Re: [PATCH] mm: extend reuse_swap_page range as much as possible

[Huang\, Ying]

<zhouxianrong@xxxxxxxxxx> writes:

> From: zhouxianrong <zhouxianrong@xxxxxxxxxx>
>
> origanlly reuse_swap_page requires that the sum of page's
> mapcount and swapcount less than or equal to one.
> in this case we can reuse this page and avoid COW currently.
>
> now reuse_swap_page requires only that page's mapcount
> less than or equal to one and the page is not dirty in
> swap cache. in this case we do not care its swap count.
>
> the page without dirty in swap cache means that it has
> been written to swap device successfully for reclaim before
> and then read again on a swap fault. in this case the page
> can be reused even though its swap count is greater than one
> and postpone the COW on other successive accesses to the swap
> cache page later rather than now.
>
> i did this patch test in kernel 4.4.23 with arm64 and none huge
> memory. it work fine.

Why do you need this?  You saved copying one page from memory to memory
(COW) now, at the cost of reading a page from disk to memory later?

Best Regards,
Huang, Ying

> Signed-off-by: zhouxianrong <zhouxianrong@xxxxxxxxxx>
> ---
>  mm/swapfile.c |    9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index bf91dc9..c21cf07 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1543,22 +1543,27 @@ static int page_trans_huge_map_swapcount(struct page *page, int *total_mapcount,
>  bool reuse_swap_page(struct page *page, int *total_map_swapcount)
>  {
>  	int count, total_mapcount, total_swapcount;
> +	int dirty;
>  
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
>  	if (unlikely(PageKsm(page)))
>  		return false;
> +	dirty = PageDirty(page);
>  	count = page_trans_huge_map_swapcount(page, &total_mapcount,
>  					      &total_swapcount);
>  	if (total_map_swapcount)
>  		*total_map_swapcount = total_mapcount + total_swapcount;
> -	if (count == 1 && PageSwapCache(page) &&
> +	if ((total_mapcount <= 1 && !dirty) ||
> +		(count == 1 && PageSwapCache(page) &&
>  	    (likely(!PageTransCompound(page)) ||
>  	     /* The remaining swap count will be freed soon */
> -	     total_swapcount == page_swapcount(page))) {
> +	     total_swapcount == page_swapcount(page)))) {
>  		if (!PageWriteback(page)) {
>  			page = compound_head(page);
>  			delete_from_swap_cache(page);
>  			SetPageDirty(page);
> +			if (!dirty)
> +				return true;
>  		} else {
>  			swp_entry_t entry;
>  			struct swap_info_struct *p;