From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0C7BFCA5FAD for ; Tue, 29 Sep 2026 17:59:01 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id E9A656B008A; Tue, 29 Sep 2026 13:59:00 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id E6FC46B008C; Tue, 29 Sep 2026 13:59:00 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D85546B0092; Tue, 29 Sep 2026 13:59:00 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id B03D66B008A for ; Tue, 29 Sep 2026 13:59:00 -0400 (EDT) Received: from smtpin16.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay01.hostedemail.com (Postfix) with ESMTP id 189D21C346E for ; Tue, 29 Sep 2026 17:59:00 +0000 (UTC) X-FDA: 85267560840.16.573500B Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf18.hostedemail.com (Postfix) with ESMTP id 8637A1C0008 for ; Tue, 29 Sep 2026 17:58:58 +0000 (UTC) Authentication-Results: imf18.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=UWIEExcf; spf=pass (imf18.hostedemail.com: domain of tj@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=tj@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1790704738; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=D8IsblU8nmE28Btv5b/ZkJyD4K5wSxbscAgK5JZqxKw=; b=fPb4B2Jr8ERjkc7ebfm5rGhMns2yoOfT2t3WTgTGTZSi2JQNGHN5AARKAZUfRI5T/KDwfH 35qprTIa1PsVIIc2GmdudjTrx3gYU1RmSf41yUG9gIEPtObrPe2xMbf66U4xibhv88wSdO dairyrgiw5iMSdVhiQWchcPDVLbWx4U= ARC-Authentication-Results: i=1; imf18.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=UWIEExcf; spf=pass (imf18.hostedemail.com: domain of tj@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=tj@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1790704738; b=pQyx8o0c0CO6M3dUN/hdIy9d2iCoknxl04GcpDkTs3KbN/llMbWeD6QpAPk2feJxQOoOmZ tnS95lX86F6M1+BJGHKDAWuvfIt9/f8+fjeuq67MB74qBEg5zkbS/a2wciOHOx3rqFbEWb zGB+ZntRfipQyquKNcUUdF5l9l4ogDY= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id DCFEC60239; Tue, 29 Sep 2026 17:58:57 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 69E7B1F000FF; Tue, 29 Sep 2026 17:58:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790704737; bh=D8IsblU8nmE28Btv5b/ZkJyD4K5wSxbscAgK5JZqxKw=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=UWIEExcfN6jUovdJmatCAMu5OMD6YHLVKZLm4cwex0sANOgJ6vPEjlkalfQRzeyk1 HIAuV12wZ9IO9kr8cpQEcoScoW432HEkDlXR4y5ZpNlxQaXm/jw05ouD1qd1CsVN9m 3Gt001PNnIvK6A0HEBpRnJ5EDl9PT4w/lSUe+3be5PkkjdYNbwA4nUkHZ3J7G6mZls CUuSPXxVNpzgnTJkJyx6pcWtIwMHe0a7gvAeVwu2ddCGSxiFyMqDXJvS8qYjYDgJt9 1HW3sDhHDfkJPJqJ4beFmeBW+7KVeMURYf+f7sveglgtD2EZmYctQc2ubNmGnhu3D2 TGhaDjjZy1qTw== Date: Tue, 29 Sep 2026 07:58:56 -1000 Message-ID: From: Tejun Heo To: Liz Fong-Jones Cc: Christian Brauner , Jan Kara , Alexander Viro , Jens Axboe , Andrew Morton , Johannes Weiner , Roman Gushchin , Shakeel Butt , Xin Yin , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, ian@honeycomb.io Subject: Re: [PATCH v3 1/2] writeback: let foreign flushes reach dying cgwbs In-Reply-To: <20260928-wb-dying-cgwb-flush-v3-1-e35374884667@honeycomb.io> References: <20260928-wb-dying-cgwb-flush-v3-0-e35374884667@honeycomb.io> <20260928-wb-dying-cgwb-flush-v3-1-e35374884667@honeycomb.io> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 8637A1C0008 X-Rspam-User: X-Stat-Signature: xk3uandequifnsmfrow9ruj77ay71iim X-HE-Tag: 1790704738-759801 X-HE-Meta: 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 Iv1VCTTC ESa9sSl25tR0/vEPQc+O0MOQ86KRzUxdhlvIzStrHVCAqALB/sxfViHorxq479W1m6LLmHiDwJs9Wn4vkPgMP1zKim/jzLCxqtdiYjn15JIcsELSfJ2McOSg72u1jrSlE+IypOBOrDYxBdzZ40K69sAVnuAgPyvXEpGqmlFId6Utl2qqV2w3EJ9HJP2dad8ZNjHQ7cYssvL5WP94rlck8f8NTna587ex6wc1mZRQjUqdbVeDaHiUEij6DZJ0VB8UqeO30 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hello, Liz. On Tue, Sep 29, 2026 at 01:40:49AM +0000, Liz Fong-Jones wrote: > if (test_bit(WB_registered, &bdi->wb.state) && > blkcg_cgwb_list->next && memcg_cgwb_list->next) { > - /* we might have raced another instance of this function */ > - ret = radix_tree_insert(&bdi->cgwb_tree, memcg_css->id, wb); > + /* > + * We might have raced another instance of this function. A > + * dying wb keeps its slot until released; take it over. > + */ > + slot = radix_tree_lookup_slot(&bdi->cgwb_tree, memcg_css->id); > + if (!slot) { > + ret = radix_tree_insert(&bdi->cgwb_tree, memcg_css->id, wb); > + } else { > + old_wb = radix_tree_deref_slot_protected(slot, &cgwb_lock); > + if (wb_dying(old_wb)) { > + radix_tree_replace_slot(&bdi->cgwb_tree, slot, wb); This can also replace the wb of a removed memcg: 1. A cgroup with io enabled is removed. Killing its io css makes cgroup_get_e_css() return the parent's right away, but memcg_cgwb_list->next stays set until wb_memcg_offline(). 2. In between, wb_get_create() for the memcg, e.g. from __inode_attach_wb() or inode_switch_wbs(), kills the dirty wb on blkcg mismatch and a new wb takes over its slot. 3. wb_memcg_offline() kills the new wb. Foreign flushes for the memcg now find the new wb while the dirty inodes stay on the old one, as css_is_dying() keeps wbc_attach_and_unlock_inode() from switching them, so the stall comes back. Can you also fail the link when the memcg is dying? if (test_bit(WB_registered, &bdi->wb.state) && blkcg_cgwb_list->next && memcg_cgwb_list->next && !css_is_dying(memcg_css)) { CSS_DYING is set before the io css is killed. Testing it here, after the blkcg lookup and under cgwb_lock, catches every removal that could have made the old wb replaceable. Thanks. -- tejun