Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0A84A3B4E87; Tue, 21 Jul 2026 21:50:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784670636; cv=none; b=rFWFhPW6oVcpSWOuaeoFpyZRuC6YKXuSvJ2HZNO9ZLFjZ6r9JLYlOvq4CXusb37tVnUccelnGEB1Pyy1YzGEQunXq75hpah8g3IVuwJw0Q7cW/pEfQALnUZavMhO6z8n+YJmtZvNp0uqNXFJ976XI6T9QHdbe40OTSEn/MRns5k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784670636; c=relaxed/simple; bh=CPNNK9aYwkw8Yv0gHSIwiPNgwXlMkCoThRrtrTBDsMc=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=HmpMtzekTjfWvcL6w7rUpPqtCE7+uJmQHwjZ/pKlNqGC0q1zHKFaQWbCwNQgSjJSdF9CHfAY9xw2R0lpLegM8cA09UqWmTFRhHqU4Lm0qlhcU2exHoQGBb/CP3nV46WFQ++Ibslu7aelySP5vHRsA7GmC9XTvzdPSKqrubHploI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=VNuNsxOx; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="VNuNsxOx" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3609C1F000E9; Tue, 21 Jul 2026 21:50:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1784670634; bh=JJfXYl2TMxJ3UddHowFtd4vjXnXj1g3l5L2k+aYIc0Q=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=VNuNsxOxbvbskGImAez0SpvoSE3Zjlcj+jxQKzmDLKpYsbaND28jX5u/ArHFrfWHk cfuPZ28Bh6WOcSrDNIgtVKNdTci9+SiVGYLnduV1qAFy2IEtViUiUdHw0X0ePT9Xvl kMymRAPFDx2FPNgLVQdpKdn0dqTdjyzHKGnkVbFQ= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Tejun Heo , Jan Kara , Sasha Levin Subject: [PATCH 6.1 1006/1067] writeback: Avoid contention on wb->list_lock when switching inodes Date: Tue, 21 Jul 2026 17:26:47 +0200 Message-ID: <20260721152447.074408502@linuxfoundation.org> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260721152424.521567757@linuxfoundation.org> References: <20260721152424.521567757@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.1-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jan Kara [ Upstream commit e1b849cfa6b61f1c866a908c9e8dd9b5aaab820b ] There can be multiple inode switch works that are trying to switch inodes to / from the same wb. This can happen in particular if some cgroup exits which owns many (thousands) inodes and we need to switch them all. In this case several inode_switch_wbs_work_fn() instances will be just spinning on the same wb->list_lock while only one of them makes forward progress. This wastes CPU cycles and quickly leads to softlockup reports and unusable system. Instead of running several inode_switch_wbs_work_fn() instances in parallel switching to the same wb and contending on wb->list_lock, run just one work item per wb and manage a queue of isw items switching to this wb. Acked-by: Tejun Heo Signed-off-by: Jan Kara Stable-dep-of: cba38ec4cbd3 ("writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()") Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman --- fs/fs-writeback.c | 99 ++++++++++++++++++++++++--------------- include/linux/backing-dev-defs.h | 4 + include/linux/writeback.h | 2 mm/backing-dev.c | 5 + 4 files changed, 74 insertions(+), 36 deletions(-) --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -343,7 +343,8 @@ static struct bdi_writeback *inode_to_wb } struct inode_switch_wbs_context { - struct rcu_work work; + /* List of queued switching contexts for the wb */ + struct llist_node list; /* * Multiple inodes can be switched at once. The switching procedure @@ -353,7 +354,6 @@ struct inode_switch_wbs_context { * array embedded into struct inode_switch_wbs_context. Otherwise * an inode could be left in a non-consistent state. */ - struct bdi_writeback *new_wb; struct inode *inodes[]; }; @@ -462,13 +462,11 @@ skip_switch: return switched; } -static void