From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f199.google.com (mail-qk0-f199.google.com [209.85.220.199]) by kanga.kvack.org (Postfix) with ESMTP id E6DB26B0006 for ; Tue, 10 Apr 2018 14:19:40 -0400 (EDT) Received: by mail-qk0-f199.google.com with SMTP id j130so8980298qke.13 for ; Tue, 10 Apr 2018 11:19:40 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id m63si53933qkf.474.2018.04.10.11.19.39 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 10 Apr 2018 11:19:39 -0700 (PDT) Date: Tue, 10 Apr 2018 21:19:31 +0300 From: "Michael S. Tsirkin" Subject: Re: [PATCH v29 1/4] mm: support reporting free page blocks Message-ID: <20180410211719-mutt-send-email-mst@kernel.org> References: <1522031994-7246-1-git-send-email-wei.w.wang@intel.com> <1522031994-7246-2-git-send-email-wei.w.wang@intel.com> <20180326142254.c4129c3a54ade686ee2a5e21@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180326142254.c4129c3a54ade686ee2a5e21@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Wei Wang , virtio-dev@lists.oasis-open.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-mm@kvack.org, mhocko@kernel.org, pbonzini@redhat.com, liliang.opensource@gmail.com, yang.zhang.wz@gmail.com, quan.xu0@gmail.com, nilal@redhat.com, riel@redhat.com, huangzhichao@huawei.com On Mon, Mar 26, 2018 at 02:22:54PM -0700, Andrew Morton wrote: > On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang wrote: > > > This patch adds support to walk through the free page blocks in the > > system and report them via a callback function. Some page blocks may > > leave the free list after zone->lock is released, so it is the caller's > > responsibility to either detect or prevent the use of such pages. > > > > One use example of this patch is to accelerate live migration by skipping > > the transfer of free pages reported from the guest. A popular method used > > by the hypervisor to track which part of memory is written during live > > migration is to write-protect all the guest memory. So, those pages that > > are reported as free pages but are written after the report function > > returns will be captured by the hypervisor, and they will be added to the > > next round of memory transfer. > > > > ... > > > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -4912,6 +4912,102 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask) > > show_swap_cache_info(); > > } > > > > +/* > > + * Walk through a free page list and report the found pfn range via the > > + * callback. > > + * > > + * Return 0 if it completes the reporting. Otherwise, return the non-zero > > + * value returned from the callback. > > + */ > > +static int walk_free_page_list(void *opaque, > > + struct zone *zone, > > + int order, > > + enum migratetype mt, > > + int (*report_pfn_range)(void *, > > + unsigned long, > > + unsigned long)) > > +{ > > + struct page *page; > > + struct list_head *list; > > + unsigned long pfn, flags; > > + int ret = 0; > > + > > + spin_lock_irqsave(&zone->lock, flags); > > + list = &zone->free_area[order].free_list[mt]; > > + list_for_each_entry(page, list, lru) { > > + pfn = page_to_pfn(page); > > + ret = report_pfn_range(opaque, pfn, 1 << order); > > + if (ret) > > + break; > > + } > > + spin_unlock_irqrestore(&zone->lock, flags); > > + > > + return ret; > > +} > > + > > +/** > > + * walk_free_mem_block - Walk through the free page blocks in the system > > + * @opaque: the context passed from the caller > > + * @min_order: the minimum order of free lists to check > > + * @report_pfn_range: the callback to report the pfn range of the free pages > > + * > > + * If the callback returns a non-zero value, stop iterating the list of free > > + * page blocks. Otherwise, continue to report. > > + * > > + * Please note that there are no locking guarantees for the callback and > > + * that the reported pfn range might be freed or disappear after the > > + * callback returns so the caller has to be very careful how it is used. > > + * > > + * The callback itself must not sleep or perform any operations which would > > + * require any memory allocations directly (not even GFP_NOWAIT/GFP_ATOMIC) > > + * or via any lock dependency. It is generally advisable to implement > > + * the callback as simple as possible and defer any heavy lifting to a > > + * different context. > > + * > > + * There is no guarantee that each free range will be reported only once > > + * during one walk_free_mem_block invocation. > > + * > > + * pfn_to_page on the given range is strongly discouraged and if there is > > + * an absolute need for that make sure to contact MM people to discuss > > + * potential problems. > > + * > > + * The function itself might sleep so it cannot be called from atomic > > + * contexts. > > I don't see how walk_free_mem_block() can sleep. > > > + * In general low orders tend to be very volatile and so it makes more > > + * sense to query larger ones first for various optimizations which like > > + * ballooning etc... This will reduce the overhead as well. > > + * > > + * Return 0 if it completes the reporting. Otherwise, return the non-zero > > + * value returned from the callback. > > + */ > > +int walk_free_mem_block(void *opaque, > > + int min_order, > > + int (*report_pfn_range)(void *opaque, > > + unsigned long pfn, > > + unsigned long num)) > > +{ > > + struct zone *zone; > > + int order; > > + enum migratetype mt; > > + int ret; > > + > > + for_each_populated_zone(zone) { > > + for (order = MAX_ORDER - 1; order >= min_order; order--) { > > + for (mt = 0; mt < MIGRATE_TYPES; mt++) { > > + ret = walk_free_page_list(opaque, zone, > > + order, mt, > > + report_pfn_range); > > + if (ret) > > + return ret; > > + } > > + } > > + } > > + > > + return 0; > > +} > > +EXPORT_SYMBOL_GPL(walk_free_mem_block); > > This looks like it could take a long time. Will we end up needing to > add cond_resched() in there somewhere? Andrew, were your questions answered? If yes could I bother you for an ack on this? > > static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref) > > { > > zoneref->zone = zone; > > -- > > 2.7.4