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

[Michal Hocko]

On Fri 30-10-15 16:01:40, Minchan Kim wrote:
> 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@xxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>

Yes this makes a lot of sense.

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

One nit below.

> ---
>  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;

This could go into !pte_present if block

> +	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

-- 
Michal Hocko
SUSE Labs
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