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Tue, 04 Aug 2026 07:03:46 -0700 (PDT) Date: Tue, 4 Aug 2026 10:03:41 -0400 From: Johannes Weiner To: Barry Song Cc: "David Hildenbrand (Arm)" , Joanne Koong , akpm@linux-foundation.org, ljs@kernel.org, usama.arif@linux.dev, alex@ghiti.fr, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, lance.yang@linux.dev, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, willy@infradead.org, linux-mm@kvack.org Subject: Re: [PATCH v1 2/2] mm/memory: add anonymous mTHP folios to deferred split list Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Queue-Id: B1B59C000F X-Stat-Signature: tycofoigxu7qwqhgdbam41x7pf3cn1b3 X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1785852229-684366 X-HE-Meta: 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 i/M53L6i SDqE921s+TpDGWvBNO97McKd08rE0TwQmZusqh4FS0/kuMnp5yObAW960vR10bUnni45LKCXWmJcR8Fi6w2M5A7qXeOQApfCCCkZS1RlC6SV5CoD5KB152XcZoMiRajLxYkKQOfaPqNEfVE2xsm1vX9q2XlTSzmm65TSNg8zkOesm8kapkPUWSo3wQ/7N/bbYLJ/h+iWiu9BoJTtFlmRRGi0+0cN8lQUpIdR6aZEkdMyuhvODhBFm8fYwR6jN3aOSg9JUuGGsggjXMuXjlA4X/LS1+fKyMOwDEuLTDXGk0m8736dAVMmUfFjwtDODsO7SrGWvydLnE3FyGLVyNlip3xqtqhiqnqckpWQH/WvUc3mzXn4TPPRwVgrxgjWEg64SVp+iCR+kfOpLArXxMtnw8usjypkW6MrkITc9aaTkYhk3jExVspsE4wnT5sPIpG2f1c6K+wBLYVBGdhJne+/neBv/K28eptrFyfuG8wjxeB3zvWI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Aug 04, 2026 at 10:03:14AM +0800, Barry Song wrote: > On Tue, Aug 4, 2026 at 8:48 AM Johannes Weiner wrote: > > > [...] > > > > > > #define LARGE_FOLIO_ZERO_SCAN_MIN_SIZE SZ_2M > > > > > > if (folio_size(folio) >= LARGE_FOLIO_ZERO_SCAN_MIN_SIZE) > > > deferred_split_folio(folio, false); > > > > > > If, someday, people find that 1 MiB also helps, they can provide > > > data to support it. > > > > I am very confused. Did you not see my proposal above? > > > > Why not this? > > Hi Johannes, > > For arm64, if the base page size is 64KB, a PMD would be 512MB, > and PMD-1 would be 256MB. Usama mentioned 2MB, which is just > order-5, not PMD-1 on arm64. > > BTW, I assume khugepaged_max_ptes_none is intended for collapse, > not splitting. I am a bit concerned that reusing it for this > purpose would be quite disruptive. It already is: static bool thp_underused(struct folio *folio) { int num_zero_pages = 0, num_filled_pages = 0; int i; if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1) return false; if (folio_contain_hwpoisoned_page(folio)) return false; for (i = 0; i < folio_nr_pages(folio); i++) { if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) { if (++num_zero_pages > khugepaged_max_ptes_none) return true; } else { /* * Another path for early exit once the number * of non-zero filled pages exceeds threshold. */ if (++num_filled_pages >= HPAGE_PMD_NR - khugepaged_max_ptes_none) return false; } } return false; } That's ABI and setups are relying on it. All I'm proposing is to only queue pages that the shrinker would actually split under currently existing rules. That's a mostly transparent optimization, not a new policy. No new sysctls. No new policy hardcoded in kernel code. Default behavior remains unchanged.