ext4: avoid uneeded calls to ext4_mb_load_buddy() while reading mb_groups

Currently ext4_mb_load_buddy is called for every group, irrespective
of whether the group info is already in memory, while reading
/proc/fs/ext4/<partition>/mb_groups proc file.  For the purpose of
mb_groups proc file, it is unnecessary to load the file group info
from disk if it was loaded in past.  These calls to ext4_mb_load_buddy
make reading the mb_groups proc file expensive.

Also, the locks around ext4_get_group_info are not required.

This patch modifies the code to call ext4_mb_load_buddy only if the
group info had never been loaded into memory in past. It also removes
the mb group locking around ext4_get_group_info call.

Signed-off-by: Aditya Kali <adityakali@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 15 insertions, 8 deletions

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 1cd6994fc446..9f1e655979b9 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2077,8 +2077,9 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, void *v)
         struct super_block *sb = seq->private;
         ext4_group_t group = (ext4_group_t) ((unsigned long) v);
         int i;
-        int err;
+        int err, buddy_loaded = 0;
         struct ext4_buddy e4b;
+        struct ext4_group_info *grinfo;
         struct sg {
                 struct ext4_group_info info;
                 ext4_grpblk_t counters[16];
@@ -2095,15 +2096,21 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, void *v)
 
         i = (sb->s_blocksize_bits + 2) * sizeof(sg.info.bb_counters[0]) +
                 sizeof(struct ext4_group_info);
-        err = ext4_mb_load_buddy(sb, group, &e4b);
-        if (err) {
-                seq_printf(seq, "#%-5u: I/O error\n", group);
-                return 0;
+        grinfo = ext4_get_group_info(sb, group);
+        /* Load the group info in memory only if not already loaded. */
+        if (unlikely(EXT4_MB_GRP_NEED_INIT(grinfo))) {
+                err = ext4_mb_load_buddy(sb, group, &e4b);
+                if (err) {
+                        seq_printf(seq, "#%-5u: I/O error\n", group);
+                        return 0;
+                }
+                buddy_loaded = 1;
         }
-        ext4_lock_group(sb, group);
+
         memcpy(&sg, ext4_get_group_info(sb, group), i);
-        ext4_unlock_group(sb, group);
-        ext4_mb_unload_buddy(&e4b);
+
+        if (buddy_loaded)
+                ext4_mb_unload_buddy(&e4b);
 
         seq_printf(seq, "#%-5u: %-5u %-5u %-5u [", group, sg.info.bb_free,
                         sg.info.bb_fragments, sg.info.bb_first_free);