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b=m39OTSXGNZNIqCEztgvYpbKdbg8exALPQw7LXuaZ+cFC4pTdZYLSKC2Zdjd81Vyer eZpFzbESNoK57ZUzT63Kbqvgt1CMyurYUx2euQpsisN/d4NC/346wAtZsoptWrU4Bx uRX4gEsirlSzXf+lc+Rns6jlQ7EWFJcNt9l7zBzO/7jjfAGfLlses1qCqZ2PnXXuYW HsohxPZh4YLf6b2zK1D0YtNlqhIZJHYa8PyNjgE2x5N+oRAcrUXbuEmqvxa4E75PVC dpN563eX06NOCoyUTkrJGWoANofJdSgpOtzf+lUCyE0JhqCyCjG6ZGZGK36cUa9/jQ Dnw4JlCRv97UQ== Message-ID: <42d1a163-3502-49b8-9a27-7df9d06eee64@kernel.org> Date: Fri, 2 Oct 2026 10:23:47 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6] mm/page_alloc: avoid direct reclaim and compaction for costly __GFP_NORETRY allocations Content-Language: en-US To: Andrew Morton , Salvatore Dipietro Cc: linux-kernel@vger.kernel.org, hch@infradead.org, abuehaze@amazon.com, alisaidi@amazon.com, blakgeof@amazon.com, brauner@kernel.org, dgc@kernel.org, dipietro.salvatore@gmail.com, djwong@kernel.org, hannes@cmpxchg.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-xfs@vger.kernel.org, mhocko@suse.com, ritesh.list@gmail.com, rvvandan@amazon.com, stable@vger.kernel.org, surenb@google.com, willy@infradead.org, ziy@nvidia.com, David Hildenbrand , Christoph Hellwig , Dmitry Ilvokhin , Brendan Jackman References: <20261001082152.2879289-1-dipiets@amazon.it> <20261001182942.ce99faa753fb9d9bd2efec1e@linux-foundation.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: eb8y46kqixho4n9whrigduznexrxno4d X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 60EDBA0005 X-HE-Tag: 1790929435-492227 X-HE-Meta: U2FsdGVkX18gNWRj+qeE+wPporAwjBNbMUklRCFhDwqHDomu0LaFzv4KEyF31Zl1vzj2QA8eLmjsErHNb9/7awZ+F/fuRZ01b6dzn2EbB2PxOPK70Aow/GWU/tzGMKRrVLud1MbXtODbGaaYRcp55iRNwN32AXYfhupgo1MVqBJM0ec3bslY+1t6S8+u/gMIiowWL9s8PcVWs16vz/Wvi/yN4rQbE37WaVw7yMWDJ7Gy+SpfjjXEMJPA/kyt53eK1p9CeRP+yqGtA1wDXzFQnJQOpcaWQ4W3bStJujRvKK5E2AJnnDW3oJ/HkG3mKJLgfRWtKtZuJAWLXOHp7QxRPjEYgeAEc8It5dfSLGBRGc1r5i2yPXjdnkk3IKtuBoTpVUZdlsgu7wQMPjGjD+gu0GNfHzesiZRCGRldz86oxkbgUXM6RXAGxA0OavQZTyfChbRJJc+08leZ4FmXZPKPU7fHnsrOlGAxnjoUqo6cMS/+OMMNGp42TEM/pscmwbAgczByuAjVjgak34fSsV56huhbS3YXtr5wgj0MK/ZjREatQonbXyrtkkL/3kXZlGzoso+b/E3g2n8f9LLVlz2PufKE1jIJT8U5YyfP9+31/8Q+8OjwWilsGvKkezy1hao6rTpx9bkDEU719JfZ/Dg9juTBiGwustlaBe92WZlFtX+7PKYLd1aojkUbgGR6RnHkO6vWEeoC9u66PLqJ0Ke7zScwjFjmKRzS1VDuWoSHTjeDPel4kMWxae8lF94yAbpqWnSgNvAPMcJKEWcPezD7DzfIP4pbED+kr6etOoD6BRKD5g8XEuSosvrafpYchCGF18aLzKhy5kC7rDdTsxINPfVOJbuG8xHi7lEYtCMx6MUk7QG7o+Nk64lszWAiP+5+0JyV0s1FA9IBwgImSwSrgZzpPWPhO7oW6En3CPjymTUQybDmGDtd95rFPQAuzA2N20Jx4T3IqAOOgXzYNhv Xtz1rVD5 