ext4: fallback to complex scan if aligned scan doesn't work

Currently in case the goal length is a multiple of stripe size we use
ext4_mb_scan_aligned() to find the stripe size aligned physical blocks.
In case we are not able to find any, we again go back to calling
ext4_mb_choose_next_group() to search for a different suitable block
group. However, since the linear search always begins from the start,
most of the times we end up with the same BG and the cycle continues.

With large fliesystems, the CPU can be stuck in this loop for hours
which can slow down the whole system. Hence, until we figure out a
better way to continue the search (rather than starting from beginning)
in ext4_mb_choose_next_group(), lets just fallback to
ext4_mb_complex_scan_group() in case aligned scan fails, as it is much
more likely to find the needed blocks.

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/ee033f6dfa0a7f2934437008a909c3788233950f.1702455010.git.ojaswin@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -2889,14 +2889,19 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 			ac->ac_groups_scanned++;
 			if (cr == CR_POWER2_ALIGNED)
 				ext4_mb_simple_scan_group(ac, &e4b);
-			else if ((cr == CR_GOAL_LEN_FAST ||
-				 cr == CR_BEST_AVAIL_LEN) &&
-				 sbi->s_stripe &&
-				 !(ac->ac_g_ex.fe_len %
-				 EXT4_B2C(sbi, sbi->s_stripe)))
-				ext4_mb_scan_aligned(ac, &e4b);
-			else
-				ext4_mb_complex_scan_group(ac, &e4b);
+			else {
+				bool is_stripe_aligned = sbi->s_stripe &&
+					!(ac->ac_g_ex.fe_len %
+					  EXT4_B2C(sbi, sbi->s_stripe));
+
+				if ((cr == CR_GOAL_LEN_FAST ||
+				     cr == CR_BEST_AVAIL_LEN) &&
+				    is_stripe_aligned)
+					ext4_mb_scan_aligned(ac, &e4b);
+
+				if (ac->ac_status == AC_STATUS_CONTINUE)
+					ext4_mb_complex_scan_group(ac, &e4b);
+			}
 
 			ext4_unlock_group(sb, group);
 			ext4_mb_unload_buddy(&e4b);