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


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