mm: add support for discard of unused ptes (45961722) · Commits · 戴 / test

In a virtualized environment and given an appropriate interface the guest can mark pages as unused while they are free (for the s390 implementation see git commit "guest page hinting light"). For the host the unused state is a property of the pte. This patch adds the primitive 'pte_unused' and code to the host swap out handler so that pages marked as unused by all mappers are not swapped out but discarded instead, thus saving one IO for swap out and potentially another one for swap in. [ Martin Schwidefsky: patch reordering and simplification ] Signed-off-by:

Konstantin Weitz <konstantin.weitz@gmail.com> Signed-off-by:

Martin Schwidefsky <schwidefsky@de.ibm.com>

include/asm-generic/pgtable.h

+13 −0

Original line number	Diff line number	Diff line
		@@ -193,6 +193,19 @@ static inline int pte_same(pte_t pte_a, pte_t pte_b)
		}
		#endif

		#ifndef __HAVE_ARCH_PTE_UNUSED
		/*
		* Some architectures provide facilities to virtualization guests
		* so that they can flag allocated pages as unused. This allows the
		* host to transparently reclaim unused pages. This function returns
		* whether the pte's page is unused.
		*/
		static inline int pte_unused(pte_t pte)
		{
		return 0;
		}
		#endif

		#ifndef __HAVE_ARCH_PMD_SAME
		#ifdef CONFIG_TRANSPARENT_HUGEPAGE
		static inline int pmd_same(pmd_t pmd_a, pmd_t pmd_b)

mm/rmap.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -1165,6 +1165,16 @@ int try_to_unmap_one(struct page page, struct vm_area_struct vma,
		}
		set_pte_at(mm, address, pte,
		swp_entry_to_pte(make_hwpoison_entry(page)));
		} else if (pte_unused(pteval)) {
		/*
		* The guest indicated that the page content is of no
		* interest anymore. Simply discard the pte, vmscan
		* will take care of the rest.
		*/
		if (PageAnon(page))
		dec_mm_counter(mm, MM_ANONPAGES);
		else
		dec_mm_counter(mm, MM_FILEPAGES);
		} else if (PageAnon(page)) {
		swp_entry_t entry = { .val = page_private(page) };
		pte_t swp_pte;

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