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b=M3/XZUzitaumcj1/KnoWZmvdgSRdUerVUHWtOpQ7yjJ9shktx2IW88h69FXGVVjaY 8k4nIov1fxmbgMmfpel6aLZC0xLOjiRntKLQkMVPCcyhc+jMy1LDRY2abpSVJuueFB 7qZI65LexUAeY4fXm6dD0mntx9qy+3+10s8di10Y+JvfkOzGyEnZwqEtKDqrF2mmbt IH2tPqS4izlT3qB33WP8/2V5B+eheGO37bHrEMgDnZQOW2KwDM7NmdB0cimK6rFlb8 sjtVMBMje1aPmGYR0ou1y07Sf9da7o8kSvc6C0/AqShXSg0cIoGSs1iPZUDN0knrns KAf1FIO5ylFcw== Message-ID: <2aedfd17-17e6-4dfe-8ae5-c7342ead708b@kernel.org> Date: Tue, 26 May 2026 15:13:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/page_alloc: fix defrag_mode for non-reclaimable allocations To: Dmitry Ilvokhin , Andrew Morton Cc: Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260520122228.201550-1-d@ilvokhin.com> <20260521165910.e7dea6a4e591d66293d2bd47@linux-foundation.org> From: "Vlastimil Babka (SUSE)" Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 5FDD51A0004 X-Stat-Signature: r4hstykc3he987wqntkpojg34c6yy6b5 X-HE-Tag: 1779801194-61189 X-HE-Meta: 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 unUaiFoE opdxEQc9WaFA28w18PfKVoAiIhzSYzGN12l1rlnCZYHYbnBGpvYkO0rVJiJB1De0BfHN+y5H2YNLO0+jjnuZfvAwGqBnIH86Cg9kqYIhjSQOU+M01rmBr/wjwl1YJJIoMCjjvPTfYKDwwRfGkEIlOIXceOg5D+SEfxBf6r0020mOaH1cPuSUp8XVuga2ulwp/Trd4gE3r3LwF1E3Okhcr6DybJA9OdUvEPJ04y6G+cqC5bePFwA6UAqWCfOov+k0xhsc3AtB4PSXj3zL9IJrkh1dC87b3xwtCntroOTgK/oQv6t9gvpNGdZOFQfEZ2TizEbq/azGvJjWlsfKirdAN4bIMWMmpCg5p2dAts0Yttb3u3tw= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/22/26 3:05 PM, Dmitry Ilvokhin wrote: > On Thu, May 21, 2026 at 04:59:10PM -0700, Andrew Morton wrote: >> On Wed, 20 May 2026 12:22:28 +0000 Dmitry Ilvokhin wrote: >> >>> When defrag_mode is enabled, ALLOC_NOFRAGMENT is enforced to prevent >>> migratetype fallbacks and keep pageblocks clean. The allocator relies on >>> reclaim and compaction to free pages of the correct type before allowing >>> fallback as a last resort. >>> >>> However, non-reclaimable allocations such as GFP_ATOMIC cannot invoke >>> direct reclaim or compaction. With defrag_mode=1, these allocations hit >>> the !can_direct_reclaim bailout in __alloc_pages_slowpath() with >>> ALLOC_NOFRAGMENT still set, and fail without ever attempting a fallback. >>> >>> This causes a large number of SLUB allocation failures for >>> skbuff_head_cache under network-heavy workloads, despite free memory >>> being available in other migratetype freelists. >> >> That sounds painful. >> >>> Clear ALLOC_NOFRAGMENT and retry for allocations that request kswapd >>> reclaim but cannot do direct reclaim themselves (GFP_ATOMIC). Purely >>> speculative allocations like GFP_TRANSHUGE_LIGHT that don't set >>> __GFP_KSWAPD_RECLAIM are left to fail, since they have reasonable >>> fallbacks and should not cause fragmentation. >> >> How serious is this to our users when running real-world workloads? > > We observed it on a few of the Meta workloads that adopted > defrag_mode=1. Do you (or Johannes) have some observations to share about what motivated those to adopt it, what kind of workloads benefit and how? Because I have no idea who uses this mode and what are the expectations. Thanks, Vlastimil