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mm: add support for discard of unused ptes
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 45e576b "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 <>
Signed-off-by: Martin Schwidefsky <>
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konne88 authored and Martin Schwidefsky committed Feb 21, 2014
1 parent 53e857f commit 45961722f8e30ceab9d135b1ddc0947d53aef7c3
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@@ -193,6 +193,19 @@ static inline int pte_same(pte_t pte_a, pte_t pte_b)

* 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;

static inline int pmd_same(pmd_t pmd_a, pmd_t pmd_b)
@@ -1165,6 +1165,16 @@ int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
set_pte_at(mm, address, pte,
} 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);
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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