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 B2DE2440905 for ; Fri, 14 Jul 2017 15:17:29 -0400 (EDT) Received: by mail-qk0-f199.google.com with SMTP id g6so10851292qkf.15 for ; Fri, 14 Jul 2017 12:17:29 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id w64si8389380qkd.327.2017.07.14.12.17.28 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 14 Jul 2017 12:17:28 -0700 (PDT) Date: Fri, 14 Jul 2017 22:17:13 +0300 From: "Michael S. Tsirkin" Subject: Re: [PATCH v12 6/8] mm: support reporting free page blocks Message-ID: <20170714181523-mutt-send-email-mst@kernel.org> References: <1499863221-16206-1-git-send-email-wei.w.wang@intel.com> <1499863221-16206-7-git-send-email-wei.w.wang@intel.com> <20170714123023.GA2624@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170714123023.GA2624@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Wei Wang , linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, kvm@vger.kernel.org, linux-mm@kvack.org, david@redhat.com, cornelia.huck@de.ibm.com, akpm@linux-foundation.org, mgorman@techsingularity.net, aarcange@redhat.com, amit.shah@redhat.com, pbonzini@redhat.com, liliang.opensource@gmail.com, virtio-dev@lists.oasis-open.org, yang.zhang.wz@gmail.com, quan.xu@aliyun.com On Fri, Jul 14, 2017 at 02:30:23PM +0200, Michal Hocko wrote: > On Wed 12-07-17 20:40:19, Wei Wang wrote: > > This patch adds support for reporting blocks of pages on the free list > > specified by the caller. > > > > As pages can leave the free list during this call or immediately > > afterwards, they are not guaranteed to be free after the function > > returns. The only guarantee this makes is that the page was on the free > > list at some point in time after the function has been invoked. > > > > Therefore, it is not safe for caller to use any pages on the returned > > block or to discard data that is put there after the function returns. > > However, it is safe for caller to discard data that was in one of these > > pages before the function was invoked. > > I do not understand what is the point of such a function and how it is > used because the patch doesn't give us any user (I haven't checked other > patches yet). > > But just from the semantic point of view this sounds like a horrible > idea. The only way to get a free block of pages is to call the page > allocator. I am tempted to give it Nack right on those grounds but I > would like to hear more about what you actually want to achieve. Basically it's a performance hint to the hypervisor. For example, these pages would be good candidates to move around as they are not mapped into any running applications. As such, it's important not to slow down other parts of the system too much - otherwise we are speeding up one part of the system while we slow down other parts of it, which is why it's trying to drop the lock as soon a possible. As long as hypervisor does not assume it can drop these pages, and as long it's correct in most cases. we are OK even if the hint is slightly wrong because hypervisor notifications are racing with allocations. There are patches to do more tricks - if hypervisor tracks all memory writes we might actually use this hint to discard data - but that is just implementation detail. > > Signed-off-by: Wei Wang > > Signed-off-by: Liang Li > > --- > > include/linux/mm.h | 5 +++ > > mm/page_alloc.c | 96 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > 2 files changed, 101 insertions(+) > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > index 46b9ac5..76cb433 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > @@ -1835,6 +1835,11 @@ extern void free_area_init_node(int nid, unsigned long * zones_size, > > unsigned long zone_start_pfn, unsigned long *zholes_size); > > extern void free_initmem(void); > > > > +#if IS_ENABLED(CONFIG_VIRTIO_BALLOON) > > +extern int report_unused_page_block(struct zone *zone, unsigned int order, > > + unsigned int migratetype, > > + struct page **page); > > +#endif > > /* > > * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK) > > * into the buddy system. The freed pages will be poisoned with pattern > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > index 64b7d82..8b3c9dd 100644 > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -4753,6 +4753,102 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask) > > show_swap_cache_info(); > > } > > > > +#if IS_ENABLED(CONFIG_VIRTIO_BALLOON) > > + > > +/* > > + * Heuristically get a page block in the system that is unused. > > + * It is possible that pages from the page block are used immediately after > > + * report_unused_page_block() returns. It is the caller's responsibility > > + * to either detect or prevent the use of such pages. > > + * > > + * The free list to check: zone->free_area[order].free_list[migratetype]. > > + * > > + * If the caller supplied page block (i.e. **page) is on the free list, offer > > + * the next page block on the list to the caller. Otherwise, offer the first > > + * page block on the list. > > + * > > + * Note: it is not safe for caller to use any pages on the returned > > + * block or to discard data that is put there after the function returns. > > + * However, it is safe for caller to discard data that was in one of these > > + * pages before the function was invoked. > > + * > > + * Return 0 when a page block is found on the caller specified free list. > > + */ > > +int report_unused_page_block(struct zone *zone, unsigned int order, > > + unsigned int migratetype, struct page **page) > > +{ > > + struct zone *this_zone; > > + struct list_head *this_list; > > + int ret = 0; > > + unsigned long flags; > > + > > + /* Sanity check */ > > + if (zone == NULL || page == NULL || order >= MAX_ORDER || > > + migratetype >= MIGRATE_TYPES) > > + return -EINVAL; > > + > > + /* Zone validity check */ > > + for_each_populated_zone(this_zone) { > > + if (zone == this_zone) > > + break; > > + } > > + > > + /* Got a non-existent zone from the caller? */ > > + if (zone != this_zone) > > + return -EINVAL; > > Huh, what do you check for here? Why don't you simply > populated_zone(zone)? > > > + > > + spin_lock_irqsave(&this_zone->lock, flags); > > + > > + this_list = &zone->free_area[order].free_list[migratetype]; > > + if (list_empty(this_list)) { > > + *page = NULL; > > + ret = 1; > > + goto out; > > + } > > + > > + /* The caller is asking for the first free page block on the list */ > > + if ((*page) == NULL) { > > + *page = list_first_entry(this_list, struct page, lru); > > + ret = 0; > > + goto out; > > + } > > + > > + /* > > + * The page block passed from the caller is not on this free list > > + * anymore (e.g. a 1MB free page block has been split). In this case, > > + * offer the first page block on the free list that the caller is > > + * asking for. > > + */ > > + if (PageBuddy(*page) && order != page_order(*page)) { > > + *page = list_first_entry(this_list, struct page, lru); > > + ret = 0; > > + goto out; > > + } > > + > > + /* > > + * The page block passed from the caller has been the last page block > > + * on the list. > > + */ > > + if ((*page)->lru.next == this_list) { > > + *page = NULL; > > + ret = 1; > > + goto out; > > + } > > + > > + /* > > + * Finally, fall into the regular case: the page block passed from the > > + * caller is still on the free list. Offer the next one. > > + */ > > + *page = list_next_entry((*page), lru); > > + ret = 0; > > +out: > > + spin_unlock_irqrestore(&this_zone->lock, flags); > > + return ret; > > +} > > +EXPORT_SYMBOL(report_unused_page_block); > > + > > +#endif > > + > > static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref) > > { > > zoneref->zone = zone; > > -- > > 2.7.4 > > > > -- > > To unsubscribe, send a message with 'unsubscribe linux-mm' in > > the body to majordomo@kvack.org. For more info on Linux MM, > > see: http://www.linux-mm.org/ . > > Don't email: email@kvack.org > > -- > Michal Hocko > SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org