mm: compaction: introduce sync-light migration for use by compaction

This patch adds a lightweight sync migrate operation MIGRATE_SYNC_LIGHT
mode that avoids writing back pages to backing storage.  Async compaction
maps to MIGRATE_ASYNC while sync compaction maps to MIGRATE_SYNC_LIGHT.
For other migrate_pages users such as memory hotplug, MIGRATE_SYNC is
used.

This avoids sync compaction stalling for an excessive length of time,
particularly when copying files to a USB stick where there might be a
large number of dirty pages backed by a filesystem that does not support
->writepages.

[aarcange@redhat.com: This patch is heavily based on Andrea's work]
[akpm@linux-foundation.org: fix fs/nfs/write.c build]
[akpm@linux-foundation.org: fix fs/btrfs/disk-io.c build]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Andy Isaacson <adi@hexapodia.org>
Cc: Nai Xia <nai.xia@gmail.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

5 files changed, 47 insertions, 39 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index fb291585e1b..71a58f67f48 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -557,7 +557,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
                 nr_migrate = cc->nr_migratepages;
                 err = migrate_pages(&cc->migratepages, compaction_alloc,
                                 (unsigned long)cc, false,
-                                cc->sync);
+                                cc->sync ? MIGRATE_SYNC_LIGHT : MIGRATE_ASYNC);
                 update_nr_listpages(cc);
                 nr_remaining = cc->nr_migratepages;
 
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 06d3479513a..56080ea3614 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1557,7 +1557,7 @@ int soft_offline_page(struct page *page, int flags)
                                             page_is_file_cache(page));
                 list_add(&page->lru, &pagelist);
                 ret = migrate_pages(&pagelist, new_page, MPOL_MF_MOVE_ALL,
-                                                                0, true);
+                                                        0, MIGRATE_SYNC);
                 if (ret) {
                         putback_lru_pages(&pagelist);
                         pr_info("soft offline: %#lx: migration failed %d, type %lx\n",
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 2168489c0bc..6629fafd6ce 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -809,7 +809,7 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
                 }
                 /* this function returns # of failed pages */
                 ret = migrate_pages(&source, hotremove_migrate_alloc, 0,
-                                                                true, true);
+                                                        true, MIGRATE_SYNC);
                 if (ret)
                         putback_lru_pages(&source);
         }
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index e3d58f08846..06b145fb64a 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -942,7 +942,7 @@ static int migrate_to_node(struct mm_struct *mm, int source, int dest,
 
         if (!list_empty(&pagelist)) {
                 err = migrate_pages(&pagelist, new_node_page, dest,
-                                                                false, true);
+                                                        false, MIGRATE_SYNC);
                 if (err)
                         putback_lru_pages(&pagelist);
         }
diff --git a/mm/migrate.c b/mm/migrate.c
index 4e86f3bacb8..9871a56d82c 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -218,12 +218,13 @@ out:
 
 #ifdef CONFIG_BLOCK
 /* Returns true if all buffers are successfully locked */
-static bool buffer_migrate_lock_buffers(struct buffer_head *head, bool sync)
+static bool buffer_migrate_lock_buffers(struct buffer_head *head,
+                                                        enum migrate_mode mode)
 {
         struct buffer_head *bh = head;
 
         /* Simple case, sync compaction */
-        if (sync) {
+        if (mode != MIGRATE_ASYNC) {
                 do {
                         get_bh(bh);
                         lock_buffer(bh);
@@ -259,7 +260,7 @@ static bool buffer_migrate_lock_buffers(struct buffer_head *head, bool sync)
 }
 #else
 static inline bool buffer_migrate_lock_buffers(struct buffer_head *head,
-                                                                bool sync)
+                                                        enum migrate_mode mode)
 {
         return true;
 }
@@ -275,7 +276,7 @@ static inline bool buffer_migrate_lock_buffers(struct buffer_head *head,
  */
 static int migrate_page_move_mapping(struct address_space *mapping,
                 struct page *newpage, struct page *page,
-                struct buffer_head *head, bool sync)
+                struct buffer_head *head, enum migrate_mode mode)
 {
         int expected_count;
         void **pslot;
@@ -311,7 +312,8 @@ 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 (!sync && head && !buffer_migrate_lock_buffers(head, sync)) {
+        if (mode == MIGRATE_ASYNC && head &&
+                        !buffer_migrate_lock_buffers(head, mode)) {
                 page_unfreeze_refs(page, expected_count);
                 spin_unlock_irq(&mapping->tree_lock);
                 return -EAGAIN;
@@ -472,13 +474,14 @@ EXPORT_SYMBOL(fail_migrate_page);
  * Pages are locked upon entry and exit.
  */
 int migrate_page(struct address_space *mapping,
-                struct page *newpage, struct page *page, bool sync)
+                struct page *newpage, struct page *page,
+                enum migrate_mode mode)
 {
         int rc;
 
