From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757597AbdLQWSt (ORCPT ); Sun, 17 Dec 2017 17:18:49 -0500 Received: from bombadil.infradead.org ([65.50.211.133]:47720 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752207AbdLQWSr (ORCPT ); Sun, 17 Dec 2017 17:18:47 -0500 Date: Sun, 17 Dec 2017 14:18:42 -0800 From: Matthew Wilcox To: "Wang, Wei W" Cc: Tetsuo Handa , "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" , "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" Subject: Re: [PATCH v19 3/7] xbitmap: add more operations Message-ID: <20171217221842.GA6683@bombadil.infradead.org> References: <5A311C5E.7000304@intel.com> <201712132316.EJJ57332.MFOSJHOFFVLtQO@I-love.SAKURA.ne.jp> <5A31F445.6070504@intel.com> <201712150129.BFC35949.FFtFOLSOJOQHVM@I-love.SAKURA.ne.jp> <20171214181219.GA26124@bombadil.infradead.org> <201712160121.BEJ26052.HOFFOOQFMLtSVJ@I-love.SAKURA.ne.jp> <20171215184915.GB27160@bombadil.infradead.org> <20171215192203.GC27160@bombadil.infradead.org> <286AC319A985734F985F78AFA26841F739387C1D@shsmsx102.ccr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <286AC319A985734F985F78AFA26841F739387C1D@shsmsx102.ccr.corp.intel.com> User-Agent: Mutt/1.9.1 (2017-09-22) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Dec 17, 2017 at 01:47:21PM +0000, Wang, Wei W wrote: > On Saturday, December 16, 2017 3:22 AM, Matthew Wilcox wrote: > > On Fri, Dec 15, 2017 at 10:49:15AM -0800, Matthew Wilcox wrote: > > > Here's the API I'm looking at right now. The user need take no lock; > > > the locking (spinlock) is handled internally to the implementation. > > Another place I saw your comment " The xb_ API requires you to handle your own locking" which seems conflict with the above "the user need take no lock". > Doesn't the caller need a lock to avoid concurrent accesses to the ida bitmap? Yes, the xb_ implementation requires you to handle your own locking. The xbit_ API that I'm proposing will take care of the locking for you. There's also no preallocation in the API. > We'll change it to "bool xb_find_set(.., unsigned long *result)", returning false indicates no "1" bit is found. I put a replacement proposal in the next paragraph: bool xbit_find_set(struct xbitmap *, unsigned long *start, unsigned long max); Maybe 'start' is the wrong name for that parameter. Let's call it 'bit'. It's both "where to start" and "first bit found". > > - xbit_clear() can't return an error. Neither can xbit_zero(). > > I found the current xbit_clear implementation only returns 0, and there isn't an error to be returned from this function. In this case, is it better to make the function "void"? Yes, I think so. My only qualm is that I've been considering optimising the memory consumption when an entire 1024-bit chunk is full; instead of keeping a pointer to a 128-byte entry full of ones, store a special value in the radix tree which means "every bit is set". The downside is that we then have to pass GFP flags to xbit_clear() and xbit_zero(), and they can fail. It's not clear to me whether that's a good tradeoff. > Are you suggesting to rename the current xb_ APIs to the above xbit_ names (with parameter changes)? > > Why would we need xbit_alloc, which looks like ida_get_new, I think set/clear should be adequate to the current usages. I'm intending on replacing the xb_ and ida_ implementations with this one. It removes the preload API which makes it easier to use, and it handles the locking for you. But I need to get the XArray (which replaces the radix tree) finished first.