From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f200.google.com (mail-pf0-f200.google.com [209.85.192.200]) by kanga.kvack.org (Postfix) with ESMTP id C09F86B0006 for ; Mon, 26 Mar 2018 17:22:57 -0400 (EDT) Received: by mail-pf0-f200.google.com with SMTP id j12so11692110pff.18 for ; Mon, 26 Mar 2018 14:22:57 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id f69si12117212pfd.196.2018.03.26.14.22.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 26 Mar 2018 14:22:56 -0700 (PDT) Date: Mon, 26 Mar 2018 14:22:54 -0700 From: Andrew Morton Subject: Re: [PATCH v29 1/4] mm: support reporting free page blocks Message-Id: <20180326142254.c4129c3a54ade686ee2a5e21@linux-foundation.org> In-Reply-To: <1522031994-7246-2-git-send-email-wei.w.wang@intel.com> References: <1522031994-7246-1-git-send-email-wei.w.wang@intel.com> <1522031994-7246-2-git-send-email-wei.w.wang@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Wei Wang Cc: virtio-dev@lists.oasis-open.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-mm@kvack.org, mst@redhat.com, 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, 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? > static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref) > { > zoneref->zone = zone; > -- > 2.7.4