From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792706549; b=nu9T2WtVEjIX5f6s23Pt71VHK0XOQKI8MwFpDwmeTi/QQir13ZsPpO0EvkbVlG+1eFd9 BekoyhxjTpxD0w9yTfQACNjUWzKF3VuSFwYoC5NCYR8Fz626nwEwDnj5CCk9GcH44Hr06 x+rB3ygGw4GGLwnzjWfh7BODt/t9F4xw9bUSVOj5J2degrcsJf12xCkbmjDOC7B/vCcBi 8FX+/yFOxJcOLJes5zVTLQlXmx8AowEdQ4ivvTJwA8W6m7f0Xf6SrAygFPWEFpbUp6HBf 6zlekoxYa5a0SwSLQ+xm0KhKRXeZ0K8/ulnqdPTxdTb+VihagQdlX4SlI1cQV3S+dPw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792706549; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-Id:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=JjaHi74G05A3micuDJmoCwFLOLh7P45YvU+FurY2Pj0=; b=g5rAf5pUJfYZozvt/FFQ5AVVYFoqc1W2jpwkFwK/2b0sDiaaoPxUodvQp64CevpbuhKr vepp4mK5MV8iDOdvpvD6xt6z+h3r/BEf/B96vYuC2cClf4A0tpfBPAS9Az0yg24v3/T9v a6kmqVSqvzrAF62Gl+XpcJgWmrROdmqwicHJb4gaCzlsWFN009h1DU4Wg755kBF48/u4n +8QSeZTUZ7iF3OHtahHiwyHHyGFolGd9BeMmnF7o3k7uJ0ryEGmFDdEbd+4ayVv6F6lzs /XJr5xIvKX/VUG/aWwDG4Yi2Gt0vULtp4lGAL4EfOLDffLtsxL8sd9C3Fn3NgdN4AJA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=k8inY7Kc; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E8685606A31A for ; Thu, 22 Oct 2026 22:02:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=k8inY7Kc Received: by smtp.kernel.org (Postfix) with ESMTPSA id 956C51F031E1; Thu, 22 Oct 2026 22:02:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792706548; bh=JjaHi74G05A3micuDJmoCwFLOLh7P45YvU+FurY2Pj0=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=k8inY7KcNfDHOkqo0imhkQE+tEBqL+diazAO17P2RG2P27Hmt+VuhS+fcaFY5e5Aa ynsTXJBiaiHXUlL6fPauUlOtjC5RvwDBeYPrQdUiioyXKohxu4MU058CoouTpmffFQ WfyAyjHX064J5cETuXCP6mkewx8tdsb4MoKwsQt5naCnba4RSF/pNd3tRyGbMv9MQu C8LZuU8KQJw7u7l5uP0POqVHgz7XGC8Cw7Me1TYY+pYW7cEBSmCjOCrM7asMy1b8ha Vs+/1/zWRjUFpZe+1MJsAhPNUHhwwnyLdx4boCX36SJAcJ3NseWV1EcoJ/y8bfehqB bP8B4jm1JbC7A== From: "Barry Song (Xiaomi)" To: akpm@linux-foundation.org, linux-mm@kvack.org Cc: hui.zhu@linux.dev, axelrasmussen@google.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, david@kernel.org, hannes@cmpxchg.org, linux-kernel@vger.kernel.org, ljs@kernel.org, mhocko@kernel.org, qi.zheng@linux.dev, ryncsn@gmail.com, shakeel.butt@linux.dev, weixugc@google.com, yuanchu@google.com, zhuhui@kylinos.cn, baoquan.he@linux.dev, liuxiang.277@bytedance.com, wangzicheng@honor.com, lianux.mm@gmail.com, kasong@tencent.com, liam@infradead.org, surenb@google.com, mhocko@suse.com, rppt@kernel.org, zhangbo56@xiaomi.com, lyugaofei@xiaomi.com, ridong.chen@linux.dev, kunwu.chan@gmail.com Subject: [RFC PATCH v5 03/14] mm/mglru: reclaim from the oldest populated gen Date: Fri, 23 Oct 2026 06:01:52 +0800 Message-Id: <20261022220203.91911-4-baohua@kernel.org> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: <20261022220203.91911-1-baohua@kernel.org> References: <20261022220203.91911-1-baohua@kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Kairui Song Currently MGLRU tries to keep the gen numbers of the two types in sync, avoiding reducing the gen number of one type when the other type has more gens, as described in the comment in try_to_inc_min_seq(). This makes scan_folios() stuck reclaiming the type with fewer gens: it always scanned the oldest gen of the type, even when that gen was empty, so reclaim made no progress on that type unless forced aging created a new gen and eliminated the dead empty gen. There are other approaches, like lifting the gen number synchronization [1]. However, that would break the gen layout used by lru_gen_folio_seq(): newly added folios would no longer have a well-defined hotness according to their gen number, which reduces accuracy. It is also not pressure-aware, since with gentle reclaim we still want to respect the gen layout (hotness). Instead, reclaim from whichever populated oldest gen is left, but only when the default reclaim priority can't satisfy the reclaimer's requirement. This is typical for swappiness limitations: e.g., low swappiness asks the reclaimer to reclaim from file, but file gens are drained, so the reclaim cycle fails and priority is raised. At