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Date: Wed, 5 Sep 2012 11:53:40 +0100
From: Mel Gorman <mgorman@...e.de>
To: Minchan Kim <minchan@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Marek Szyprowski <m.szyprowski@...sung.com>,
Michal Nazarewicz <mina86@...a86.com>,
Rik van Riel <riel@...hat.com>
Subject: Re: [RFC] mm: support MIGRATE_DISCARD
On Fri, Aug 24, 2012 at 01:25:30PM +0900, Minchan Kim wrote:
> This patch introudes MIGRATE_DISCARD mode in migration.
s/introudes/introduces/
It's also not a "mode" like the other MIGRATE_ modes but is instead
a modifier.
> It drops *unmapped 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.
It does assume that there is clean unmapped page cache available. This will
be the case if the workload was a streaming read for example. I don't know
enough about embedded workloads to categorise them unfortunately.
> 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@...sung.com>
> Cc: Michal Nazarewicz <mina86@...a86.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
> include/linux/migrate_mode.h | 11 ++++++---
> mm/migrate.c | 56 ++++++++++++++++++++++++++++++++++--------
> mm/page_alloc.c | 2 +-
> 3 files changed, 55 insertions(+), 14 deletions(-)
>
> diff --git a/include/linux/migrate_mode.h b/include/linux/migrate_mode.h
> index ebf3d89..8e44e30 100644
> --- a/include/linux/migrate_mode.h
> +++ b/include/linux/migrate_mode.h
> @@ -6,11 +6,16 @@
> * on most operations but not ->writepage as the potential stall time
> * is too significant
> * MIGRATE_SYNC will block when migrating pages
> + * MIGRTATE_DISCARD will discard clean cache page instead of migration
> + *
s/MIGRTATE_DISCARD/MIGRATE_DISCARD/
Your changelog says that it discards unmapped clean page cache pages.
This says it unmaps clean page cache pages. Which is it?
> + * MIGRATE_ASYNC, MIGRATE_SYNC_LIGHT, MIGRATE_SYNC shouldn't be used
> + * together with OR flag.
> */
> enum migrate_mode {
> - MIGRATE_ASYNC,
> - MIGRATE_SYNC_LIGHT,
> - MIGRATE_SYNC,
> + MIGRATE_ASYNC = 1 << 0,
> + MIGRATE_SYNC_LIGHT = 1 << 1,
> + MIGRATE_SYNC = 1 << 2,
> + MIGRATE_DISCARD = 1 << 3,
> };
>
This change means that migrate_mode is no longer an enum and it would be
more suitable to make it a bitwise type similar to what reclaim_mode_t
used to be.
> #endif /* MIGRATE_MODE_H_INCLUDED */
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 77ed2d7..90be7a9 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -225,7 +225,7 @@ static bool buffer_migrate_lock_buffers(struct buffer_head *head,
> struct buffer_head *bh = head;
>
> /* Simple case, sync compaction */
> - if (mode != MIGRATE_ASYNC) {
> + if (!(mode & MIGRATE_ASYNC)) {
> do {
> get_bh(bh);
> lock_buffer(bh);
> @@ -313,7 +313,7 @@ static int migrate_page_move_mapping(struct address_space *mapping,
> * the mapping back due to an elevated page count, we would have to
> * block waiting on other references to be dropped.
> */
> - if (mode == MIGRATE_ASYNC && head &&
> + if (mode & MIGRATE_ASYNC && head &&
> !buffer_migrate_lock_buffers(head, mode)) {
Expect this generated a compiler warning
if ((mode & MIGRATE_ASYNC) && head &&
> page_unfreeze_refs(page, expected_count);
> spin_unlock_irq(&mapping->tree_lock);
> @@ -521,7 +521,7 @@ int buffer_migrate_page(struct address_space *mapping,
> * with an IRQ-safe spinlock held. In the sync case, the buffers
> * need to be locked now
> */
> - if (mode != MIGRATE_ASYNC)
> + if (!(mode & MIGRATE_ASYNC))
> BUG_ON(!buffer_migrate_lock_buffers(head, mode));
>
> ClearPagePrivate(page);
> @@ -603,7 +603,7 @@ static int fallback_migrate_page(struct address_space *mapping,
> {
> if (PageDirty(page)) {
> /* Only writeback pages in full synchronous migration */
> - if (mode != MIGRATE_SYNC)
> + if (!(mode & MIGRATE_SYNC))
> return -EBUSY;
> return writeout(mapping, page);
> }
> @@ -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;
> +
WARN_ON(PageDirty(page));
> + 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, enum migrate_mode mode)
> {
> @@ -685,9 +698,12 @@ 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)
> + if (!force || mode & MIGRATE_ASYNC)
may also need parens around mode & MIGRATE_ASYNC here.
> goto out;
>
> /*
> @@ -733,7 +749,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> * the retry loop is too short and in the sync-light case,
> * the overhead of stalling is too much
> */
> - if (mode != MIGRATE_SYNC) {
> + if (!(mode & MIGRATE_SYNC)) {
> rc = -EBUSY;
> goto uncharge;
> }
> @@ -799,12 +815,32 @@ 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;
> +
This should be broken out into another function migratemode_to_ttuflags()
similar to allocflags_to_migratetype.
> /* 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);
>
If discard_mode == true then ttu_flags does not have TTU_MIGRATION set.
That means a migration PTE is not put in place. Now, I expect this is
deliberate because you are about to discard the page but it should be
commented on.
> 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;
> + }
>
I like the idea of what you're doing but not how you are doing
it. I do not like that this patch changes migrate_pages() into
migrate_but_sometimes_reclaim_pages_depending_on_flags().
I would *much* prefer if CMA created a new helper that took a page list.
In the first pass it would create a second list and add all clean page
cache pages and send that to shrink_page_list() in mm/vmscan.c. The second
pass would send the remaining pages to migrate_pages(). That would be
functionally similar but share common core code, churn migrate.c less,
avoid the need for changing an enum to a bitwise and generally be easier
to follow. The cost will be *marginally* higher as two scans of the list
are necessary but tiny in comparison to the cost of migration.
> if (rc && remap_swapcache)
> remove_migration_ptes(page, page);
> @@ -907,7 +943,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
> rc = -EAGAIN;
>
> if (!trylock_page(hpage)) {
> - if (!force || mode != MIGRATE_SYNC)
> + if (!force || !(mode & MIGRATE_SYNC))
> goto out;
> lock_page(hpage);
> }
> 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
>
--
Mel Gorman
SUSE Labs
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