d-4e67-821a-8bbc10a22c0d@kernel.org> Date: Thu, 19 Mar 2026 08:37:11 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/4] mm: bypass mmap_miss heuristic for VM_EXEC readahead To: Jan Kara , Usama Arif Cc: Andrew Morton , ryan.roberts@arm.com, 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, 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> <20260310145406.3073394-3-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-Server: rspam04 X-Rspamd-Queue-Id: 4CCC440006 X-Stat-Signature: yatmj3fawh5pi38xw7ikg7um36pdcd3i X-Rspam-User: X-HE-Tag: 1773905839-270793 X-HE-Meta: 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 3XAzOupE UAxkS7ewYbb3f9TBWxXcJCh51lBJiyBbACv7ikLM0FGGKD72WDlIqxO+sq/WcAJ33sGEbo9x9zIoLO9OE/OC+zML1s0yKp3qaFCJn8gDjvGsi1wRngQmCvXb99yqVbSqIwzwnQMj+NFVkHLwWhf8K5x3oX2VlFAmHGAytQYm3A+tOl64sbeeuKX+6WO3rrP04U6Bi7HeOepEh8BKN9OkO2qN9epHpb4LY9Sde1n/ixZTN8ffnt2daIlwYD3p768Mt6b6lasu7Qk7979Akj/4qEKRNFePJXEIk+udRAashq2xSFUzt0LHkeU5qEFMwyxcjvwQVhosIs4/UgP1wXC2bL1TVW4yRrqA8++tDzVq4SMwWj/g02feru2xYBPXryuEaqMX5jqFqwhN5rWs= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/18/26 17:43, Jan Kara wrote: > On Tue 10-03-26 07:51:15, Usama Arif wrote: >> The mmap_miss counter in do_sync_mmap_readahead() tracks whether >> readahead is useful for mmap'd file access. It is incremented by 1 on >> every page cache miss in do_sync_mmap_readahead(), and decremented in >> two places: >> >> - filemap_map_pages(): decremented by N for each of N pages >> successfully mapped via fault-around (pages found already in cache, >> evidence readahead was useful). Only pages not in the workingset >> count as hits. >> >> - do_async_mmap_readahead(): decremented by 1 when a page with >> PG_readahead is found in cache. >> >> When the counter exceeds MMAP_LOTSAMISS (100), all readahead is >> disabled, including the targeted VM_EXEC readahead [1] that requests >> arch-preferred folio orders for contpte mapping. >> >> On arm64 with 64K base pages, both decrement paths are inactive: >> >> 1. 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. >> >> 2. 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 page faults, disabling all subsequent >> exec readahead. >> >> Fix this by moving the VM_EXEC readahead block above the mmap_miss >> check. The exec readahead path is targeted. It reads a single folio at >> the fault location with async_size=0, not speculative prefetch, so the >> mmap_miss heuristic designed to throttle wasteful speculative readahead >> should not gate it. The page would need to be faulted in regardless, >> the only question is at what order. >> >> [1] https://lore.kernel.org/all/20250430145920.3748738-6-ryan.roberts@arm.com/ >> >> Signed-off-by: Usama Arif > > I can see the problem but I'm not sure what you propose is the right fix. > If you move the VM_EXEC logic earlier, you'll effectively disable > VM_HUGEPAGE handling for VM_EXEC vmas which I don't think we want. So > shouldn't we rather disable mmap_miss logic for VM_EXEC vmas like: > > if (!(vm_flags & (VM_SEQ_READ | VM_EXEC))) { > ... > } > That sounds reasonable to me. -- Cheers, David