the default priority the scan quota is 1/4096 of the evictable size, and the old behavior is kept there to respect the hotness distribution. Once priority is raised, the quota is at least 1/2048, so the hotness bias introduced by reclaiming from a younger gen is bounded by that quota, which seems fair. Global reclaim for MGLRU may start with DEF_PRIORITY / 2 (1/64); the ratio is still reasonable at that point, and when global pressure is more critical, we should reclaim harder. Note that scan_folios() still refuses to reclaim a type left with only MIN_NR_GENS gens, so a completely drained type still requires aging. This change only avoids scanning empty gens when populated reclaimable gens remain. Link: https://lore.kernel.org/linux-mm/20260726122123.7614-4-baohua@kernel.org/ [1] Signed-off-by: Kairui Song Signed-off-by: Barry Song --- mm/vmscan.c | 30 +++++++++++++++++++----------- 1 file changed, 19 insertions(+), 11 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 500ef23949fd..d7eb15533de1 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -3451,11 +3451,9 @@ static int __folio_inc_gen(struct folio *folio, int old_gen, bool *increased) } /* protect pages accessed multiple times through file descriptors */ -static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio) +static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio, int old_gen) { - int type = folio_is_file_lru(folio); - struct lru_gen_folio *lrugen = &lruvec->lrugen; - int new_gen, old_gen = lru_gen_from_seq(lrugen->min_seq[type]); + int new_gen; bool gen_increased; new_gen = __folio_inc_gen(folio, old_gen, &gen_increased); @@ -4836,7 +4834,7 @@ void lru_gen_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent, ******************************************************************************/ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_control *sc, - int tier_idx) + int min_gen, int tier_idx) { int gen = folio_lru_gen(folio); int type = folio_is_file_lru(folio); @@ -4854,14 +4852,14 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c return false; /* promoted */ - if (gen != lru_gen_from_seq(lrugen->min_seq[type])) { + if (gen != min_gen) { list_move(&folio->lru, &lrugen->folios[gen][type][zone]); return true; } /* protected */ if (tier > tier_idx || refs + workingset == BIT(LRU_REFS_WIDTH) + 1) { - gen = folio_inc_gen(lruvec, folio); + gen = folio_inc_gen(lruvec, folio, min_gen); list_move(&folio->lru, &lrugen->folios[gen][type][zone]); /* don't count the workingset being lazily promoted */ @@ -4876,7 +4874,7 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c /* ineligible */ if (zone > sc->reclaim_idx) { - gen = folio_inc_gen(lruvec, folio); + gen = folio_inc_gen(lruvec, folio, min_gen); list_move_tail(&folio->lru, &lrugen->folios[gen][type][zone]); return true; } @@ -4919,17 +4917,27 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec, int scanned = 0; int isolated = 0; int skipped = 0; + unsigned long min_seq; unsigned long remaining = nr_to_scan; struct lru_gen_folio *lrugen = &lruvec->lrugen; VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH); VM_WARN_ON_ONCE(!list_empty(list)); + /* + * Respect the gen hotness distribution at low priority to avoid + * aging and scan cost, but prefer to satisfy the reclaimers request + * when under pressure to respect swappiness more and reduce IO cost. + */ + if (sc->priority == DEF_PRIORITY) + min_seq = lrugen->min_seq[type]; + else + min_seq = lruvec_populated_min_seq(lruvec, type, lrugen->max_seq - MIN_NR_GENS + 1); + if (get_nr_gens(lruvec, type) == MIN_NR_GENS) return 0; - gen = lru_gen_from_seq(lrugen->min_seq[type]); - + gen = lru_gen_from_seq(min_seq); for (i = MAX_NR_ZONES; i > 0; i--) { LIST_HEAD(moved); int skipped_zone = 0; @@ -4947,7 +4955,7 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec, scanned += delta; - if (sort_folio(lruvec, folio, sc, tier)) + if (sort_folio(lruvec, folio, sc, gen, tier)) sorted += delta; else if (isolate_folio(lruvec, folio, sc)) { list_add(&folio->lru, list); -- 2.34.1