[RFC 08/11] ext4: Don't skip prefetching BLOCK_UNINIT groups

Ojaswin Mujoo
2023-01-27T12:39:18Z

Currently, ext4_mb_prefetch() and ext4_mb_prefetch_fini() skip
BLOCK_UNINIT groups since fetching their bitmaps doesn't need disk IO.
As a consequence, we end not initializing the buddy structures and CR0/1
lists for these BGs, even though it can be done without any disk IO
overhead. Hence, don't skip such BGs during prefetch and prefetch_fini.

This improves the accuracy of CR0/1 allocation as earlier, we could have
essentially empty BLOCK_UNINIT groups being ignored by CR0/1 due to their buddy
not being initialized, leading to slower CR2 allocations. With this patch CR0/1
will be able to discover these groups as well, thus improving performance.

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
fs/ext4/mballoc.c | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 14529d2fe65f..48726a831264 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2557,9 +2557,7 @@ ext4_group_t ext4_mb_prefetch(struct super_block *sb, ext4_group_t group,
*/
if (!EXT4_MB_GRP_TEST_AND_SET_READ(grp) &&
EXT4_MB_GRP_NEED_INIT(grp) &&
- ext4_free_group_clusters(sb, gdp) > 0 &&
- !(ext4_has_group_desc_csum(sb) &&
- (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)))) {
+ ext4_free_group_clusters(sb, gdp) > 0 ) {
bh = ext4_read_block_bitmap_nowait(sb, group, true);
if (bh && !IS_ERR(bh)) {
if (!buffer_uptodate(bh) && cnt)
@@ -2600,9 +2598,7 @@ void ext4_mb_prefetch_fini(struct super_block *sb, ext4_group_t group,
grp = ext4_get_group_info(sb, group);

if (EXT4_MB_GRP_NEED_INIT(grp) &&
- ext4_free_group_clusters(sb, gdp) > 0 &&
- !(ext4_has_group_desc_csum(sb) &&
- (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)))) {
+ ext4_free_group_clusters(sb, gdp) > 0) {
if (ext4_mb_init_group(sb, group, GFP_NOFS))
break;
}
--
2.31.1