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Date: Thu, 10 Feb 2011 23:02:50 -0800 (PST) From: Hugh Dickins <hughd@...gle.com> To: Andrea Arcangeli <aarcange@...hat.com> cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>, Daisuke Nishimura <nishimura@....nes.nec.co.jp>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "hannes@...xchg.org" <hannes@...xchg.org> Subject: Re: [mmotm] BUG: Bad page state in process khugepaged ? On Wed, 9 Feb 2011, Andrea Arcangeli wrote: > On Wed, Feb 09, 2011 at 03:52:46PM +0900, KAMEZAWA Hiroyuki wrote: > > On Wed, 9 Feb 2011 15:50:01 +0900 > > Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote: > > > > > > > > > > In hex, pc->flags was 7A00000000004 and this means PCG_USED bit is set. > > > > This implies page_remove_rmap() may not be called but ->mapping is NULL. Hmm? > > > > (7A is encoding of section number.) > > > > > > > Sigh.. it seems another freed-but-not-uncharged problem.. > > > > > > > Ah, ok, this is maybe caused by this. I'm sorry that I missed this. > > == > > static inline int free_pages_check(struct page *page) > > { > > if (unlikely(page_mapcount(page) | > > (page->mapping != NULL) | > > (atomic_read(&page->_count) != 0) | > > (page->flags & PAGE_FLAGS_CHECK_AT_FREE) | > > (mem_cgroup_bad_page_check(page)))) { <==========(*) > > bad_page(page); > > return 1; > > == > > > > Then, ok, this is a memcgroup and hugepage issue. > > > > I'll look into. > > Yes, the rest of the info on the page looked ok and shouldn't have > triggered a bad_page call. Thanks so much for looking into it. There is a separate little issue here, Andrea. Although we went to some trouble for bad_page() to take the page out of circulation yet let the system continue, your VM_BUG_ON(!PageBuddy) inside __ClearPageBuddy(page), from two callsites in bad_page(), is turning it into a fatal error when CONFIG_DEBUG_VM. You could that only MM developers switch CONFIG_DEBUG_VM=y, and they would like bad_page() to be fatal; maybe, but if so we should do that as an intentional patch, rather than as an unexpected side-effect ;) I noticed this a few days ago, but hadn't quite decided whether just to remove the VM_BUG_ON, or move it to __ClearPageBuddy's third callsite, or... doesn't matter much. I do also wonder if PageBuddy would better be _mapcount -something else: if we've got a miscounted page (itself unlikely of course), there's a chance that its _mapcount will be further decremented after it has been freed: whereupon it will go from -1 to -2, PageBuddy at present. The special avoidance of PageBuddy being that it can pull a whole block of pages into misuse if its mistaken. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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