Re: [PATCH] mm: do not use page_count without a page pin

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2012/06/11 16:44), Andrea Arcangeli wrote:
> Hi,
>
> On Mon, Jun 11, 2012 at 04:20:17PM +0900, Kamezawa Hiroyuki wrote:
>> (2012/06/11 9:17), Minchan Kim wrote:
>>> d179e84ba fixed the problem[1] in vmscan.c but same problem is here.
>>> Let's fix it.
>>>
>>> [1] http://comments.gmane.org/gmane.linux.kernel.mm/65844
>>>
>>> I copy and paste d179e84ba's contents for description.
>>>
>>> "It is unsafe to run page_count during the physical pfn scan because
>>> compound_head could trip on a dangling pointer when reading
>>> page->first_page if the compound page is being freed by another CPU."
>>>
>>> Cc: Andrea Arcangeli<aarcange@redhat.com>
>>> Cc: Mel Gorman<mgorman@suse.de>
>>> Cc: Michal Hocko<mhocko@suse.cz>
>>> Cc: KAMEZAWA Hiroyuki<kamezawa.hiroyu@jp.fujitsu.com>
>>> Signed-off-by: Minchan Kim<minchan@kernel.org>
>>> ---
>>>    mm/page_alloc.c |    6 +++++-
>>>    1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 266f267..019c4fe 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -5496,7 +5496,11 @@ __count_immobile_pages(struct zone *zone, struct page *page, int count)
>>>    			continue;
>>>
>>>    		page = pfn_to_page(check);
>>> -		if (!page_count(page)) {
>>> +		/*
>>> +		 * We can't use page_count withou pin a page
>>> +		 * because another CPU can free compound page.
>>> +		 */
>>> +		if (!atomic_read(&page->_count)) {
>>>    			if (PageBuddy(page))
>>>    				iter += (1<<   page_order(page)) - 1;
>>>    			continue;
>> Nice Catch.
>
> Agreed!
>
>> Other than the comment fix already pointed out..
>> Hmm...BTW, it seems this __count_xxx doesn't have any code for THP/Hugepage..
>> so, we need more fixes for better code, I think.
>> Hmm, Don't we need !PageTail() check and 'skip thp' code ?
>
> So the page->_count for tail pages is guaranteed zero at all times
> (tail page refcounting is done on _mapcount).
>
> We could add a comment that "this check already skips compound tails
> of THP because their page->_count is zero at all times".
>

Thank you for clarification.

I'll look into this later. Fortunately, our team has memory-hotplug
team again for our next server and should revisit this :)
I'll give an input to them.

Thanks,
-Kame

> Instead of a comment we could consider defining an inline function
> with a special name that does atomic_read(&page->_count) and use it
> when we intend to the regular or compound head count and return 0 on
> tails. It would make it easier to identify these places later if we
> ever want to change the refcounting mechanism, but it may be overkill,
> it's up to you.
>
> Tail pages also can't be PageLRU.
>
> The code after the patch should already skip thp tails fine (it won't
> skip heads but I believe that's intentional, but one problem that
> remains is that the heads should increase found by more than 1...).
>
> Thanks,
> Andrea


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