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Howlett" , Mike Rapoport , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 4/4] mm: page_alloc: fix non-movable reclaim storm in defrag_mode Message-ID: References: <20260626182215.1107966-1-hannes@cmpxchg.org> <20260626182215.1107966-5-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Stat-Signature: 8mgrrsmxp39oaqwh4s6de8hp6g79h7ab X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 9A050C0009 X-HE-Tag: 1782499412-659519 X-HE-Meta: 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 ZLEELkyv x3oML1pbmmiJNJfQyPpH9YdNVgFsKCCQ6d7PSRfQ199VexR02sdRMvJXNzRVLkTpvFNEbC6CW2aWIlqg0z0ypbnE2shn8TSuA6R2yTBos5cHh9t3VLrOIsBM3iaHxWy4VR8MjCx7e/0MnM9l1+Jb1o8D6Z8wupJ8yAJkpJhVlnn9CpIWa68hGpullWMZxDRr/TUcs0rcjgcogojeFjMGSDbJKAoCwR2QMmO/bSbEKaOefl7TebWY1pF6QUhk/jUSjudnsXfefcQzam1BYlvVNf2G8gQl9J9neELmMCCRqQTbMmUc34k+94sSnLXVc42psuvr49ZNhZAd7sLYrRMLy1w3jxEcMyhHqN2wwYnJLG3OKlt/a3B5+uPib6CDFVcg3xcapYHqSFATtM9oUY3+v3bgDxu/ao+Jvpdfoc4eFzTc03BnPqyw5IVhH+frONZTJrmj1JX35Caoj/cs= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Jun 26, 2026 at 02:29:24PM -0400, Zi Yan wrote: > On Fri Jun 26, 2026 at 2:21 PM EDT, Johannes Weiner wrote: > > As we deployed defrag_mode into Meta production, pressure spikes and > > excessive swapping were observed on some workloads. Tracing confirmed > > that this is unmovable/reclaimable requests spinning in the allocator > > and direct reclaim, causing excessive amounts of swap. > > > > The initial plan for defrag_mode was to rely on kswapd/kcompactd to > > produce blocks, and if those are overwhelmed under high pressure, let > > the allocator fall back (__rmqueue_steal()) after its retry loops. > > However, that retrying results in more reclaim on some of these > > workloads than we'd hoped, sometimes excessively so, spurred on by the > > !costly order conditions in should_reclaim_retry(). > > > > The storms are dependent on the request type. Reclaim will inevitably > > make room in existing movable blocks, since that's where the LRU pages > > live. So if movable requests retry on reclaim, they make progress. > > > > When non-movable requests spin in reclaim that isn't productive. They > > cannot use the individually freed pages, and the process is unlikely > > to accidentally free whole blocks to meet the ALLOC_NOFRAGMENT bar. > > They spin and overreclaim excessively, which tanks performance and > > triggers userspace guards like swap exhaustion or pressure based OOM. > > > > To fix this, send non-movable requests, regardless of order, into > > pageblock reclaim/compaction. This way, they help move things along to > > meet the ALLOC_NOFRAGMENT bar. After this patch, the reclaim storms > > and excess OOM rates are no longer observed in production. > > > > The longer-term plan is still to have all requests, including the > > movable ones, help make blocks to spread the cost of defragmenting > > more evenly and fairly; combined with proper watermarking to reduce > > allocation latencies in the common case. However, doing this naively > > unearths scaling and concurrency limitations in compaction that need > > to be addressed first. Promoting just non-movables for now is the > > minimally viable bug fix for the above issue. > > > > Fixes: f38356df6474 ("mm: page_alloc: introduce defrag_mode") > > Should be > Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode"). > Since I cannot find f38356df6474 in the tree. Oops, indeed. I managed to pull that commit from the old development branch I still had locally.