Re: [PATCH 1/2] mm: hotplug: implement non-movable version of get_user_pages()called get_user_pages_non_movable()

[Simon Jeons]

On 02/05/2013 09:32 PM, Mel Gorman wrote:
> On Tue, Feb 05, 2013 at 11:57:22AM +0000, Mel Gorman wrote:
> > > > +				migrate_pre_flag = 1;
> > > > +			}
> > > > +
> > > > +			if (!isolate_lru_page(pages[i])) {
> > > > +				inc_zone_page_state(pages[i], NR_ISOLATED_ANON +
> > > > +						 page_is_file_cache(pages[i]));
> > > > +				list_add_tail(&pages[i]->lru, &pagelist);
> > > > +			} else {
> > > > +				isolate_err = 1;
> > > > +				goto put_page;
> > > > +			}
> > isolate_lru_page() takes the LRU lock every time.
> Credit to Michal Hocko for bringing this up but with the number of
> other issues I missed that this is also broken with respect to huge page
> handling. hugetlbfs pages will not be on the LRU so the isolation will mess
> up and the migration has to be handled differently.  Ordinarily hugetlbfs
> pages cannot be allocated from ZONE_MOVABLE but it is possible to configure
> it to be allowed via /proc/sys/vm/hugepages_treat_as_movable. If this
> encounters a hugetlbfs page, it'll just blow up.

As you said, hugetlbfs pages are not in LRU list, then how can encounter 
a hugetlbfs page and blow up?

> The other is that this almost certainly broken for transhuge page
> handling. gup returns the head and tail pages and ordinarily this is ok

When need gup thp? in kvm case?

> because the caller only cares about the physical address. Migration will
> also split a hugepage if it receives it but you are potentially adding
> tail pages to a list here and then migrating them. The split of the first
> page will get very confused. I'm not exactly sure what the result will be
> but it won't be pretty.
>
> Was THP enabled when this was tested? Was CONFIG_DEBUG_LIST enabled
> during testing?

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