From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f71.google.com (mail-wm0-f71.google.com [74.125.82.71]) by kanga.kvack.org (Postfix) with ESMTP id BBD00440905 for ; Fri, 14 Jul 2017 08:54:35 -0400 (EDT) Received: by mail-wm0-f71.google.com with SMTP id 62so9062039wmw.13 for ; Fri, 14 Jul 2017 05:54:35 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id v24si6336636wrd.208.2017.07.14.05.54.34 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 14 Jul 2017 05:54:34 -0700 (PDT) Date: Fri, 14 Jul 2017 14:54:30 +0200 From: Michal Hocko Subject: Re: [PATCH v12 6/8] mm: support reporting free page blocks Message-ID: <20170714125429.GP2618@dhcp22.suse.cz> 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: Wei Wang Cc: 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, 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 14-07-17 14:30:23, 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. OK, so I gave it another thought and giving a page which is still on the free list to a random module is just a free ticket to a disaster. Nacked-by: Michal Hocko > > > 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 -- 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