ext4: add mb_debug logging when there are lost chunks

Lost chunks are when some other process raced with the current thread
to grab a particular block allocation.  Add mb_debug log for
developers who wants to see how often this is happening for a
particular workload.

Signed-off-by: Chunguang Xu <brookxu@tencent.com>
Link: https://lore.kernel.org/r/0a165ac0-1912-aebd-8a0d-b42e7cd1aea1@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -2309,6 +2309,7 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 	struct ext4_sb_info *sbi;
 	struct super_block *sb;
 	struct ext4_buddy e4b;
+	unsigned int lost;
 
 	sb = ac->ac_sb;
 	sbi = EXT4_SB(sb);
@@ -2456,22 +2457,24 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 		 * We've been searching too long. Let's try to allocate
 		 * the best chunk we've found so far
 		 */
-
 		ext4_mb_try_best_found(ac, &e4b);
 		if (ac->ac_status != AC_STATUS_FOUND) {
 			/*
 			 * Someone more lucky has already allocated it.
 			 * The only thing we can do is just take first
 			 * found block(s)
-			printk(KERN_DEBUG "EXT4-fs: someone won our chunk\n");
 			 */
+			lost = (unsigned int)atomic_inc_return(&sbi->s_mb_lost_chunks);
+			mb_debug(sb, "lost chunk, group: %u, start: %d, len: %d, lost: %u\n",
+				 ac->ac_b_ex.fe_group, ac->ac_b_ex.fe_start,
+				 ac->ac_b_ex.fe_len, lost);
+
 			ac->ac_b_ex.fe_group = 0;
 			ac->ac_b_ex.fe_start = 0;
 			ac->ac_b_ex.fe_len = 0;
 			ac->ac_status = AC_STATUS_CONTINUE;
 			ac->ac_flags |= EXT4_MB_HINT_FIRST;
 			cr = 3;
-			atomic_inc(&sbi->s_mb_lost_chunks);
 			goto repeat;
 		}
 	}