ext4: Fix small file fragmentation

For small file block allocations, mballoc uses per cpu prealloc
space.  Use goal block when searching for the right prealloc
space.  Also make sure ext4_da_writepages tries to write
all the pages for small files in single attempt

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

2 files changed, 61 insertions, 13 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index d1906d9a22de..7e91913e325b 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2282,13 +2282,12 @@ static int ext4_da_writepages_trans_blocks(struct inode *inode)
 static int ext4_da_writepages(struct address_space *mapping,
                               struct writeback_control *wbc)
 {
-        struct inode *inode = mapping->host;
         handle_t *handle = NULL;
-        int needed_blocks;
-        int ret = 0;
-        long to_write;
         loff_t range_start = 0;
-        long pages_skipped = 0;
+        struct inode *inode = mapping->host;
+        int needed_blocks, ret = 0, nr_to_writebump = 0;
+        long to_write, pages_skipped = 0;
+        struct ext4_sb_info *sbi = EXT4_SB(mapping->host->i_sb);
 
         /*
          * No pages to write? This is mainly a kludge to avoid starting
@@ -2297,6 +2296,16 @@ static int ext4_da_writepages(struct address_space *mapping,
          */
         if (!mapping->nrpages || !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
                 return 0;
+        /*
+         * Make sure nr_to_write is >= sbi->s_mb_stream_request
+         * This make sure small files blocks are allocated in
+         * single attempt. This ensure that small files
+         * get less fragmented.
+         */
+        if (wbc->nr_to_write < sbi->s_mb_stream_request) {
+                nr_to_writebump = sbi->s_mb_stream_request - wbc->nr_to_write;
+                wbc->nr_to_write = sbi->s_mb_stream_request;
+        }
 
         if (!wbc->range_cyclic)
                 /*
@@ -2377,7 +2386,7 @@ restart_loop:
         }
 
 out_writepages:
-        wbc->nr_to_write = to_write;
+        wbc->nr_to_write = to_write - nr_to_writebump;
         wbc->range_start = range_start;
         return ret;
 }
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 865e9ddb44d4..e0e3a5eb1ddb 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -3282,6 +3282,35 @@ static void ext4_mb_use_group_pa(struct ext4_allocation_context *ac,
 }
 
 /*
+ * Return the prealloc space that have minimal distance
+ * from the goal block. @cpa is the prealloc
+ * space that is having currently known minimal distance
+ * from the goal block.
+ */
+static struct ext4_prealloc_space *
+ext4_mb_check_group_pa(ext4_fsblk_t goal_block,
+                        struct ext4_prealloc_space *pa,
+                        struct ext4_prealloc_space *cpa)
+{
+        ext4_fsblk_t cur_distance, new_distance;
+
+        if (cpa == NULL) {
+                atomic_inc(&pa->pa_count);
+                return pa;
+        }
+        cur_distance = abs(goal_block - cpa->pa_pstart);
+        new_distance = abs(goal_block - pa->pa_pstart);
+
+        if (cur_distance < new_distance)
+                return cpa;
+
+        /* drop the previous reference */
+        atomic_dec(&cpa->pa_count);
+        atomic_inc(&pa->pa_count);
+        return pa;
+}
+
+/*
  * search goal blocks in preallocated space
  */
 static noinline_for_stack int
@@ -3290,7 +3319,8 @@ ext4_mb_use_preallocated(struct ext4_allocation_context *ac)
         int order, i;
         struct ext4_inode_info *ei = EXT4_I(ac->ac_inode);
         struct ext4_locality_group *lg;
-        struct ext4_prealloc_space *pa;
+        struct ext4_prealloc_space *pa, *cpa = NULL;
+        ext4_fsblk_t goal_block;
 
         /* only data can be preallocated */
         if (!(ac->ac_flags & EXT4_MB_HINT_DATA))
@@ -3333,6 +3363,13 @@ ext4_mb_use_preallocated(struct ext4_allocation_context *ac)
                 /* The max size of hash table is PREALLOC_TB_SIZE */
                 order = PREALLOC_TB_SIZE - 1;
 
+        goal_block = ac->ac_g_ex.fe_group * EXT4_BLOCKS_PER_GROUP(ac->ac_sb) +
+                     ac->ac_g_ex.fe_start +
+                     le32_to_cpu(EXT4_SB(ac->ac_sb)->s_es->s_first_data_block);
+        /*
+         * search for the prealloc space that is having
+         * minimal distance from the goal block.
+         */
         for (i = order; i < PREALLOC_TB_SIZE; i++) {
                 rcu_read_lock();
                 list_for_each_entry_rcu(pa, &lg->lg_prealloc_list[i],
@@ -3340,17 +3377,19 @@ ext4_mb_use_preallocated(struct ext4_allocation_context *ac)
                         spin_lock(&pa->pa_lock);
                         if (pa->pa_deleted == 0 &&
                                         pa->pa_free >= ac->ac_o_ex.fe_len) {
-                                atomic_inc(&pa->pa_count);
-                                ext4_mb_use_group_pa(ac, pa);
-                                spin_unlock(&pa->pa_lock);
-                                ac->ac_criteria = 20;
-                                rcu_read_unlock();
-                                return 1;
+
+                                cpa = ext4_mb_check_group_pa(goal_block,
+                                                                pa, cpa);
                         }
                         spin_unlock(&pa->pa_lock);
                 }
                 rcu_read_unlock();
         }
+        if (cpa) {
+                ext4_mb_use_group_pa(ac, cpa);
+                ac->ac_criteria = 20;
+                return 1;
+        }
         return 0;
 }