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

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Date:   Thu, 10 Aug 2017 09:53:50 +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: [virtio-dev] Re: [PATCH v13 4/5] mm: support reporting free page
 blocks

On Thu 10-08-17 15:38:34, Wei Wang wrote:
> On 08/10/2017 03:05 PM, Michal Hocko wrote:
> >On Tue 08-08-17 14:34:25, Wei Wang wrote:
> >>On 08/08/2017 02:12 PM, Wei Wang wrote:
> >>>On 08/03/2017 05:11 PM, Michal Hocko wrote:
> >>>>On Thu 03-08-17 14:38:18, Wei Wang wrote:
> >>>>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;
> >>>Can we directly use for_each_populated_zone(zone) here?
> >yes, my example couldn't because I was still assuming per-node API
> >
> >>>>spin_lock_irqsave(&zone->lock, flags);
> >>>>        for (order = min_order; order < MAX_ORDER; ++order) {
> >>>
> >>>This appears to be covered by for_each_migratetype_order(order, mt) below.
> >yes but
> >#define for_each_migratetype_order(order, type) \
> >	for (order = 0; order < MAX_ORDER; order++) \
> >		for (type = 0; type < MIGRATE_TYPES; type++)
> >
> >so you would have to skip orders < min_order
> 
> Yes, that's why we have a new macro
> 
> #define for_each_migratetype_order_decend(min_order, order, type) \
>  for (order = MAX_ORDER - 1; order < MAX_ORDER && order >= min_order; \
>  order--) \
>     for (type = 0; type < MIGRATE_TYPES; type++)
> 
> If you don't like the macro, we can also directly use it in the code.
> 
> I think it would be better to report the larger free page block first, since
> the callback has an opportunity (though just a theoretical possibility, good
> to
> take that into consideration if possible) to skip reporting the given free
> page
> block to the hypervisor as the ring gets full. Losing the small block is
> better
> than losing the larger one, in terms of the optimization work.

I see. But I think this is so specialized that opencoding the macro
would be easier to read.

-- 
Michal Hocko
SUSE Labs