From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pl1-f200.google.com (mail-pl1-f200.google.com [209.85.214.200]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5E0A23AB291 for ; Sat, 8 Aug 2026 04:06:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.200 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786161982; cv=none; b=k49JeGKQn0SRnBbr+7x3daoEWXFLMZGFDlSthr88/cprsCm+Frhr8x8JxFL6iGZ+l/ER7Z8AISc6rAmtq3nL45z63n6avfI80LvIkI2aHJ3robw/2b7MItYWDbgZ7dNbgsi7A72cE9iq5Npf1+LzT3trK2Wzxh8U7hqB4yReNeg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786161982; c=relaxed/simple; bh=xA+Anu1G01kalxTG8fE/ijwz0xdzhG/T7miN8n1j8/Q=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=OCjGy+ew8tS///MbEloZS3UVmQlDXMeBtfTJ2HqxkWIeNUZtJjQK4kUCfumrhKWmfYITPsIJWHkdTADIUmMKnkEwSlFKGXOq6HNF3mffj3a/XIIMfrBFJu8xL4GPQV14EdDw2TEJPR8eTHXZzAUkSt1xPyXkmqR3Do6FexoMU8Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--ankitkap.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=fY5zmUgL; arc=none smtp.client-ip=209.85.214.200 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--ankitkap.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="fY5zmUgL" Received: by mail-pl1-f200.google.com with SMTP id d9443c01a7336-2d004f13426so3792795ad.3 for ; Fri, 07 Aug 2026 21:06:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1786161980; x=1786766780; darn=vger.kernel.org; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:from:to:cc:subject:date:message-id:reply-to :content-type; bh=cp2CV40UiAeLycXOv282ow6mV6SoZ8nWxaGYowz7qXM=; b=fY5zmUgLOWZo3V6GvY3eBSCrhds82RzCJQDYAds0393xSRU/cA9sv6zI5sF+Cg1/7C HK3BAl1d79RLha36volF9XRK/pIgCnZcA87/x+2sO/f2ZpEls8jUwKy3KmeON6r1XNVj ZQ7XtNzBaLCi8waIGTmZ5J978Yh3r1ASMJC7P3D45GfpGRBFHpf4/aoLsw5UE0YpVMZf +z4wXsv+ILTvf78t4FZrNVy73mpSvZNPQeKXU1B0fHQBk3B3N+getQZ0laI2dcEDN97t Tjuu7t1BQaxO8C3+LhTO9ZBlctihLYupWrQeXLWhd4uruh4nHMqgxR14HysVe03cD6HK ExVg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1786161980; x=1786766780; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=cp2CV40UiAeLycXOv282ow6mV6SoZ8nWxaGYowz7qXM=; b=OpzAulGDmlkqn+Ub2xGmeMItWYYpLGe5AGZEm5Jm2/5ut8NUmcYF1fJEM9d/wl88W1 O7JTOL4XjuGMUNQHOaIwiBoXREU4u6A08bsXODbbPGtwu+4Pos+M1gxMep3QeyusmuXU QqeAbIo91NC3Gq7Kk77b4Ufx2ZveYqYBNAjLX8HIJ4zTkO0rptsyQ0OV1DD9q0u/BWCc jzpAhEUKcVlyR5hT7pC6tphvKiGRspvd9mO1iDJ/gPUc7pFuvZdfqZQULL6BOJiwpH4Y J1ekulVgt9ZTwSgnzu+zU4V6P9uvGDjOwIG84XjOiON8QcR7iW3TQrdySf1Wy+pm13hn 2j/Q== X-Forwarded-Encrypted: i=1; AHgh+Ro1M/3xqRr5eCvhbBTclZH33pqz61Qqj1t1eQtrFOVpjDv9Axo8fE9Q4HMluyFIV0Zlz6amRsNFS9I1W98=@vger.kernel.org X-Gm-Message-State: AOJu0YyqyiFqCdoPkGwYATCQTQIffWpysha9UoegUySfgeNSOKqa9Ip+ esLVlKTLL4S+dxBVaWoTcWBQEcPXIVjlHknHrc86zI41EKOa5ENX+MQ8RQS/V1tvc1zK7XJP1sl TNSrbeXs79OqpGg== X-Received: from plas19.prod.google.com ([2002:a17:903:2013:b0:2d0:34d:e973]) (user=ankitkap job=prod-delivery.src-stubby-dispatcher) by 2002:a17:902:e84b:b0:2c9:b396:1a55 with SMTP id d9443c01a7336-2d0ca76a3bamr304649095ad.12.1786161979362; Fri, 07 Aug 2026 21:06:19 -0700 (PDT) Date: Sat, 8 Aug 2026 04:05:49 +0000 In-Reply-To: <20260808040549.2778125-1-ankitkap@google.com> Precedence: bulk X-Mailing-List: linux-bcache@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260808040549.2778125-1-ankitkap@google.com> X-Mailer: git-send-email 2.55.0.679.g6767b8d81c-goog Message-ID: <20260808040549.2778125-3-ankitkap@google.com> Subject: [PATCH v3 2/2] bcache: inspect active bypass writes lock-free via RCU From: Ankit Kapoor To: Coly Li , linux-bcache@vger.kernel.org Cc: Kent Overstreet , linux-kernel@vger.kernel.org, Ankit Kapoor Content-Type: text/plain; charset="UTF-8" Checking for active bypass writes on the cache miss read path currently requires acquiring page-level spinlocks across the target sector range. While contention is low under normal operation, acquiring a lock in the latency-sensitive read path introduces unnecessary overhead. Optimize the read path by using RCU to inspect active bypass write counters lock-free. Writers continue to use page-level spinlocks to synchronize counter updates and allocations, while readers inspect the tracking array under rcu_read_lock(). Suggested-by: Coly Li Signed-off-by: Ankit Kapoor --- drivers/md/bcache/bcache.h | 14 +++++++-- drivers/md/bcache/request.c | 58 +++++++++++++++++++++++++------------ drivers/md/bcache/super.c | 9 ++++-- 3 files changed, 56 insertions(+), 25 deletions(-) diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h index 2e50526a52fc..2ecff48d8902 100644 --- a/drivers/md/bcache/bcache.h +++ b/drivers/md/bcache/bcache.h @@ -299,10 +299,18 @@ enum stop_on_failure { BCH_CACHED_DEV_STOP_MODE_MAX, }; +extern struct kmem_cache *bch_bypass_cache; + +struct bch_bypass_counts { + struct rcu_head rcu; + struct cached_dev *dc; + u32 counts[PAGE_SIZE / sizeof(u32)]; +}; + struct bch_bypass_page { - u32 *counts; - unsigned int active; - spinlock_t lock; + struct bch_bypass_counts __rcu *counts; + unsigned int active; + spinlock_t lock; }; struct cached_dev { diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c index bfd28b68f499..9221db669050 100644 --- a/drivers/md/bcache/request.c +++ b/drivers/md/bcache/request.c @@ -24,6 +24,7 @@ #define CUTOFF_CACHE_READA 90 struct kmem_cache *bch_search_cache; +struct kmem_cache *bch_bypass_cache; static CLOSURE_CALLBACK(bch_data_insert_start); @@ -852,22 +853,28 @@ static void bch_bypass_write_start(struct cached_dev *dc, sector_t sector, unsig unsigned long pg_idx = bypass_chunk_to_page(chunk); unsigned long pg_off = bypass_chunk_to_offset(chunk); struct bch_bypass_page *pg = &dc->bypass_pages[pg_idx]; - u32 *new_counts; - u32 *dup_counts = NULL; + struct bch_bypass_counts *new_counts; + struct bch_bypass_counts *dup_counts = NULL; + struct bch_bypass_counts *counts; unsigned long flags; spin_lock_irqsave(&pg->lock, flags); - if (!pg->counts) { + counts = rcu_dereference_protected(pg->counts, lockdep_is_held(&pg->lock)); + if (!counts) { spin_unlock_irqrestore(&pg->lock, flags); new_counts = mempool_alloc(&dc->bypass_mempool, GFP_NOIO); - memset(new_counts, 0, PAGE_SIZE); + memset(new_counts->counts, 0, PAGE_SIZE); + new_counts->dc = dc; spin_lock_irqsave(&pg->lock, flags); - if (pg->counts) + counts = rcu_dereference_protected(pg->counts, lockdep_is_held(&pg->lock)); + if (counts) { dup_counts = new_counts; - else - pg->counts = new_counts; + } else { + counts = new_counts; + rcu_assign_pointer(pg->counts, counts); + } } - pg->counts[pg_off]++; + WRITE_ONCE(counts->counts[pg_off], counts->counts[pg_off] + 1); pg->active++; spin_unlock_irqrestore(&pg->lock, flags); @@ -876,6 +883,13 @@ static void bch_bypass_write_start(struct cached_dev *dc, sector_t sector, unsig } } +static void bch_bypass_counts_free_rcu(struct rcu_head *rcu) +{ + struct bch_bypass_counts *counts = container_of(rcu, struct bch_bypass_counts, rcu); + + mempool_free(counts, &counts->dc->bypass_mempool); +} + static void bch_bypass_write_end(struct cached_dev *dc, sector_t sector, unsigned