Re: [PATCH 2/2] mm: support MIGRATE_DISCARD

[lihanhui <hanhui.li@gmail.com>]

在 2012-9-5，下午4:11，Minchan Kim <minchan@kernel.org> 写道：

> This patch introudes MIGRATE_DISCARD mode in migration.
> It drops *clean cache pages* instead of migration so that
> migration latency could be reduced by avoiding (memcpy + page remapping).
> It's useful for CMA because latency of migration is very important rather
> than eviction of background processes's workingset. In addition, it needs
> less free pages for migration targets so it could avoid memory reclaiming
> to get free pages, which is another factor increase latency.
> 
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Michal Nazarewicz <mina86@mina86.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> include/linux/migrate_mode.h |    7 +++++++
> mm/migrate.c                 |   41 ++++++++++++++++++++++++++++++++++++++---
> mm/page_alloc.c              |    2 +-
> 3 files changed, 46 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/migrate_mode.h b/include/linux/migrate_mode.h
> index 8848cad..4eb1646 100644
> --- a/include/linux/migrate_mode.h
> +++ b/include/linux/migrate_mode.h
> @@ -14,6 +14,13 @@
>  */
> #define MIGRATE_SYNC		((__force migrate_mode_t)0x4)
> 
> +/*
> + * MIGRTATE_DISCARD will discard clean cache page instead of migration.
> + * MIGRATE_ASYNC, MIGRATE_SYNC_LIGHT, MIGRATE_SYNC shouldn't be used
> + * together with OR flag in current implementation.
> + */
> +#define MIGRATE_DISCARD		((__force migrate_mode_t)0x8)
> +
> typedef unsigned __bitwise__ migrate_mode_t;
> 
> #endif		/* MIGRATE_MODE_H_INCLUDED */
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 28d464b..2de7709 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -678,6 +678,19 @@ static int move_to_new_page(struct page *newpage, struct page *page,
> 	return rc;
> }
> 
> +static int discard_page(struct page *page)
> +{
> +	int ret = -EAGAIN;
> +
> +	struct address_space *mapping = page_mapping(page);
> +	if (page_has_private(page))
> +		if (!try_to_release_page(page, GFP_KERNEL))
> +			return ret;
> +	if (remove_mapping(mapping, page))
> +		ret = 0;
> +	return ret;
> +}
> +
> static int __unmap_and_move(struct page *page, struct page *newpage,
> 			int force, bool offlining, migrate_mode_t mode)
> {
> @@ -685,6 +698,9 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> 	int remap_swapcache = 1;
> 	struct mem_cgroup *mem;
> 	struct anon_vma *anon_vma = NULL;
> +	enum ttu_flags ttu_flags;
> +	bool discard_mode = false;
> +	bool file = false;
> 
> 	if (!trylock_page(page)) {
> 		if (!force || (mode & MIGRATE_ASYNC))
> @@ -799,12 +815,31 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> 		goto skip_unmap;
> 	}
> 
> +	file = page_is_file_cache(page);
> +	ttu_flags = TTU_IGNORE_ACCESS;
> +retry:
> +	if (!(mode & MIGRATE_DISCARD) || !file || PageDirty(page))
> +		ttu_flags |= (TTU_MIGRATION | TTU_IGNORE_MLOCK);
> +	else
> +		discard_mode = true;
> +
> 	/* Establish migration ptes or remove ptes */
> -	try_to_unmap(page, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
> +	rc = try_to_unmap(page, ttu_flags);
> 
> skip_unmap:
> -	if (!page_mapped(page))
> -		rc = move_to_new_page(newpage, page, remap_swapcache, mode);
> +	if (rc == SWAP_SUCCESS) {
> +		if (!discard_mode) {
> +			rc = move_to_new_page(newpage, page,
> +					remap_swapcache, mode);
> +		} else {
> +			rc = discard_page(page);
> +			goto uncharge;
> +		}
> +	} else if (rc == SWAP_MLOCK && discard_mode) {
> +		mode &= ~MIGRATE_DISCARD;
> +		discard_mode = false;
> +		goto retry;
> +	}
> 
> 	if (rc && remap_swapcache)
> 		remove_migration_ptes(page, page);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ba3100a..e14b960 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5670,7 +5670,7 @@ static int __alloc_contig_migrate_range(unsigned long start, unsigned long end)
> 
> 		ret = migrate_pages(&cc.migratepages,
> 				    __alloc_contig_migrate_alloc,
> -				    0, false, MIGRATE_SYNC);
> +				    0, false, MIGRATE_SYNC|MIGRATE_DISCARD);
> 	}
> 
> 	putback_lru_pages(&cc.migratepages);
> -- 
> 1.7.9.5
> 
> --
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