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

[Minchan Kim]

Hi Andrea,

On Mon, Jun 11, 2012 at 09:44:40AM +0200, 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@xxxxxxxxxx>
> > > Cc: Mel Gorman<mgorman@xxxxxxx>
> > > Cc: Michal Hocko<mhocko@xxxxxxx>
> > > Cc: KAMEZAWA Hiroyuki<kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > > Signed-off-by: Minchan Kim<minchan@xxxxxxxxxx>
> > > ---
> > >   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).

Sure.

> 
> We could add a comment that "this check already skips compound tails
> of THP because their page->_count is zero at all times".

No problem.

> 
> 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.

That's a good idea but it's not proper time because I don't have much time
for it and other patch[1] is pended by this.

I hope it could be another nice clean up patch later. :)

[1] https://lkml.org/lkml/2012/6/11/169

> 
> 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...).

I can't fail to parse your last sentense.
Could you elaborate it more?

AFAIUC, you mean we have to increase reference count of head page?
If so, it's not in __count_immobile_pages because it is already race-likely function
so it shouldn't be critical although race happens.

If I miss something, please let me know it.


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