         BUG_ON(PageWriteback(page));    /* Writeback must be complete */
 
-        rc = migrate_page_move_mapping(mapping, newpage, page, NULL, sync);
+        rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode);
 
         if (rc)
                 return rc;
@@ -495,17 +498,17 @@ EXPORT_SYMBOL(migrate_page);
  * exist.
  */
 int buffer_migrate_page(struct address_space *mapping,
-                struct page *newpage, struct page *page, bool sync)
+                struct page *newpage, struct page *page, enum migrate_mode mode)
 {
         struct buffer_head *bh, *head;
         int rc;
 
         if (!page_has_buffers(page))
-                return migrate_page(mapping, newpage, page, sync);
+                return migrate_page(mapping, newpage, page, mode);
 
         head = page_buffers(page);
 
-        rc = migrate_page_move_mapping(mapping, newpage, page, head, sync);
+        rc = migrate_page_move_mapping(mapping, newpage, page, head, mode);
 
         if (rc)
                 return rc;
@@ -515,8 +518,8 @@ 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 (sync)
-                BUG_ON(!buffer_migrate_lock_buffers(head, sync));
+        if (mode != MIGRATE_ASYNC)
+                BUG_ON(!buffer_migrate_lock_buffers(head, mode));
 
         ClearPagePrivate(page);
         set_page_private(newpage, page_private(page));
@@ -593,10 +596,11 @@ static int writeout(struct address_space *mapping, struct page *page)
  * Default handling if a filesystem does not provide a migration function.
  */
 static int fallback_migrate_page(struct address_space *mapping,
-        struct page *newpage, struct page *page, bool sync)
+        struct page *newpage, struct page *page, enum migrate_mode mode)
 {
         if (PageDirty(page)) {
-                if (!sync)
+                /* Only writeback pages in full synchronous migration */
+                if (mode != MIGRATE_SYNC)
                         return -EBUSY;
                 return writeout(mapping, page);
         }
@@ -609,7 +613,7 @@ static int fallback_migrate_page(struct address_space *mapping,
             !try_to_release_page(page, GFP_KERNEL))
                 return -EAGAIN;
 
-        return migrate_page(mapping, newpage, page, sync);
+        return migrate_page(mapping, newpage, page, mode);
 }
 
 /*
@@ -624,7 +628,7 @@ static int fallback_migrate_page(struct address_space *mapping,
  *  == 0 - success
  */
 static int move_to_new_page(struct page *newpage, struct page *page,
-                                        int remap_swapcache, bool sync)
+                                int remap_swapcache, enum migrate_mode mode)
 {
         struct address_space *mapping;
         int rc;
@@ -645,7 +649,7 @@ static int move_to_new_page(struct page *newpage, struct page *page,
 
         mapping = page_mapping(page);
         if (!mapping)
-                rc = migrate_page(mapping, newpage, page, sync);
+                rc = migrate_page(mapping, newpage, page, mode);
         else if (mapping->a_ops->migratepage)
                 /*
                  * Most pages have a mapping and most filesystems provide a
@@ -654,9 +658,9 @@ static int move_to_new_page(struct page *newpage, struct page *page,
                  * is the most common path for page migration.
                  */
                 rc = mapping->a_ops->migratepage(mapping,
-                                                newpage, page, sync);
+                                                newpage, page, mode);
         else
-                rc = fallback_migrate_page(mapping, newpage, page, sync);
+                rc = fallback_migrate_page(mapping, newpage, page, mode);
 
         if (rc) {
                 newpage->mapping = NULL;
@@ -671,7 +675,7 @@ static int move_to_new_page(struct page *newpage, struct page *page,
 }
 
 static int __unmap_and_move(struct page *page, struct page *newpage,
-                                int force, bool offlining, bool sync)
+                        int force, bool offlining, enum migrate_mode mode)
 {
         int rc = -EAGAIN;
         int remap_swapcache = 1;
@@ -680,7 +684,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
         struct anon_vma *anon_vma = NULL;
 
         if (!trylock_page(page)) {
-                if (!force || !sync)
+                if (!force || mode == MIGRATE_ASYNC)
                         goto out;
 
