Re: [PATCH] mm: skip the page buddy block instead of one page

[Minchan Kim <minchan@kernel.org>]

Hi Mel,

On Thu, Aug 15, 2013 at 12:30:19PM +0100, Mel Gorman wrote:
> On Thu, Aug 15, 2013 at 01:17:55PM +0900, Minchan Kim wrote:
> > Hello,
> > 
> 
> Well, this thread managed to get out of control for no good reason!
> 
> > > > <SNIP>
> > > > So, what's the result by that?
> > > > As I said, it's just skipping (pageblock_nr_pages -1) at worst case
> > > 
> > > Hi Minchan,
> > > I mean if the private is set to a large number, it will skip 2^private 
> > > pages, not (pageblock_nr_pages -1). I find somewhere will use page->private, 
> > > such as fs. Here is the comment about parivate.
> > > /* Mapping-private opaque data:
> > >  * usually used for buffer_heads
> > >  * if PagePrivate set; used for
> > >  * swp_entry_t if PageSwapCache;
> > >  * indicates order in the buddy
> > >  * system if PG_buddy is set.
> > >  */
> > 
> > Please read full thread in detail.
> > 
> > Mel suggested following as
> > 
> > if (PageBuddy(page)) {
> >         int nr_pages = (1 << page_order(page)) - 1;
> >         if (PageBuddy(page)) {
> >                 nr_pages = min(nr_pages, MAX_ORDER_NR_PAGES - 1);
> >                 low_pfn += nr_pages;
> >                 continue;
> >         }
> > }
> > 
> > min(nr_pages, xxx) removes your concern but I think Mel's version
> > isn't right. It should be aligned with pageblock boundary so I 
> > suggested following.
> > 
> 
> Why? We're looking for pages to migrate. If the page is free and at the
> maximum order then there is no point searching in the middle of a free
> page.

isolate_migratepages_range API works with [low_pfn, end_pfn)
and we can't guarantee page_order in normal compaction path
so I'd like to limit the skipping by end_pfn conservatively.

> 
> > if (PageBuddy(page)) {
> > #ifdef CONFIG_MEMORY_ISOLATION
> > 	unsigned long order = page_order(page);
> > 	if (PageBuddy(page)) {
> > 		low_pfn += (1 << order) - 1;
> > 		low_pfn = min(low_pfn, end_pfn);
> > 	}
> > #endif
> > 	continue;
> > }
> > 
> > so worst case is (pageblock_nr_pages - 1).
> 
> No it isn't. The worst case it that the whole region being searched is
> skipped. For THP allocations, it would happen to work as being the
> pageblock boundary but it is not required by the API. I expect that
> end_pfn is not necessarily the next pageblock boundary for CMA
> allocations.

Mel, as I said eariler, CMA and memory-hotplug don't have a race
problem of page_order so we can consider only normal compaction path
like high order allocation(ex, THP). So, about this race problem,
worst case is the number of (pageblock_nr_pages - 1) skipping.

> 
> -- 
> Mel Gorman
> SUSE Labs
> 
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-- 
Kind regards,
Minchan Kim

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