[PATCH v4 04/16] mm: free swp_entry in madvise_free

Minchan Kim
2015-11-20T08:02:52Z

When I test below piece of code with 12 processes(ie, 512M * 12 = 6G
consume) on my (3G ram + 12 cpu + 8G swap, the madvise_free is siginficat
slower (ie, 2x times) than madvise_dontneed.

loop = 5;
mmap(512M);
while (loop--) {
memset(512M);
madvise(MADV_FREE or MADV_DONTNEED);
}

The reason is lots of swapin.

1) dontneed: 1,612 swapin
2) madvfree: 879,585 swapin

If we find hinted pages were already swapped out when syscall is called,
it's pointless to keep the swapped-out pages in pte.
Instead, let's free the cold page because swapin is more expensive
than (alloc page + zeroing).

With this patch, it reduced swapin from 879,585 to 1,878 so elapsed time

1) dontneed: 6.10user 233.50system 0:50.44elapsed
2) madvfree: 6.03user 401.17system 1:30.67elapsed
2) madvfree + below patch: 6.70user 339.14system 1:04.45elapsed

Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
mm/madvise.c | 26 +++++++++++++++++++++++++-
1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 7b5c6f648fdb..c8e23102fc99 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -270,6 +270,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
spinlock_t *ptl;
pte_t *pte, ptent;
struct page *page;
+ int nr_swap = 0;

split_huge_page_pmd(vma, addr, pmd);
if (pmd_trans_unstable(pmd))
@@ -280,8 +281,24 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
for (; addr != end; pte++, addr += PAGE_SIZE) {
ptent = *pte;

- if (!pte_present(ptent))
+ if (pte_none(ptent))
continue;
+ /*
+ * If the pte has swp_entry, just clear page table to
+ * prevent swap-in which is more expensive rather than
+ * (page allocation + zeroing).
+ */
+ if (!pte_present(ptent)) {
+ swp_entry_t entry;
+
+ entry = pte_to_swp_entry(ptent);
+ if (non_swap_entry(entry))
+ continue;
+ nr_swap--;
+ free_swap_and_cache(entry);
+ pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
+ continue;
+ }

page = vm_normal_page(vma, addr, ptent);
if (!page)
@@ -327,6 +344,13 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
}
}

+ if (nr_swap) {
+ if (current->mm == mm)
+ sync_mm_rss(mm);
+
+ add_mm_counter(mm, MM_SWAPENTS, nr_swap);
+ }
+
arch_leave_lazy_mmu_mode();
pte_unmap_unlock(pte - 1, ptl);
cond_resched();