From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 756601E8342; Thu, 19 Mar 2026 07:37:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773905839; cv=none; b=ljzHcg23sqPki/spJ7UGbVpMH/hAJ9Byj6xw4wGccZJBPFLB7LTX03Ph7rAX1fHvGT5q4mG7whEuRGU+ZMDABPfIBqmi2GuZKThJ3UE9294JK5NCEszbddacOZPCUrFlTT1Etd85/eFTd0/daTi8A0LBKHDX3PWDKULAu7emtH4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773905839; c=relaxed/simple; bh=qRMfpUzApsAdbW6LZfIeTNJxNnO9wM97sAR9cQMuBA8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=TVXx/KMa8D3ew1JR7+iJch7hdnSUJwuvkOzyltkQizQ4uJ/aIuWWey2LOWrSpWTGO3OisTDAEm14zRzJjvxb4KEXCIZH9XMScnb0nRqCcPBgG52/zBPYBaWdpCNgNmt/ASkgvT91PYnMlRaHdSJK9rhBHrfGd3SeZqNlaJ8gUKo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YctH8PgT; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="YctH8PgT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 94B68C19424; Thu, 19 Mar 2026 07:37:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773905839; bh=qRMfpUzApsAdbW6LZfIeTNJxNnO9wM97sAR9cQMuBA8=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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