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SubjectRe: [PATCH 1/2] mm: add support for discard of unused ptes
On Thu, 25 Jul 2013 10:54:20 +0200 Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:

> From: Konstantin Weitz <konstantin.weitz@gmail.com>
>
> 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 45e576b1c3d00206 "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.
>
> ...
>
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -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)
> diff --git a/mm/rmap.c b/mm/rmap.c
> index cd356df..2291f25 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1234,6 +1234,16 @@ 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) };

Obviously harmless. Please include this in whatever tree carries
"[PATCH 2/2] s390/kvm: support collaborative memory management".


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