Re: [mmotm] BUG: Bad page state in process khugepaged ?

[Hugh Dickins <hughd@google.com>]

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@mxp.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

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