Re: [PATCH V4 4/6] mm: reclaim MADV_FREE pages

[Minchan Kim]

On Wed, Feb 22, 2017 at 10:50:42AM -0800, Shaohua Li wrote:
> When memory pressure is high, we free MADV_FREE pages. If the pages are
> not dirty in pte, the pages could be freed immediately. Otherwise we
> can't reclaim them. We put the pages back to anonumous LRU list (by
> setting SwapBacked flag) and the pages will be reclaimed in normal
> swapout way.
> 
> We use normal page reclaim policy. Since MADV_FREE pages are put into
> inactive file list, such pages and inactive file pages are reclaimed
> according to their age. This is expected, because we don't want to
> reclaim too many MADV_FREE pages before used once pages.
> 
> Based on Minchan's original patch
> 
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Shaohua Li <shli@xxxxxx>
> ---
>  include/linux/rmap.h |  2 +-
>  mm/huge_memory.c     |  2 ++
>  mm/madvise.c         |  1 +
>  mm/rmap.c            | 10 ++++++++--
>  mm/vmscan.c          | 34 ++++++++++++++++++++++------------
>  5 files changed, 34 insertions(+), 15 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index e2cd8f9..2bfd8c6 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -300,6 +300,6 @@ static inline int page_mkclean(struct page *page)
>  #define SWAP_AGAIN	1
>  #define SWAP_FAIL	2
>  #define SWAP_MLOCK	3
> -#define SWAP_LZFREE	4
> +#define SWAP_DIRTY	4

Could you write down about SWAP_DIRTY in try_to_unmap's description?

< snip >

> diff --git a/mm/rmap.c b/mm/rmap.c
> index c621088..083f32e 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1424,6 +1424,12 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  				dec_mm_counter(mm, MM_ANONPAGES);
>  				rp->lazyfreed++;
>  				goto discard;
> +			} else if (!PageSwapBacked(page)) {
> +				/* dirty MADV_FREE page */
> +				set_pte_at(mm, address, pvmw.pte, pteval);
> +				ret = SWAP_DIRTY;
> +				page_vma_mapped_walk_done(&pvmw);
> +				break;
>  			}
>  
>  			if (swap_duplicate(entry) < 0) {
> @@ -1525,8 +1531,8 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
>  
>  	if (ret != SWAP_MLOCK && !page_mapcount(page)) {
>  		ret = SWAP_SUCCESS;
> -		if (rp.lazyfreed && !PageDirty(page))
> -			ret = SWAP_LZFREE;
> +		if (rp.lazyfreed && PageDirty(page))
> +			ret = SWAP_DIRTY;

Hmm, I don't understand why we need to introduce new return value.
Can't we set SetPageSwapBacked and return SWAP_FAIL in try_to_unmap_one?

>  	}
>  	return ret;
>  }
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 68ea50d..830981a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c

< snip >
>  			goto keep_locked;
>  
>  		/* Double the slab pressure for mapped and swapcache pages */
> -		if (page_mapped(page) || PageSwapCache(page))
> +		if ((page_mapped(page) || PageSwapCache(page)) &&
> +		    !(PageAnon(page) && !PageSwapBacked(page)))
>  			sc->nr_scanned++;
>  
>  		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
> @@ -1118,8 +1120,10 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		/*
>  		 * Anonymous process memory has backing store?
>  		 * Try to allocate it some swap space here.
> +		 * Lazyfree page could be freed directly
>  		 */
> -		if (PageAnon(page) && !PageSwapCache(page)) {
> +		if (PageAnon(page) && !PageSwapCache(page) &&
> +		    PageSwapBacked(page)) {
>  			if (!(sc->gfp_mask & __GFP_IO))
>  				goto keep_locked;
>  			if (!add_to_swap(page, page_list))
> @@ -1140,9 +1144,12 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		 * The page is mapped into the page tables of one or more
>  		 * processes. Try to unmap it here.
>  		 */
> -		if (page_mapped(page) && mapping) {
> +		if (page_mapped(page)) {
>  			switch (ret = try_to_unmap(page,
>  				ttu_flags | TTU_BATCH_FLUSH)) {
> +			case SWAP_DIRTY:
> +				SetPageSwapBacked(page);
> +				/* fall through */
>  			case SWAP_FAIL:
>  				nr_unmap_fail++;
>  				goto activate_locked;