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

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Date:	Wed, 20 Feb 2013 17:58:22 +0800
From:	Simon Jeons <simon.jeons@...il.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Lin Feng <linfeng@...fujitsu.com>, bcrl@...ck.org,
	viro@...iv.linux.org.uk, khlebnikov@...nvz.org, walken@...gle.com,
	kamezawa.hiroyu@...fujitsu.com, minchan@...nel.org,
	riel@...hat.com, rientjes@...gle.com,
	isimatu.yasuaki@...fujitsu.com, wency@...fujitsu.com,
	laijs@...fujitsu.com, jiang.liu@...wei.com, mhocko@...e.cz,
	linux-mm@...ck.org, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: hotplug: implement non-movable version of get_user_pages()
 called get_user_pages_non_movable()

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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