inode_switch_wbs_work_fn(struct work_struct *work) +static void process_inode_switch_wbs(struct bdi_writeback *new_wb, + struct inode_switch_wbs_context *isw) { - struct inode_switch_wbs_context *isw = - container_of(to_rcu_work(work), struct inode_switch_wbs_context, work); struct backing_dev_info *bdi = inode_to_bdi(isw->inodes[0]); struct bdi_writeback *old_wb = isw->inodes[0]->i_wb; - struct bdi_writeback *new_wb = isw->new_wb; unsigned long nr_switched = 0; struct inode **inodep; @@ -528,6 +526,38 @@ relock: atomic_dec(&isw_nr_in_flight); } +void inode_switch_wbs_work_fn(struct work_struct *work) +{ + struct bdi_writeback *new_wb = container_of(work, struct bdi_writeback, + switch_work); + struct inode_switch_wbs_context *isw, *next_isw; + struct llist_node *list; + + /* + * Grab out reference to wb so that it cannot get freed under us + * after we process all the isw items. + */ + wb_get(new_wb); + while (1) { + list = llist_del_all(&new_wb->switch_wbs_ctxs); + /* Nothing to do? */ + if (!list) + break; + /* + * In addition to synchronizing among switchers, I_WB_SWITCH + * tells the RCU protected stat update paths to grab the i_page + * lock so that stat transfer can synchronize against them. + * Let's continue after I_WB_SWITCH is guaranteed to be + * visible. + */ + synchronize_rcu(); + + llist_for_each_entry_safe(isw, next_isw, list, list) + process_inode_switch_wbs(new_wb, isw); + } + wb_put(new_wb); +} + static bool inode_prepare_wbs_switch(struct inode *inode, struct bdi_writeback *new_wb) { @@ -557,6 +587,13 @@ static bool inode_prepare_wbs_switch(str return true; } +static void wb_queue_isw(struct bdi_writeback *wb, + struct inode_switch_wbs_context *isw) +{ + if (llist_add(&isw->list, &wb->switch_wbs_ctxs)) + queue_work(isw_wq, &wb->switch_work); +} + /** * inode_switch_wbs - change the wb association of an inode * @inode: target inode @@ -570,6 +607,7 @@ static void inode_switch_wbs(struct inod struct backing_dev_info *bdi = inode_to_bdi(inode); struct cgroup_subsys_state *memcg_css; struct inode_switch_wbs_context *isw; + struct bdi_writeback *new_wb = NULL; /* noop if seems to be already in progress */ if (inode->i_state & I_WB_SWITCH) @@ -594,40 +632,34 @@ static void inode_switch_wbs(struct inod if (!memcg_css) goto out_free; - isw->new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC); + new_wb = wb_get_create(bdi, memcg_css, GFP_ATOMIC); css_put(memcg_css); - if (!isw->new_wb) + if (!new_wb) goto out_free; - if (!inode_prepare_wbs_switch(inode, isw->new_wb)) + if (!inode_prepare_wbs_switch(inode, new_wb)) goto out_free; isw->inodes[0] = inode; - /* - * In addition to synchronizing among switchers, I_WB_SWITCH tells - * the RCU protected stat update paths to grab the i_page - * lock so that stat transfer can synchronize against them. - * Let's continue after I_WB_SWITCH is guaranteed to be visible. - */ - INIT_RCU_WORK(&isw->work, inode_switch_wbs_work_fn); - queue_rcu_work(isw_wq, &isw->work); + wb_queue_isw(new_wb, isw); return; out_free: atomic_dec(&isw_nr_in_flight); - if (isw->new_wb) - wb_put(isw->new_wb); + if (new_wb) + wb_put(new_wb); kfree(isw); } -static bool isw_prepare_wbs_switch(struct inode_switch_wbs_context *isw, +static bool isw_prepare_wbs_switch(struct bdi_writeback *new_wb, + struct inode_switch_wbs_context *isw, struct list_head *list, int *nr) { struct inode *inode; list_for_each_entry(inode, list, i_io_list) { - if (!inode_prepare_wbs_switch(inode, isw->new_wb)) + if (!inode_prepare_wbs_switch(inode, new_wb)) continue; isw->inodes[*nr] = inode; @@ -651,6 +683,7 @@ bool cleanup_offline_cgwb(struct bdi_wri { struct cgroup_subsys_state *memcg_css; struct inode_switch_wbs_context *isw; + struct bdi_writeback *new_wb; int nr; bool restart = false; @@ -663,12 +696,12 @@ bool cleanup_offline_cgwb(struct bdi_wri for (memcg_css = wb->memcg_css->parent; memcg_css; memcg_css = memcg_css->parent) { - isw->new_wb = wb_get_create(wb->bdi, memcg_css, GFP_KERNEL); - if (isw->new_wb) + new_wb = wb_get_create(wb->bdi, memcg_css, GFP_KERNEL); + if (new_wb) break; } - if (unlikely(!isw->new_wb)) - isw->new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */ + if (unlikely(!new_wb)) + new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */ nr = 0; spin_lock(&wb->list_lock); @@ -680,27 +713,21 @@ bool cleanup_offline_cgwb(struct bdi_wri * bandwidth restrictions, as writeback of inode metadata is not * accounted for. */ - restart = isw_prepare_wbs_switch(isw, &wb->b_attached, &nr); + restart = isw_prepare_wbs_switch(new_wb, isw, &wb->b_attached, &nr); if (!restart) - restart = isw_prepare_wbs_switch(isw, &wb->b_dirty_time, &nr); + restart = isw_prepare_wbs_switch(new_wb, isw, &wb->b_dirty_time, + &nr); spin_unlock(&wb->list_lock); /* no attached inodes? bail out */ if (nr == 0) { atomic_dec(&isw_nr_in_flight); - wb_put(isw->new_wb); + wb_put(new_wb); kfree(isw); return restart; } - /* - * In addition to synchronizing among switchers, I_WB_SWITCH tells - * the RCU protected stat update paths to grab the i_page - * lock so that stat transfer can synchronize against them. - * Let's continue after I_WB_SWITCH is guaranteed to be visible. - */ - INIT_RCU_WORK(&isw->work, inode_switch_wbs_work_fn); - queue_rcu_work(isw_wq, &isw->work); + wb_queue_isw(new_wb, isw); return restart; } --- a/include/linux/backing-dev-defs.h +++ b/include/linux/backing-dev-defs.h @@ -154,6 +154,10 @@ struct bdi_writeback { struct list_head blkcg_node; /* anchored at blkcg->cgwb_list */ struct list_head b_attached; /* attached inodes, protected by list_lock */ struct list_head offline_node; /* anchored at offline_cgwbs */ + struct work_struct switch_work; /* work used to perform inode switching + * to this wb */ + struct llist_head switch_wbs_ctxs; /* queued contexts for + * writeback switching */ union { struct work_struct release_work; --- a/include/linux/writeback.h +++ b/include/linux/writeback.h @@ -288,6 +288,8 @@ static inline void wbc_init_bio(struct w bio_associate_blkg_from_css(bio, wbc->wb->blkcg_css); } +void inode_switch_wbs_work_fn(struct work_struct *work); + #else /* CONFIG_CGROUP_WRITEBACK */ static inline void inode_attach_wb(struct inode *inode, struct page *page) --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -415,6 +415,7 @@ static void cgwb_release_workfn(struct w wb_exit(wb); bdi_put(bdi); WARN_ON_ONCE(!list_empty(&wb->b_attached)); + WARN_ON_ONCE(work_pending(&wb->switch_work)); call_rcu(&wb->rcu, cgwb_free_rcu); } @@ -491,6 +492,8 @@ static int cgwb_create(struct backing_de wb->memcg_css = memcg_css; wb->blkcg_css = blkcg_css; INIT_LIST_HEAD(&wb->b_attached); + INIT_WORK(&wb->switch_work, inode_switch_wbs_work_fn); + init_llist_head(&wb->switch_wbs_ctxs); INIT_WORK(&wb->release_work, cgwb_release_workfn); set_bit(WB_registered, &wb->state); bdi_get(bdi); @@ -624,6 +627,8 @@ static int cgwb_bdi_init(struct backing_ if (!ret) { bdi->wb.memcg_css = &root_mem_cgroup->css; bdi->wb.blkcg_css = blkcg_root_css; + INIT_WORK(&bdi->wb.switch_work, inode_switch_wbs_work_fn); + init_llist_head(&bdi->wb.switch_wbs_ctxs); } return ret; }