linux-kernel - Re: [PATCH mm-unstable v2 8/8] mm/hugetlb: convert demote_free_huge

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Date:   Fri, 13 Jan 2023 09:16:23 -0600
From:   Sidhartha Kumar <sidhartha.kumar@...cle.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, songmuchun@...edance.com,
        jhubbard@...dia.com
Subject: Re: [PATCH mm-unstable v2 8/8] mm/hugetlb: convert
 demote_free_huge_page to folios

On 1/10/23 6:00 PM, Mike Kravetz wrote:
> On 01/10/23 21:40, Matthew Wilcox wrote:
>> On Tue, Jan 10, 2023 at 03:28:21PM -0600, Sidhartha Kumar wrote:
>>> @@ -3505,6 +3505,7 @@ static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
>>>   {
>>>   	int nr_nodes, node;
>>>   	struct page *page;
>>> +	struct folio *folio;
>>>   
>>>   	lockdep_assert_held(&hugetlb_lock);
>>>   
>>> @@ -3518,8 +3519,8 @@ static int demote_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
>>>   		list_for_each_entry(page, &h->hugepage_freelists[node], lru) {
>>>   			if (PageHWPoison(page))
>>>   				continue;
>>> -
>>> -			return demote_free_huge_page(h, page);
>>> +			folio = page_folio(page);
>>> +			return demote_free_hugetlb_folio(h, folio);
>>>   		}
>>>   	}
>>
>> Can't this be
>> 		list_for_each_entry(folio, &h->hugepage_freelists[node], lru)
>>
>> which avoids the call to page_folio() here.
>>
>> I think the call to PageHWPoison is actually wrong here.  That would
>> only check the hwpoison bit on the first page, whereas we want to know
>> about the hwpoison bit on any page (don't we?)  So this should be
>> folio_test_has_hwpoisoned()?
>>
>> Or is that a THP-thing that is different for hugetlb pages?
> 
> I believe it is different for hugetlb pages.  See hugetlb_set_page_hwpoison()
> where it sets PageHWPoison on head page as well as allocating a raw_hwp_page

I agree, this line in hugetlb_set_page_hwpoison (which is now 
folio_set_hugetlb_hwpoison) sets the HWPoison flag on the head page

int ret = folio_test_set_hwpoison(folio) ? -EHWPOISON : 0;

so the correct code in demote_pool_huge_page() would be:

	list_for_each_entry(folio, &h->hugepage_freelists[node], lru) {
			if (folio_test_hwpoison(folio))
				continue;
			return demote_free_hugetlb_folio(h, folio);
		}


> to track the actual page with poison.  Note that we can not directly flag
> hugetlb 'subpages' because we may not have the struct pages due to vmemmap
> optimization.  Adding Naoya just to be sure.
> 
> Do agree that this could be list_for_each_entry(folio ...

Will make this change in v3.

Thanks,
Sidhartha Kumar