[PATCH 4/8] mm: free swp_entry in madvise_free

[Minchan Kim <minchan@kernel.org>]

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: 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 640311704e31..663bd9fa0ae0 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -270,6 +270,8 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 	spinlock_t *ptl;
 	pte_t *pte, ptent;
 	struct page *page;
+	swp_entry_t entry;
+	int nr_swap = 0;
 
 	split_huge_page_pmd(vma, addr, pmd);
 	if (pmd_trans_unstable(pmd))
@@ -280,8 +282,22 @@ 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)) {
+			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)
@@ -313,6 +329,14 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
 		set_pte_at(mm, addr, pte, ptent);
 		tlb_remove_tlb_entry(tlb, pte, 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();
-- 
1.9.1