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