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[198.145.29.99]) by mx.google.com with ESMTPS id u46si50288426pgn.578.2019.08.07.12.00.39 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 07 Aug 2019 12:00:39 -0700 (PDT) Received-SPF: pass (google.com: domain of akpm@linux-foundation.org designates 198.145.29.99 as permitted sender) client-ip=198.145.29.99; Authentication-Results: mx.google.com; dkim=pass header.i=@kernel.org header.s=default header.b=YlgkEarD; spf=pass (google.com: domain of akpm@linux-foundation.org designates 198.145.29.99 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org Received: from akpm3.svl.corp.google.com (unknown [104.133.8.65]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 8554B21EF2; Wed, 7 Aug 2019 19:00:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1565204438; bh=H8GQL47oZOjHGMHt6mc7yPh3DyXvvruf/VfI2fdq1XA=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=YlgkEarDGN0iPkWPaV/pbw5ncD7rQmx4yJoKPQ9nlm4ouA1+CTa9RxYW+98Vy24gI qJ6E09hPzeZhxEQ6ywpXQDT2SLGtRgRNv3zVoSJY1Fz2VgEHGrOKuc89UnlAoTAMXS uihlSf1Qg+aa309BG0I7DD0CeJ2fpdTb3Y3u8rXk= Date: Wed, 7 Aug 2019 12:00:37 -0700 From: Andrew Morton To: Tejun Heo Cc: axboe@kernel.dk, jack@suse.cz, hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, guro@fb.com Subject: Re: [PATCH 2/4] bdi: Add bdi->id Message-Id: <20190807120037.72018c136db40e88d89c05d1@linux-foundation.org> In-Reply-To: <20190807183151.GM136335@devbig004.ftw2.facebook.com> References: <20190803140155.181190-1-tj@kernel.org> <20190803140155.181190-3-tj@kernel.org> <20190806160102.11366694af6b56d9c4ca6ea3@linux-foundation.org> <20190807183151.GM136335@devbig004.ftw2.facebook.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 7 Aug 2019 11:31:51 -0700 Tejun Heo wrote: > Hello, > > On Tue, Aug 06, 2019 at 04:01:02PM -0700, Andrew Morton wrote: > > On Sat, 3 Aug 2019 07:01:53 -0700 Tejun Heo wrote: > > > There currently is no way to universally identify and lookup a bdi > > > without holding a reference and pointer to it. This patch adds an > > > non-recycling bdi->id and implements bdi_get_by_id() which looks up > > > bdis by their ids. This will be used by memcg foreign inode flushing. > > > > Why is the id non-recycling? Presumably to address some > > lifetime/lookup issues, but what are they? > > The ID by itself is used to point to the bdi from cgroup and idr > recycles really aggressively. Combined with, for example, loop device > based containers, stale pointing can become pretty common. We're > having similar issues with cgroup IDs. OK, but why is recycling a problem? For example, file descriptors recycle as aggressively as is possible, and that doesn't cause any trouble. Presumably recycling is a problem with cgroups because of some sort of stale reference problem?