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AFNElJ9O8NdpLXvH8DJQVa68+nVEit3FB6kIep8+97FLK1MTsxzW7WEKPXsLEiHOsGhkAWtmpcnqhl4K1g==@kvack.org X-Gm-Message-State: AOJu0YwfinaXKSOC1UV6MnbRwKTEFx5tPX7Gnll3/UfCvxws6FfAY3/u N4YlgRXaPaYVIxqAwGSz4Adx+viqWGMHhqySq3xMdaciszXCCCkBR1GuwrlMdR77/DfV1a83sg9 agCwTWA== X-Received: from pgbdk2.prod.google.com ([2002:a05:6a02:c82:b0:c85:9dd2:d11e]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:cc97:b0:3b4:5c6f:cf30 with SMTP id adf61e73a8af0-3bd4b25b347mr3615613637.38.1782402027934; Thu, 25 Jun 2026 08:40:27 -0700 (PDT) Date: Thu, 25 Jun 2026 08:40:27 -0700 In-Reply-To: <6ed7d12a-c3a1-4572-8385-754e6d5b8b44@kernel.org> Mime-Version: 1.0 References: <20260618-gmem-inplace-conversion-v8-0-9d2959357853@google.com> <20260618-gmem-inplace-conversion-v8-18-9d2959357853@google.com> <6ed7d12a-c3a1-4572-8385-754e6d5b8b44@kernel.org> Message-ID: Subject: Re: [PATCH v8 18/46] KVM: guest_memfd: Handle lru_add fbatch refcounts during conversion safety check From: Sean Christopherson To: "David Hildenbrand (Arm)" Cc: Ackerley Tng , aik@amd.com, andrew.jones@linux.dev, binbin.wu@linux.intel.com, brauner@kernel.org, chao.p.peng@linux.intel.com, jmattson@google.com, jthoughton@google.com, michael.roth@amd.com, oupton@kernel.org, pankaj.gupta@amd.com, qperret@google.com, rick.p.edgecombe@intel.com, rientjes@google.com, shivankg@amd.com, steven.price@arm.com, tabba@google.com, willy@infradead.org, wyihan@google.com, yan.y.zhao@intel.com, forkloop@google.com, pratyush@kernel.org, suzuki.poulose@arm.com, aneesh.kumar@kernel.org, liam@infradead.org, Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Shuah Khan , Shuah Khan , Vishal Annapurve , Andrew Morton , Chris Li , Kairui Song , Kemeng Shi , Nhat Pham , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Youngjun Park , Qi Zheng , Shakeel Butt , Kiryl Shutsemau , Baoquan He , Jason Gunthorpe , Vlastimil Babka , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev Content-Type: text/plain; charset="us-ascii" X-Rspamd-Queue-Id: 7C3E6140007 X-Stat-Signature: x44g1hiiqzebigygiaqgcow96iwgiawm X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1782402029-717475 X-HE-Meta: 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 jhJTATkY 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jun 25, 2026, David Hildenbrand (Arm) wrote: > On 6/25/26 02:35, Sean Christopherson wrote: > > One thought I had, to avoid the IPIs that draining all per-CPU caches requires, > > was to disallow putting guest_memfd pages in folio batches, e.g. by hacking > > something into folio_may_be_lru_cached(). But due to taking a per-lru lock, > > that would penalize the relatively hot path and definitely common operation of > > faulting in guest memory. On the other hand, memory conversion is already a > > relatively slow operation and is relatively uncommon compared to page faults, > > (and likely very uncommon for real world setups). I.e. having to drain all > > caches if conversion isn't safe penalizes a relatively slow, relatively uncommon > > path. > > Yeah, the lru_add_drain_all is rather messy. > > We have similar code in > > collect_longterm_unpinnable_folios(), where we first try a lru_add_drain(), to > then escalate to a lru_add_drain_all(). > > Maybe we could factor that (suboptimal code) out to not have to reinvent the > same thing multiple times? As discussed in the guest_memfd call, we should do this straightaway, i.e. instead of merging this series as-is, so that we don't export lru_add_drain_all() only to drop the export a kernel or two later, and can instead export the helper to drain any batches for a folio (or set of folios/pages).