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From: "JP Kobryn (Meta)" Subject: Re: [PATCH] mm/lruvec: preemptively free dead folios during lru_add drain To: Barry Song , Shakeel Butt Cc: linux-mm@kvack.org, akpm@linux-foundation.org, vbabka@kernel.org, mhocko@suse.com, willy@infradead.org, hannes@cmpxchg.org, riel@surriel.com, chrisl@kernel.org, kasong@tencent.com, shikemeng@huaweicloud.com, nphamcs@gmail.com, bhe@redhat.com, youngjun.park@lge.com, qi.zheng@linux.dev, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260423164307.29805-1-jp.kobryn@linux.dev> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 4/23/26 4:53 PM, Barry Song wrote: > On Fri, Apr 24, 2026 at 7:46 AM Shakeel Butt wrote: >> >> On Fri, Apr 24, 2026 at 07:22:30AM +0800, Barry Song wrote: >>> On Fri, Apr 24, 2026 at 12:43 AM JP Kobryn (Meta) wrote: >>>> >>>> Of all observable lruvec lock contention in our fleet, we find that ~24% >>>> occurs when dead folios are present in lru_add batches at drain time. This >>>> is wasteful in the sense that the folio is added to the LRU just to be >>>> immediately removed via folios_put_refs(), incurring two unnecessary lock >>>> acquisitions. >>>> >>>> Eliminate this overhead by preemptively cleaning up dead folios before they >>>> make it into the LRU. Use folio_ref_freeze() to filter folios whose only >>>> remaining refcount is the batch ref. When dead folios are found, move them >>>> off the add batch and onto a temporary batch to be freed. >>>> >>>> During A/B testing on one of our prod instagram workloads (high-frequency >>>> short-lived requests), the patch intercepted almost all dead folios before >>>> they entered the LRU. Data collected using the mm_lru_insertion tracepoint >>>> shows the effectiveness of the patch: >>>> >>>> Per-host LRU add averages at 95% CPU load >>>> (60 hosts each side, 3 x 60s intervals) >>>> >>>> dead folios/min total folios/min dead % >>>> unpatched: 1,297,785 19,341,986 6.7097% >>>> patched: 14 19,039,996 0.0001% >>>> >>>> Within this workload, we save ~2.6M lock acquisitions per minute per host >>>> as a result. >>>> >>>> System-wide memory stats improved on the patched side also at 95% CPU load: >>>> - direct reclaim scanning reduced 7% >>>> - allocation stalls reduced 5.2% >>>> - compaction stalls reduced 12.3% >>>> - page frees reduced 4.9% >>>> >>>> No regressions were observed in requests served per second or request tail >>>> latency (p99). Both metrics showed directional improvement at higher CPU >>>> utilization (comparing 85% to 95%). >>>> >>>> Signed-off-by: JP Kobryn (Meta) >>> >>> Hi JP, >>> I’m seeing a large number of "BAD page" bugs. >>> Not sure if it’s related, but reverting this patch >>> seems to fix the issue. It seems this was missed since classic LRU was used in testing. >>> >>> [ 2869.365978] BUG: Bad page state in process uname pfn:3a5417 >>> [ 2869.365981] page: refcount:0 mapcount:0 mapping:0000000000000000 >>> index:0x724884c20 pfn:0x3a5417 >>> [ 2869.365983] flags: >>> 0x17ffffc0020908(uptodate|active|owner_2|swapbacked|node=0|zone=2|lastcpupid=0x1fffff) >> >> Hi Barry, are you using MGLRU? It seems like MGLRU set active flag in >> folio_add_lru(). > > Yes. If you are referring to this set_active, I think it is > incorrect, so I have fixed it here and am waiting for review: > > https://lore.kernel.org/linux-mm/20260418120233.7162-1-baohua@kernel.org/ > >> >> JP, we need to clean active flag but let's check what else can be set before >> folio_add_lru(). Looks like only active is the problem. If we start manually clearing flags it starts to feel messy. I get that some fix is needed though. I don't see this patch in mm-new yet so maybe we can hold off on merging there to avoid the MGLRU case. But if Barry's patch is accepted, could we re-apply? Let me know if you're thinking there are any implications beyond the active flag.