linux-kernel - Re: [PATCH 3/3] mm/memory-failure.c: dissolve truncated hugetlb page

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Date:   Tue, 12 Apr 2022 14:10:47 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "shy828301@...il.com" <shy828301@...il.com>,
        "mike.kravetz@...cle.com" <mike.kravetz@...cle.com>,
        "david@...hat.com" <david@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] mm/memory-failure.c: dissolve truncated hugetlb page

On 2022/4/12 13:59, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Tue, Apr 12, 2022 at 10:47:53AM +0800, Miaohe Lin wrote:
>> On 2022/4/11 21:13, HORIGUCHI NAOYA(堀口 直也) wrote:
>>> Hi Miaohe,
>>>
>>> On Thu, Apr 07, 2022 at 09:03:52PM +0800, Miaohe Lin wrote:
>>>> If me_huge_page meets a truncated huge page, hpage won't be dissolved
>>>
>>> I might not understand correctly what "truncated huge page" means.  If it
>>> means the page passed to me_huge_page() and truncate_error_page() is called
>>> on it, the else branch you're trying to update is not chosen, so maybe it
>>> sounds irrelevant to me?  Could you elaborate it or share the procedure to
>>> reproduce the case you care about?
>>
>> Sorry for making confusing. What 'truncated hugetlb page' means is that a hugepage is
>> truncated but still on the way to free. So HPageMigratable is still set and we might
>> come across it here. Does this make sense for you?
> 
> Yes, it does. Thank you.
> 
>>
>>>
>>>> even if we hold the last refcnt. It's because the truncated huge page
>>>> has NULL page_mapping while it's not anonymous page too. Thus we lose
>>>> the last chance to dissolve it into buddy to save healthy subpages.
>>>> Remove PageAnon check to handle these huge pages too.
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>>> ---
>>>>  mm/memory-failure.c | 3 +--
>>>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>>>
>>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>>> index bd563f47630c..3f054dbb169d 100644
>>>> --- a/mm/memory-failure.c
>>>> +++ b/mm/memory-failure.c
>>>> @@ -1046,8 +1046,7 @@ static int me_huge_page(struct page_state *ps, struct page *p)
>>>>  		 * hugepage, so we can free and dissolve it into buddy to
>>>>  		 * save healthy subpages.
>>>>  		 */
>>>> -		if (PageAnon(hpage))
>>>> -			put_page(hpage);
>>>
>>> I think that the reason of this "if (PageAnon(hpage))" is to not remove
>>> hugepages for hugetlbfs files.  Unlike anonymous hugepage, it can be
>>> accessed from file after error handling, so we can't simply dissolve it
>>> because otherwise another process reading the hugepage sees zeroed one
>>> without knowing the memory error.
>>
>> In this branch, we have precondition that page_mapping is NULL. So it can't be hugepages
>> for hugetlbfs files. It should be anonymous hugepages in most cases. If it's not anonymous
>> hugepages too, i.e. (!page_mapping(hpage) && !PageAnon(hpage)), it could be free hugepages
>> or 'truncated hugetlb page'. But we have already handled the free hugepages case, it should
>> be 'truncated hugetlb page' here. Since it's on the way to free, we should put the refcnt
>> to increase the chance that we can free and dissolve it into buddy to save healthy subpages.
>> Or am I miss something?
> 
> No, it sounds correct.  So I agree with removing the "if".  Could you also
> update the inline comment just above it in the next version?  We no longer
> need to limitedly mention "anonymous hugepage".

Sure. Will do it in next version. Many thanks!

> 
> Thanks,
> Naoya Horiguchi
>