Messages in this thread | | | Date | Wed, 26 Jun 2019 16:15:37 -0700 | From | Andrew Morton <> | Subject | Re: [PATCHv4] mm/gup: speed up check_and_migrate_cma_pages() on huge page |
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On Wed, 26 Jun 2019 21:10:00 +0800 Pingfan Liu <kernelfans@gmail.com> wrote:
> Both hugetlb and thp locate on the same migration type of pageblock, since > they are allocated from a free_list[]. Based on this fact, it is enough to > check on a single subpage to decide the migration type of the whole huge > page. By this way, it saves (2M/4K - 1) times loop for pmd_huge on x86, > similar on other archs. > > Furthermore, when executing isolate_huge_page(), it avoid taking global > hugetlb_lock many times, and meanless remove/add to the local link list > cma_page_list. > > ... > > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -1342,19 +1342,22 @@ static long check_and_migrate_cma_pages(struct task_struct *tsk, > LIST_HEAD(cma_page_list); > > check_again: > - for (i = 0; i < nr_pages; i++) { > + for (i = 0; i < nr_pages;) { > + > + struct page *head = compound_head(pages[i]); > + long step = 1; > + > + if (PageCompound(head))
I suspect this would work correctly if the PageCompound test was simply removed. Not that I'm really suggesting that it be removed - dunno.
> + step = (1 << compound_order(head)) - (pages[i] - head);
I don't understand this statement. Why does the position of this page in the pages[] array affect anything? There's an assumption about the contents of the skipped pages, I assume.
Could we please get a comment in here whcih fully explains the logic and any assumptions?
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