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=1794342051; b=Xm7k6//yIkBs8iR4cE9mblYMaz/KcMiYdPysEb5adeONjtDA35+k3kLtLQhwWSFSJ7QA Lad+icaiCszvupsGlL8IjlDyWgohMvmI8gNRXoSJcB8xn9T6R1gAdYt5G7L01+T+w5Q2J yxh4jB5JbjCI+PwSoJfHvPbazQgY6SGK5t12eCbjhL0dfBGzYBnlKxvvqbZ6pFO/6c0us +G8TY28gJUJGAT0ahYdU1sU+meBCagSeNPDtR4WFFPjzpo+wSrZsvRf6+YxVrA6OqdTES VDKJT7SSbXMNUEvbx+Vj9zBHtftudX1tYz95/bCS8MBKc4ibPUtxOJHUKXJbVcLHJdA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794342051; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-Id:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=wkOurhuE3F+xoJ1NIf+lBrhllBPRtPbJRVx8vBGAF64=; b=CcNyt7UkuPe3NepsUChODyqde8TwKJuomTzHtXE3pFowLhG0iazqXzfdvbEmzN0G77yq AU+4jnEsKs2QUTUQbvKZzD1lTnPa6kEH31IgzBppJoGUix67sTZyv8xLgGIAbytk3aSmP yXorIrSpbvSWSOlUw5QzZ+77fRvdQ/fJUcfqeR/46WktPMN+lbqRqdA7y/QKjQLbxMl7N CthyShNoplVms0rSRy6NNPya/a0dGDUkj7PbT3pY8che/u41ftqjjoCPm/ECDWqCt0N4t 8Y5u0JPShBPoRXmTBE/P1dCZc+5SqT4ZwtbOlTQeYp8ihFqIp2rKzhtZprlCaoE81Ew== 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=JIN1dvfo; 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 254C060A262A for ; Tue, 10 Nov 2026 20:20:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=JIN1dvfo Received: by smtp.kernel.org (Postfix) with ESMTPSA id 45DF91F0089F; Tue, 10 Nov 2026 20:20:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794342051; bh=wkOurhuE3F+xoJ1NIf+lBrhllBPRtPbJRVx8vBGAF64=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=JIN1dvfo7VTI5Hp2AI1OZkrS98xwyT0b0AKLVFfd90XTjdKzbsbOXNLZByf1xxS2U SXbQ9lBHQyyDJBkVEAzCvefxEqJejE32CludX4bnTTwaB/WXZ4xT0lW5vHpExvEnib gULl31rFja/5eRo+zeUinXKdQqHgvLz2kelB81SvURBmAgfssoj48/6GmdQchfzVHc FELFrd5oghsZXDOarpO5ErBTmlNU0FiPIa8phZoe5Vj2zkjNyuCwxHL18NMK0nkcdQ vxlWgVMj34D5iyjbm3RLdmirEc9bFRxVSDh6faQafo0mNXCo7VPLLgZy2IDmT4V7Pt LKvN9xtZxeBSw== From: "Barry Song (Xiaomi)" To: akpm@linux-foundation.org, linux-mm@kvack.org Cc: axelrasmussen@google.com, baolin.wang@linux.alibaba.com, baoquan.he@linux.dev, hannes@cmpxchg.org, hui.zhu@linux.dev, kasong@tencent.com, kunwu.chan@gmail.com, liam@infradead.org, lianux.mm@gmail.com, linux-kernel@vger.kernel.org, liuxiang.277@bytedance.com, lyugaofei@xiaomi.com, mhocko@kernel.org, mhocko@suse.com, qi.zheng@linux.dev, ridong.chen@linux.dev, rppt@kernel.org, ryncsn@gmail.com, shakeel.butt@linux.dev, surenb@google.com, wangzicheng@honor.com, weixugc@google.com, yuanchu@google.com, zhangbo56@xiaomi.com, zhuhui@kylinos.cn, sidchintamaneni@gmail.com, Barry Song Subject: [PATCH v6 03/15] mm/mglru: reclaim from the oldest populated gen Date: Wed, 11 Nov 2026 04:20:13 +0800 Message-Id: <20261110202025.25736-4-baohua@kernel.org> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: <20261110202025.25736-1-baohua@kernel.org> References: <20261110202025.25736-1-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 Reviewed-by: Kunwu Chan 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 4ac80968e634..593f407c0668 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -3311,11 +3311,9 @@ 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); @@ -4674,7 +4672,7 @@ 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); @@ -4692,14 +4690,14 @@ 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 */ @@ -4714,7 +4712,7 @@ 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; } @@ -4757,17 +4755,27 @@ 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; @@ -4785,7 +4793,7 @@ 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