ext4: check for a good block group before loading buddy pages

This adds a new field in ext4_group_info to cache the largest available
block range in a block group; and don't load the buddy pages until *after*
we've done a sanity check on the block group.

With large allocation requests (e.g., fallocate(), 8MiB) and relatively full
partitions, it's easy to have no block groups with a block extent large
enough to satisfy the input request length.  This currently causes the loop
during cr == 0 in ext4_mb_regular_allocator() to load the buddy bitmap pages
for EVERY block group.  That can be a lot of pages.  The patch below allows
us to call ext4_mb_good_group() BEFORE we load the buddy pages (although we
have check again after we lock the block group).

Addresses-Google-Bug: #2578108
Addresses-Google-Bug: #2704453

Signed-off-by: Curt Wohlgemuth <curtw@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

2 files changed, 58 insertions, 13 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index bf938cf7c5f0..d266003cac3e 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1678,6 +1678,7 @@ struct ext4_group_info {
         ext4_grpblk_t   bb_first_free;  /* first free block */
         ext4_grpblk_t   bb_free;        /* total free blocks */
         ext4_grpblk_t   bb_fragments;   /* nr of freespace fragments */
+        ext4_grpblk_t   bb_largest_free_order;/* order of largest frag in BG */
         struct          list_head bb_prealloc_list;
 #ifdef DOUBLE_CHECK
         void            *bb_bitmap;
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 4f2d3a9d4e21..aa499fe11687 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -658,6 +658,27 @@ static void ext4_mb_mark_free_simple(struct super_block *sb,
         }
 }
 
+/*
+ * Cache the order of the largest free extent we have available in this block
+ * group.
+ */
+static void
+mb_set_largest_free_order(struct super_block *sb, struct ext4_group_info *grp)
+{
+        int i;
+        int bits;
+
+        grp->bb_largest_free_order = -1; /* uninit */
+
+        bits = sb->s_blocksize_bits + 1;
+        for (i = bits; i >= 0; i--) {
+                if (grp->bb_counters[i] > 0) {
+                        grp->bb_largest_free_order = i;
+                        break;
+                }
+        }
+}
+
 static noinline_for_stack
 void ext4_mb_generate_buddy(struct super_block *sb,
                                 void *buddy, void *bitmap, ext4_group_t group)
@@ -700,6 +721,7 @@ void ext4_mb_generate_buddy(struct super_block *sb,
                  */
                 grp->bb_free = free;
         }
+        mb_set_largest_free_order(sb, grp);
 
         clear_bit(EXT4_GROUP_INFO_NEED_INIT_BIT, &(grp->bb_state));
 
@@ -725,6 +747,9 @@ void ext4_mb_generate_buddy(struct super_block *sb,
  * contain blocks_per_page (PAGE_CACHE_SIZE / blocksize)  blocks.
  * So it can have information regarding groups_per_page which
  * is blocks_per_page/2
+ *
+ * Locking note:  This routine takes the block group lock of all groups
+ * for this page; do not hold this lock when calling this routine!
  */
 
 static int ext4_mb_init_cache(struct page *page, char *incore)
@@ -910,6 +935,11 @@ out:
         return err;
 }
 
+/*
+ * Locking note:  This routine calls ext4_mb_init_cache(), which takes the
+ * block group lock of all groups for this page; do not hold the BG lock when
+ * calling this routine!
+ */
 static noinline_for_stack
 int ext4_mb_init_group(struct super_block *sb, ext4_group_t group)
 {
@@ -1004,6 +1034,11 @@ err:
         return ret;
 }
 
+/*
+ * Locking note:  This routine calls ext4_mb_init_cache(), which takes the
+ * block group lock of all groups for this page; do not hold the BG lock when
+ * calling this routine!
+ */
 static noinline_for_stack int
 ext4_mb_load_buddy(struct super_block *sb, ext4_group_t group,
                                         struct ext4_buddy *e4b)
@@ -1299,6 +1334,7 @@ static void mb_free_blocks(struct inode *inode, struct ext4_buddy *e4b,
                         buddy = buddy2;
                 } while (1);
         }
+        mb_set_largest_free_order(sb, e4b->bd_info);
         mb_check_buddy(e4b);
 }
 
