linux-kernel - Re: [PATCH v14 4/5] mm: support reporting free page blocks

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Date:   Mon, 21 Aug 2017 08:18:12 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Wei Wang <wei.w.wang@...el.com>, virtio-dev@...ts.oasis-open.org,
        linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        mawilcox@...rosoft.com, david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com, willy@...radead.org,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com
Subject: Re: [PATCH v14 4/5] mm: support reporting free page blocks

On Fri 18-08-17 20:23:05, Michael S. Tsirkin wrote:
> On Thu, Aug 17, 2017 at 11:26:55AM +0800, Wei Wang wrote:
[...]
> > +void walk_free_mem_block(void *opaque1,
> > +			 unsigned int min_order,
> > +			 void (*visit)(void *opaque2,
> 
> You can just avoid opaque2 completely I think, then opaque1 can
> be renamed opaque.
> 
> > +				       unsigned long pfn,
> > +				       unsigned long nr_pages))
> > +{
> > +	struct zone *zone;
> > +	struct page *page;
> > +	struct list_head *list;
> > +	unsigned int order;
> > +	enum migratetype mt;
> > +	unsigned long pfn, flags;
> > +
> > +	for_each_populated_zone(zone) {
> > +		for (order = MAX_ORDER - 1;
> > +		     order < MAX_ORDER && order >= min_order; order--) {
> > +			for (mt = 0; mt < MIGRATE_TYPES; mt++) {
> > +				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);
> > +					visit(opaque1, pfn, 1 << order);
> 
> My only concern here is inability of callback to
> 1. break out of list
> 2. remove page from the list

As I've said before this has to be a read only API. You cannot simply
fiddle with the page allocator internals under its feet.

> So I would make the callback bool, and I would use
> list_for_each_entry_safe.

If a bool would tell to break out of the loop then I agree. This sounds
useful.
-- 
Michal Hocko
SUSE Labs