                 /*
@@ -726,10 +730,12 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 
         if (PageWriteback(page)) {
                 /*
-                 * For !sync, there is no point retrying as the retry loop
-                 * is expected to be too short for PageWriteback to be cleared
+                 * Only in the case of a full syncronous migration is it
+                 * necessary to wait for PageWriteback. In the async case,
+                 * the retry loop is too short and in the sync-light case,
+                 * the overhead of stalling is too much
                  */
-                if (!sync) {
+                if (mode != MIGRATE_SYNC) {
                         rc = -EBUSY;
                         goto uncharge;
                 }
@@ -800,7 +806,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 
 skip_unmap:
         if (!page_mapped(page))
-                rc = move_to_new_page(newpage, page, remap_swapcache, sync);
+                rc = move_to_new_page(newpage, page, remap_swapcache, mode);
 
         if (rc && remap_swapcache)
                 remove_migration_ptes(page, page);
@@ -823,7 +829,8 @@ out:
  * to the newly allocated page in newpage.
  */
 static int unmap_and_move(new_page_t get_new_page, unsigned long private,
-                        struct page *page, int force, bool offlining, bool sync)
+                        struct page *page, int force, bool offlining,
+                        enum migrate_mode mode)
 {
         int rc = 0;
         int *result = NULL;
@@ -843,7 +850,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
                 if (unlikely(split_huge_page(page)))
                         goto out;
 
-        rc = __unmap_and_move(page, newpage, force, offlining, sync);
+        rc = __unmap_and_move(page, newpage, force, offlining, mode);
 out:
         if (rc != -EAGAIN) {
                 /*
@@ -891,7 +898,8 @@ out:
  */
 static int unmap_and_move_huge_page(new_page_t get_new_page,
                                 unsigned long private, struct page *hpage,
-                                int force, bool offlining, bool sync)
+                                int force, bool offlining,
+                                enum migrate_mode mode)
 {
         int rc = 0;
         int *result = NULL;
@@ -904,7 +912,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
         rc = -EAGAIN;
 
         if (!trylock_page(hpage)) {
-                if (!force || !sync)
+                if (!force || mode != MIGRATE_SYNC)
                         goto out;
                 lock_page(hpage);
         }
@@ -915,7 +923,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
         try_to_unmap(hpage, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
 
         if (!page_mapped(hpage))
-                rc = move_to_new_page(new_hpage, hpage, 1, sync);
+                rc = move_to_new_page(new_hpage, hpage, 1, mode);
 
         if (rc)
                 remove_migration_ptes(hpage, hpage);
@@ -958,7 +966,7 @@ out:
  */
 int migrate_pages(struct list_head *from,
                 new_page_t get_new_page, unsigned long private, bool offlining,
-                bool sync)
+                enum migrate_mode mode)
 {
         int retry = 1;
         int nr_failed = 0;
@@ -979,7 +987,7 @@ int migrate_pages(struct list_head *from,
 
                         rc = unmap_and_move(get_new_page, private,
                                                 page, pass > 2, offlining,
-                                                sync);
+                                                mode);
 
                         switch(rc) {
                         case -ENOMEM:
@@ -1009,7 +1017,7 @@ out:
 
 int migrate_huge_pages(struct list_head *from,
                 new_page_t get_new_page, unsigned long private, bool offlining,
-                bool sync)
+                enum migrate_mode mode)
 {
         int retry = 1;
         int nr_failed = 0;
@@ -1026,7 +1034,7 @@ int migrate_huge_pages(struct list_head *from,
 
                         rc = unmap_and_move_huge_page(get_new_page,
                                         private, page, pass > 2, offlining,
-                                        sync);
+                                        mode);
 
                         switch(rc) {
                         case -ENOMEM:
@@ -1155,7 +1163,7 @@ set_status:
         err = 0;
         if (!list_empty(&pagelist)) {
                 err = migrate_pages(&pagelist, new_page_node,
-                                (unsigned long)pm, 0, true);
+                                (unsigned long)pm, 0, MIGRATE_SYNC);
                 if (err)
                         putback_lru_pages(&pagelist);
         }