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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>