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

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Date:   Thu, 6 May 2021 12:01:34 +0800
From:   Ding Hui <dinghui@...gfor.com.cn>
To:     HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>, David Hildenbrand <david@...hat.com>
Cc:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "osalvador@...e.de" <osalvador@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm/page_alloc: fix counting of free pages after take
 off from buddy

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@...gfor.com.cn>
>>> ---
>>>    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.
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

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;
     }

     lock_page(page);
	


-- 
Thanks,
- Ding Hui