From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757951AbdLRIDZ (ORCPT ); Mon, 18 Dec 2017 03:03:25 -0500 Received: from mga04.intel.com ([192.55.52.120]:54748 "EHLO mga04.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750805AbdLRIDY (ORCPT ); Mon, 18 Dec 2017 03:03:24 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.45,421,1508828400"; d="scan'208";a="16897861" Message-ID: <5A3776C8.1040801@intel.com> Date: Mon, 18 Dec 2017 16:05:28 +0800 From: Wei Wang User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Tetsuo Handa , willy@infradead.org 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, 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 References: <5A34F193.5040700@intel.com> <201712162028.FEB87079.FOJFMQHVOSLtFO@I-love.SAKURA.ne.jp> <5A35FF89.8040500@intel.com> <201712171921.IBB30790.VOOOFMQHFSLFJt@I-love.SAKURA.ne.jp> <286AC319A985734F985F78AFA26841F739387B68@shsmsx102.ccr.corp.intel.com> <201712180016.GHD34301.MQOLOFFJHOVFtS@I-love.SAKURA.ne.jp> In-Reply-To: <201712180016.GHD34301.MQOLOFFJHOVFtS@I-love.SAKURA.ne.jp> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/17/2017 11:16 PM, Tetsuo Handa wrote: > Wang, Wei W wrote: >>> Wei Wang wrote: >>>>> But passing GFP_NOWAIT means that we can handle allocation failure. >>>>> There is no need to use preload approach when we can handle allocation failure. >>>> I think the reason we need xb_preload is because radix tree insertion >>>> needs the memory being preallocated already (it couldn't suffer from >>>> memory failure during the process of inserting, probably because >>>> handling the failure there isn't easy, Matthew may know the backstory >>>> of >>>> this) >>> According to https://lwn.net/Articles/175432/ , I think that preloading is >>> needed only when failure to insert an item into a radix tree is a significant >>> problem. >>> That is, when failure to insert an item into a radix tree is not a problem, I >>> think that we don't need to use preloading. >> It also mentions that the preload attempts to allocate sufficient memory to *guarantee* that the next radix tree insertion cannot fail. >> >> If we check radix_tree_node_alloc(), the comments there says "this assumes that the caller has performed appropriate preallocation". > If you read what radix_tree_node_alloc() is doing, you will find that > radix_tree_node_alloc() returns NULL when memory allocation failed. > > I think that "this assumes that the caller has performed appropriate preallocation" > means "The caller has to perform appropriate preallocation if the caller does not > want radix_tree_node_alloc() to return NULL". For the radix tree, I agree that we may not need preload. But ida_bitmap, which the xbitmap is based on, is allocated via preload, so I think we cannot bypass preload, otherwise, we get no ida_bitmap to use. Best, Wei