IXaGJESpU2353br8JNuidTkU8qAmzUHB9TMoMyl1GY7QvoXrSCTwycLiuEDqlnEuSv5a+6/IZq+3uSQQTBqFUB0MxCw7o9gIb04ZoFWZsctCdPE+Z0HtD7PW1kjAlztYaB/i99mv+pkccMmYh2zsDiEiSw9WP71UUmewOMKzK/b2FbDVYqK5ImPxjW0zlpl9RnhUScW++Lu9WvEH0rWfWeKWy8TOcmvEC6oh/chbyqJ4CVINHKyqVcUukNTbDTbVHORbW+R27p3fBv3DPBsWy1cDv968P2nTyGjvXWqXn2V31WmxiKIQw5f0sKTimsh2h/osOrLsaNTxa0m0ifdbng/Lu+kE55zMADdfqLi9LL4r1codAaAOcaCNPkg78Tl6yss1DfflMZKcjkMuFaat4fnAaEA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/2/26 03:29, Andrew Morton wrote: > On Thu, 1 Oct 2026 08:21:51 +0000 Salvatore Dipietro wrote: > >> Commit 5d8edfb900d5 ("iomap: Copy larger chunks from userspace") >> introduced high-order folio allocations in the iomap buffered write >> path. When memory is fragmented, each failed costly-order allocation >> enters __alloc_pages_slowpath() which runs direct reclaim, direct >> compaction and drain_all_pages(), causing a 0.38x throughput drop on >> PostgreSQL pgbench (simple-update) with 1024 clients on a 96-vCPU arm64 >> system. >> >> The root issue is that direct reclaim and direct compaction are too >> expensive for hot allocation paths that have fallbacks to smaller >> allocations. __filemap_get_folio_mpol() already marks higher-order >> allocations with __GFP_NORETRY | __GFP_NOWARN, signalling that the >> caller can handle failure. However, the page allocator still enters the >> full blocking slowpath for costly orders with __GFP_NORETRY, which is >> unnecessarily aggressive when the caller will simply retry at a lower >> order. >> Since it expands the impact of a pre-existing bug of nonblocking allocation eating reserves [1] it shouldn't proceed to mainline without a fix for that (Johannes posted it there but the review discussion got stalled). > Thanks, I'll update mm-hotfixes to this version. I've also argued for not handling it as a hotfix [2] [1] https://lore.kernel.org/all/9f415dc7-adad-4161-b20d-7c3173f50ff3@kernel.org/ [2] https://lore.kernel.org/all/c6e2c1e1-a285-406e-a2b6-c09e05df91d8@kernel.org/ > Here's what changed: > > --- a/mm/page_alloc.c~mm-page_alloc-avoid-direct-reclaim-and-compaction-for-costly-__gfp_noretry-allocations > +++ a/mm/page_alloc.c > @@ -4930,8 +4930,11 @@ retry: > * Reclaim/compaction cannot run, so defrag_mode's strategy > * of enforcing ALLOC_NOFRAGMENT cannot be fulfilled. Allow > * fallbacks rather than failing the allocation outright. > + * Not for __GFP_NORETRY: those have a cheap lower order > + * fallback, so failing beats fragmenting. > */ > if (defrag_mode && (alloc_flags & ALLOC_NOFRAGMENT) && > + !(gfp_mask & __GFP_NORETRY) && > (gfp_mask & __GFP_KSWAPD_RECLAIM)) { > alloc_flags &= ~ALLOC_NOFRAGMENT; > goto retry; > _ >