@@ -1427,6 +1463,7 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
                 e4b->bd_info->bb_counters[ord]++;
                 e4b->bd_info->bb_counters[ord]++;
         }
+        mb_set_largest_free_order(e4b->bd_sb, e4b->bd_info);
 
         mb_set_bits(EXT4_MB_BITMAP(e4b), ex->fe_start, len0);
         mb_check_buddy(e4b);
@@ -1821,16 +1858,22 @@ void ext4_mb_scan_aligned(struct ext4_allocation_context *ac,
         }
 }
 
+/* This is now called BEFORE we load the buddy bitmap. */
 static int ext4_mb_good_group(struct ext4_allocation_context *ac,
                                 ext4_group_t group, int cr)
 {
         unsigned free, fragments;
-        unsigned i, bits;
         int flex_size = ext4_flex_bg_size(EXT4_SB(ac->ac_sb));
         struct ext4_group_info *grp = ext4_get_group_info(ac->ac_sb, group);
 
         BUG_ON(cr < 0 || cr >= 4);
-        BUG_ON(EXT4_MB_GRP_NEED_INIT(grp));
+
+        /* We only do this if the grp has never been initialized */
+        if (unlikely(EXT4_MB_GRP_NEED_INIT(grp))) {
+                int ret = ext4_mb_init_group(ac->ac_sb, group);
+                if (ret)
+                        return 0;
+        }
 
         free = grp->bb_free;
         fragments = grp->bb_fragments;
@@ -1843,17 +1886,16 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
         case 0:
                 BUG_ON(ac->ac_2order == 0);
 
+                if (grp->bb_largest_free_order < ac->ac_2order)
+                        return 0;
+
                 /* Avoid using the first bg of a flexgroup for data files */
                 if ((ac->ac_flags & EXT4_MB_HINT_DATA) &&
                     (flex_size >= EXT4_FLEX_SIZE_DIR_ALLOC_SCHEME) &&
                     ((group % flex_size) == 0))
                         return 0;
 
-                bits = ac->ac_sb->s_blocksize_bits + 1;
-                for (i = ac->ac_2order; i <= bits; i++)
-                        if (grp->bb_counters[i] > 0)
-                                return 1;
-                break;
+                return 1;
         case 1:
                 if ((free / fragments) >= ac->ac_g_ex.fe_len)
                         return 1;
@@ -2024,14 +2066,11 @@ repeat:
                 group = ac->ac_g_ex.fe_group;
 
                 for (i = 0; i < ngroups; group++, i++) {
-                        struct ext4_group_info *grp;
-
                         if (group == ngroups)
                                 group = 0;
 
-                        /* quick check to skip empty groups */
-                        grp = ext4_get_group_info(sb, group);
-                        if (grp->bb_free == 0)
+                        /* This now checks without needing the buddy page */
+                        if (!ext4_mb_good_group(ac, group, cr))
                                 continue;
 
                         err = ext4_mb_load_buddy(sb, group, &e4b);
@@ -2039,8 +2078,12 @@ repeat:
                                 goto out;
 
                         ext4_lock_group(sb, group);
+
+                        /*
+                         * We need to check again after locking the
+                         * block group
+                         */
                         if (!ext4_mb_good_group(ac, group, cr)) {
-                                /* someone did allocation from this group */
                                 ext4_unlock_group(sb, group);
                                 ext4_mb_unload_buddy(&e4b);
                                 continue;
@@ -2253,6 +2296,7 @@ int ext4_mb_add_groupinfo(struct super_block *sb, ext4_group_t group,
         INIT_LIST_HEAD(&meta_group_info[i]->bb_prealloc_list);
         init_rwsem(&meta_group_info[i]->alloc_sem);
         meta_group_info[i]->bb_free_root = RB_ROOT;
+        meta_group_info[i]->bb_largest_free_order = -1;  /* uninit */
 
 #ifdef DOUBLE_CHECK
         {