Messages in this thread | | | Date | Mon, 11 Jun 2012 17:48:48 +0900 | From | Kamezawa Hiroyuki <> | Subject | Re: [PATCH] mm: do not use page_count without a page pin |
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(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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