Messages in this thread | | | Date | Wed, 30 Oct 2013 11:31:38 -0400 | From | Naoya Horiguchi <> | Subject | Re: [PATCH 2/2] mm: thp: give transparent hugepage code a separatecopy_page |
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On Tue, Oct 29, 2013 at 12:11:26AM +0200, Kirill A. Shutemov wrote: > On Mon, Oct 28, 2013 at 03:16:20PM -0700, Dave Hansen wrote: > > > > From: Dave Hansen <dave.hansen@linux.intel.com> > > > > Right now, the migration code in migrate_page_copy() uses > > copy_huge_page() for hugetlbfs and thp pages: > > > > if (PageHuge(page) || PageTransHuge(page)) > > copy_huge_page(newpage, page); > > > > So, yay for code reuse. But: > > > > void copy_huge_page(struct page *dst, struct page *src) > > { > > struct hstate *h = page_hstate(src); > > > > and a non-hugetlbfs page has no page_hstate(). This > > works 99% of the time because page_hstate() determines > > the hstate from the page order alone. Since the page > > order of a THP page matches the default hugetlbfs page > > order, it works. > > > > But, if you change the default huge page size on the > > boot command-line (say default_hugepagesz=1G), then > > we might not even *have* a 2MB hstate so page_hstate() > > returns null and copy_huge_page() oopses pretty fast > > since copy_huge_page() dereferences the hstate: > > > > void copy_huge_page(struct page *dst, struct page *src) > > { > > struct hstate *h = page_hstate(src); > > if (unlikely(pages_per_huge_page(h) > MAX_ORDER_NR_PAGES)) { > > ... > > > > This patch creates a copy_high_order_page() which can > > be used on THP pages. > > We already have copy_user_huge_page() and copy_user_gigantic_page() in > generic code (mm/memory.c). I think copy_gigantic_page() and > copy_huge_page() should be moved there too.
I agree this.
> BTW, I think pages_per_huge_page in copy_user_huge_page() is redunand: > compound_order(page) should be enough, right?
I guess that thp code is very strict on performance, so developers chose to pass it as an argument instead of calculating compound_order in each call. I think the performance gain is small (maybe invisible), but it's not a bad idea to me.
Thanks, Naoya
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