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Message-ID: <6af291a0-41fa-8112-5297-6a4cdf2337b6@sangfor.com.cn>
Date: Fri, 7 May 2021 09:46:46 +0800
From: Ding Hui <dinghui@...gfor.com.cn>
To: HORIGUCHI NAOYA(堀口 直也)
<naoya.horiguchi@....com>
Cc: David Hildenbrand <david@...hat.com>,
"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 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@...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.
>
> 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@gmail.com/
>
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
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