linux-kernel - Re: [PATCH v13 4/5] mm: support reporting free page blocks

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Message-ID: <20170803135047.GV12521@dhcp22.suse.cz>
Date:   Thu, 3 Aug 2017 15:50:47 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, mst@...hat.com, mawilcox@...rosoft.com,
        akpm@...ux-foundation.org, virtio-dev@...ts.oasis-open.org,
        david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com
Subject: Re: [PATCH v13 4/5] mm: support reporting free page blocks

On Thu 03-08-17 21:17:25, Wei Wang wrote:
> On 08/03/2017 08:41 PM, Michal Hocko wrote:
> >On Thu 03-08-17 20:11:58, Wei Wang wrote:
> >>On 08/03/2017 07:28 PM, Michal Hocko wrote:
> >>>On Thu 03-08-17 19:27:19, Wei Wang wrote:
> >>>>On 08/03/2017 06:44 PM, Michal Hocko wrote:
> >>>>>On Thu 03-08-17 18:42:15, Wei Wang wrote:
> >>>>>>On 08/03/2017 05:11 PM, Michal Hocko wrote:
> >>>>>>>On Thu 03-08-17 14:38:18, Wei Wang wrote:
> >>>>>[...]
> >>>>>>>>+static int report_free_page_block(struct zone *zone, unsigned int order,
> >>>>>>>>+				  unsigned int migratetype, struct page **page)
> >>>>>>>This is just too ugly and wrong actually. Never provide struct page
> >>>>>>>pointers outside of the zone->lock. What I've had in mind was to simply
> >>>>>>>walk free lists of the suitable order and call the callback for each one.
> >>>>>>>Something as simple as
> >>>>>>>
> >>>>>>>	for (i = 0; i < MAX_NR_ZONES; i++) {
> >>>>>>>		struct zone *zone = &pgdat->node_zones[i];
> >>>>>>>
> >>>>>>>		if (!populated_zone(zone))
> >>>>>>>			continue;
> >>>>>>>		spin_lock_irqsave(&zone->lock, flags);
> >>>>>>>		for (order = min_order; order < MAX_ORDER; ++order) {
> >>>>>>>			struct free_area *free_area = &zone->free_area[order];
> >>>>>>>			enum migratetype mt;
> >>>>>>>			struct page *page;
> >>>>>>>
> >>>>>>>			if (!free_area->nr_pages)
> >>>>>>>				continue;
> >>>>>>>
> >>>>>>>			for_each_migratetype_order(order, mt) {
> >>>>>>>				list_for_each_entry(page,
> >>>>>>>						&free_area->free_list[mt], lru) {
> >>>>>>>
> >>>>>>>					pfn = page_to_pfn(page);
> >>>>>>>					visit(opaque2, prn, 1<<order);
> >>>>>>>				}
> >>>>>>>			}
> >>>>>>>		}
> >>>>>>>
> >>>>>>>		spin_unlock_irqrestore(&zone->lock, flags);
> >>>>>>>	}
> >>>>>>>
> >>>>>>>[...]
> >>>>>>I think the above would take the lock for too long time. That's why we
> >>>>>>prefer to take one free page block each time, and taking it one by one
> >>>>>>also doesn't make a difference, in terms of the performance that we
> >>>>>>need.
> >>>>>I think you should start with simple approach and impove incrementally
> >>>>>if this turns out to be not optimal. I really detest taking struct pages
> >>>>>outside of the lock. You never know what might happen after the lock is
> >>>>>dropped. E.g. can you race with the memory hotremove?
> >>>>The caller won't use pages returned from the function, so I think there
> >>>>shouldn't be an issue or race if the returned pages are used (i.e. not free
> >>>>anymore) or simply gone due to hotremove.
> >>>No, this is just too error prone. Consider that struct page pointer
> >>>itself could get invalid in the meantime. Please always keep robustness
> >>>in mind first. Optimizations are nice but it is even not clear whether
> >>>the simple variant will cause any problems.
> >>
> >>how about this:
> >>
> >>for_each_populated_zone(zone) {
> >>               for_each_migratetype_order_decend(min_order, order, type) {
> >>                     do {
> >>      =>                  spin_lock_irqsave(&zone->lock, flags);
> >>                         ret = report_free_page_block(zone, order, type,
> >>                              &page)) {
> >>                                pfn = page_to_pfn(page);
> >>                                nr_pages = 1 << order;
> >>                                visit(opaque1, pfn, nr_pages);
> >>                          }
> >>      => spin_unlock_irqrestore(&zone->lock, flags);
> >>                     } while (!ret)
> >>}
> >>
> >>In this way, we can still keep the lock granularity at one free page block
> >>while having the struct page operated under the lock.
> >How can you continue iteration of free_list after the lock has been
> >dropped?
> 
> report_free_page_block() has handled all the possible cases after the lock
> is
> dropped. For example, if the previous reported page has not been on the free
> list, then the first node from the list of this order will be given. This is
> because
> page allocation takes page blocks from the head to end, for example:
> 
> 1,2,3,4,5,6
> if the previous reported free block is 2, when we give 2 to the report
> function
> to get the next page block, and find 1,2,3 have all gone, it will report 4,
> which
> is the head of the free list.

As I've said earlier. Start simple optimize incrementally with some
numbers to justify a more subtle code.
-- 
Michal Hocko
SUSE Labs