From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39456) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eQ9QS-0000eG-Qy for qemu-devel@nongnu.org; Sat, 16 Dec 2017 05:10:37 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eQ9QO-0002C9-S0 for qemu-devel@nongnu.org; Sat, 16 Dec 2017 05:10:36 -0500 Received: from mga04.intel.com ([192.55.52.120]:49105) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eQ9QO-0002Ae-HE for qemu-devel@nongnu.org; Sat, 16 Dec 2017 05:10:32 -0500 Message-ID: <5A34F193.5040700@intel.com> Date: Sat, 16 Dec 2017 18:12:35 +0800 From: Wei Wang MIME-Version: 1.0 References: <1513079759-14169-1-git-send-email-wei.w.wang@intel.com> <1513079759-14169-4-git-send-email-wei.w.wang@intel.com> <20171215184256.GA27160@bombadil.infradead.org> In-Reply-To: <20171215184256.GA27160@bombadil.infradead.org> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v19 3/7] xbitmap: add more operations List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Matthew Wilcox Cc: virtio-dev@lists.oasis-open.org, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-mm@kvack.org, mst@redhat.com, mhocko@kernel.org, akpm@linux-foundation.org, mawilcox@microsoft.com, david@redhat.com, penguin-kernel@I-love.SAKURA.ne.jp, cornelia.huck@de.ibm.com, mgorman@techsingularity.net, aarcange@redhat.com, amit.shah@redhat.com, pbonzini@redhat.com, liliang.opensource@gmail.com, yang.zhang.wz@gmail.com, quan.xu@aliyun.com, nilal@redhat.com, riel@redhat.com On 12/16/2017 02:42 AM, Matthew Wilcox wrote: > On Tue, Dec 12, 2017 at 07:55:55PM +0800, Wei Wang wrote: >> +int xb_preload_and_set_bit(struct xb *xb, unsigned long bit, gfp_t gfp); > I'm struggling to understand when one would use this. The xb_ API > requires you to handle your own locking. But specifying GFP flags > here implies you can sleep. So ... um ... there's no locking? In the regular use cases, people would do xb_preload() before taking the lock, and the xb_set/clear within the lock. In the virtio-balloon usage, we have a large number of bits to set with the balloon_lock being held (we're not unlocking for each bit), so we used the above wrapper to do preload and set within the balloon_lock, and passed in GFP_NOWAIT to avoid sleeping. Probably we can change to put this wrapper implementation to virtio-balloon, since it would not be useful for the regular cases. Best, Wei