int sectors) { unsigned long start_chunk = sector_to_bypass_chunk(sector); @@ -890,25 +904,27 @@ static void bch_bypass_write_end(struct cached_dev *dc, sector_t sector, unsigne unsigned long pg_idx = bypass_chunk_to_page(chunk); unsigned long pg_off = bypass_chunk_to_offset(chunk); struct bch_bypass_page *pg = &dc->bypass_pages[pg_idx]; - u32 *counts = NULL; + struct bch_bypass_counts *counts = NULL; + struct bch_bypass_counts *current_counts; unsigned long flags; spin_lock_irqsave(&pg->lock, flags); - if (WARN_ON_ONCE(!pg->counts || !pg->counts[pg_off])) { + current_counts = rcu_dereference_protected(pg->counts, lockdep_is_held(&pg->lock)); + if (WARN_ON_ONCE(!current_counts || !current_counts->counts[pg_off])) { spin_unlock_irqrestore(&pg->lock, flags); continue; } - pg->counts[pg_off]--; + WRITE_ONCE(current_counts->counts[pg_off], current_counts->counts[pg_off] - 1); pg->active--; if (!pg->active) { - counts = pg->counts; - pg->counts = NULL; + counts = current_counts; + rcu_assign_pointer(pg->counts, NULL); } spin_unlock_irqrestore(&pg->lock, flags); if (counts) - mempool_free(counts, &dc->bypass_mempool); + call_rcu(&counts->rcu, bch_bypass_counts_free_rcu); } } @@ -924,20 +940,19 @@ static bool bch_has_active_bypass_writes(struct cached_dev *dc, sector_t sector, if (WARN_ON_ONCE(end_pg_idx >= dc->bypass_num_pages)) return false; + rcu_read_lock(); for (chunk = start_chunk; chunk <= end_chunk; chunk++) { unsigned long pg_idx = bypass_chunk_to_page(chunk); unsigned long pg_off = bypass_chunk_to_offset(chunk); struct bch_bypass_page *pg = &dc->bypass_pages[pg_idx]; - unsigned long flags; + struct bch_bypass_counts *current_counts = rcu_dereference(pg->counts); - spin_lock_irqsave(&pg->lock, flags); - if (pg->counts && pg->counts[pg_off] > 0) { + if (current_counts && READ_ONCE(current_counts->counts[pg_off]) > 0) { has_active = true; - spin_unlock_irqrestore(&pg->lock, flags); break; } - spin_unlock_irqrestore(&pg->lock, flags); } + rcu_read_unlock(); return has_active; } @@ -1458,6 +1473,7 @@ void bch_flash_dev_request_init(struct bcache_device *d) void bch_request_exit(void) { + kmem_cache_destroy(bch_bypass_cache); kmem_cache_destroy(bch_search_cache); } @@ -1467,5 +1483,9 @@ int __init bch_request_init(void) if (!bch_search_cache) return -ENOMEM; + bch_bypass_cache = KMEM_CACHE(bch_bypass_counts, 0); + if (!bch_bypass_cache) + return -ENOMEM; + return 0; } diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c index b994483511de..75aa6bb2e00b 100644 --- a/drivers/md/bcache/super.c +++ b/drivers/md/bcache/super.c @@ -1350,9 +1350,12 @@ void bch_cached_dev_release(struct kobject *kobj) unsigned long i; for (i = 0; i < dc->bypass_num_pages; i++) { - if (dc->bypass_pages[i].counts) - mempool_free(dc->bypass_pages[i].counts, &dc->bypass_mempool); + struct bch_bypass_counts *counts = + rcu_dereference_protected(dc->bypass_pages[i].counts, 1); + if (counts) + mempool_free(counts, &dc->bypass_mempool); } + rcu_barrier(); kvfree(dc->bypass_pages); mempool_exit(&dc->bypass_mempool); } @@ -1433,7 +1436,7 @@ static int bch_cached_dev_bypass_init(struct cached_dev *dc, sector_t sectors) for (i = 0; i < dc->bypass_num_pages; i++) spin_lock_init(&dc->bypass_pages[i].lock); - if (mempool_init_kmalloc_pool(&dc->bypass_mempool, 16, PAGE_SIZE)) { + if (mempool_init_slab_pool(&dc->bypass_mempool, 16, bch_bypass_cache)) { kvfree(dc->bypass_pages); dc->bypass_pages = NULL; return -ENOMEM; -- 2.55.0.679.g6767b8d81c-goog