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SubjectRe: [PATCH mm-unstable v2 8/8] mm/hugetlb: convert demote_free_huge_page to folios
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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

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