Re: [RFC PATCH] mm/page_alloc: fix counting of free pages after take off from buddy

[Ding Hui]

On 2021/5/6 15:30, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Thu, May 06, 2021 at 12:01:34PM +0800, Ding Hui wrote:
> > On 2021/5/6 10:49, HORIGUCHI NAOYA(堀口 直也) wrote:
> > > On Wed, Apr 28, 2021 at 04:54:59PM +0200, David Hildenbrand wrote:
> > > > On 21.04.21 04:04, Ding Hui wrote:
> > > > > Recently we found there is a lot MemFree left in /proc/meminfo after
> > > > > do a lot of pages soft offline.
> > > > >
> > > > > I think it's incorrect since NR_FREE_PAGES should not contain HWPoison pages.
> > > > > After take_page_off_buddy, the page is no longer belong to buddy
> > > > > allocator, and will not be used any more, but we maybe missed accounting
> > > > > NR_FREE_PAGES in this situation.
> > > > >
> > > > > Signed-off-by: Ding Hui <dinghui@xxxxxxxxxxxxxx>
> > > > > ---
> > > > >     mm/page_alloc.c | 1 +
> > > > >     1 file changed, 1 insertion(+)
> > > > >
> > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > > index cfc72873961d..8d65b62784d8 100644
> > > > > --- a/mm/page_alloc.c
> > > > > +++ b/mm/page_alloc.c
> > > > > @@ -8947,6 +8947,7 @@ bool take_page_off_buddy(struct page *page)
> > > > >     			del_page_from_free_list(page_head, zone, page_order);
> > > > >     			break_down_buddy_pages(zone, page_head, page, 0,
> > > > >     						page_order, migratetype);
> > > > > +			__mod_zone_page_state(zone, NR_FREE_PAGES, -1);
> > > > >     			ret = true;
> > > > >     			break;
> > > > >     		}
> > > >
> > > > Should this use __mod_zone_freepage_state() instead?
> > >
> > > Yes, __mod_zone_freepage_state() looks better to me.
> > >
> > > And I think that maybe an additional __mod_zone_freepage_state() in
> > > unpoison_memory() is necessary to cancel the decrement.  I thought of the
> > > following, but it doesn't build because get_pfnblock_migratetype() is
> > > available only in mm/page_alloc.c, so you might want to add a small exported
> > > routine in mm/page_alloc.c and let it called from unpoison_memory().
> > >
> > >     @@ -1899,8 +1899,12 @@ int unpoison_memory(unsigned long pfn)
> > >             }
> > >             if (!get_hwpoison_page(p, flags, 0)) {
> > >     -               if (TestClearPageHWPoison(p))
> > >     +               if (TestClearPageHWPoison(p)) {
> > >     +                       int migratetype = get_pfnblock_migratetype(p, pfn);
> > >     +
> > >                             num_poisoned_pages_dec();
> > >     +                       __mod_zone_freepage_state(page_zone(p), 1, migratetype);
> > >     +               }
> > >                     unpoison_pr_info("Unpoison: Software-unpoisoned free page %#lx\n",
> > >                                      pfn, &unpoison_rs);
> > >                     return 0;
> >
> > I think there is another problem:
> > In normal case, we keep the last refcount of the hwpoison page, so
> > get_hwpoison_page should return 1. The NR_FREE_PAGES will be adjusted when
> > call put_page.
>
> I think that take_page_off_buddy() should not be called for this case
> (the error page have remaining refcount).  So it seems that no need to
> update NR_FREE_PAGES ?

Yes, take_page_off_buddy() only used for free pages, but we will call 
page_ref_inc() after that, on the other hand for in used pages, we 
increased the refcount by get_any_page(), so in both cases, the 
hwpoisoned pages have refcount great than zero.

I think there is no need to update NR_FREE_PAGES explicitly in 
unpoison_memory(), the put_page() will help us to update NR_FREE_PAGES 
and put the page back to buddy.

> > At race condition, we maybe leak the page because we does not put it back to
> > buddy in unpoison_memory, however the HWPoison flag, num_poisoned_pages,
> > NR_FREE_PAGES is adjusted correctly.
> >
> > CPU0                        CPU1
> >
> > soft_offline_page
> >    soft_offline_free_page
> >      page_handle_poison
> >        take_page_off_buddy
> >        SetPageHWPoison
> >                              unpoison_memory
> >                                if (!get_hwpoison_page(p))
> >                                  TestClearPageHWPoison
> >                                    num_poisoned_pages_dec
> >                                  __mod_zone_freepage_state
> >                                  return 0
> >                                  /* miss put the page back to buddy */
> >        page_ref_inc
> >        num_poisoned_pages_inc
>
> Thanks for checking this, unpoison_memory() is racy.  Recently we are suggesting
> to introduce mf_mutex by [1].  Although this patch is not merged to mainline yet,
> but it could be used to prevent the above race too.
>
> [1] https://lore.kernel.org/linux-mm/20210427062953.2080293-2-nao.horiguchi@xxxxxxxxx/

I'll look forward to it, thanks.

> > How about do nothing and return -EBUSY (so the caller can retry) if unpoison
> > a zero refcount page , or return 0 like 230ac719c500 ("mm/hwpoison: don't
> > try to unpoison containment-failed pages") does ?
> >
> >    @@ -1736,11 +1736,9 @@ int unpoison_memory(unsigned long pfn)
> >      }
> >
> >      if (!get_hwpoison_page(p, flags, 0)) {
> >    -       if (TestClearPageHWPoison(p))
> >    -           num_poisoned_pages_dec();
> >    -       unpoison_pr_info("Unpoison: Software-unpoisoned free page %#lx\n",
> >    +       unpoison_pr_info("Unpoison: Software-unpoisoned zero refcount page
> > %#lx\n",
> >    				 pfn, &unpoison_rs);
> >    -       return 0;
> >    +       return -EBUSY;
>
> Currently unpoison_memory() does not work as reverse operation of take_page_off_buddy()
> (it's simply broken), so implementing it at one time would be better.
> I'll take time to fix unpoison_memory().

Thanks for your work.
Actually, I'm not sure about the exactly meaning of "broken", it seems 
that the basic function of unpoison_memory() is ok if not considered the 
race conditions.


--
Thanks,
- Ding Hui