[RFC 09/11] ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs

Ojaswin Mujoo
2023-01-27T12:39:34Z

Before this patch, the call stack in ext4_run_li_request is as follows:

/*
* nr = no. of BGs we want to fetch (=s_mb_prefetch)
* prefetch_ios = no. of BGs not uptodate after
* ext4_read_block_bitmap_nowait()
*/
next_group = ext4_mb_prefetch(sb, group, nr, prefetch_ios);
ext4_mb_prefetch_fini(sb, next_group prefetch_ios);

ext4_mb_prefetch_fini() will only try to initialize buddies for BGs in
range [next_group - prefetch_ios, next_group). This is incorrect since
sometimes (prefetch_ios < nr), which causes ext4_mb_prefetch_fini() to
incorrectly ignore some of the BGs that might need initialization. This
issue is more notable now with the previous patch enabling "fetching" of
BLOCK_UNINIT BGs which are marked buffer_uptodate by default.

Fix this by passing nr to ext4_mb_prefetch_fini() instead of
prefetch_ios so that it considers the right range of groups.

Similarly, make sure we don't pass nr=0 to ext4_mb_prefetch_fini() in
ext4_mb_regular_allocator() since we might have prefetched BLOCK_UNINIT
groups that would need buddy initialization.

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
fs/ext4/mballoc.c | 4 ----
fs/ext4/super.c | 11 ++++-------
2 files changed, 4 insertions(+), 11 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 48726a831264..410c9636907b 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2702,8 +2702,6 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
if ((prefetch_grp == group) &&
(cr > CR1 ||
prefetch_ios < sbi->s_mb_prefetch_limit)) {
- unsigned int curr_ios = prefetch_ios;
-
nr = sbi->s_mb_prefetch;
if (ext4_has_feature_flex_bg(sb)) {
nr = 1 << sbi->s_log_groups_per_flex;
@@ -2712,8 +2710,6 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
}
prefetch_grp = ext4_mb_prefetch(sb, group,
nr, &prefetch_ios);
- if (prefetch_ios == curr_ios)
- nr = 0;
}

/* This now checks without needing the buddy page */
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 72ead3b56706..9dbb09cfc8f7 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3636,16 +3636,13 @@ static int ext4_run_li_request(struct ext4_li_request *elr)
ext4_group_t group = elr->lr_next_group;
unsigned int prefetch_ios = 0;
int ret = 0;
+ int nr = EXT4_SB(sb)->s_mb_prefetch;
u64 start_time;

if (elr->lr_mode == EXT4_LI_MODE_PREFETCH_BBITMAP) {
- elr->lr_next_group = ext4_mb_prefetch(sb, group,
- EXT4_SB(sb)->s_mb_prefetch, &prefetch_ios);
- if (prefetch_ios)
- ext4_mb_prefetch_fini(sb, elr->lr_next_group,
- prefetch_ios);
- trace_ext4_prefetch_bitmaps(sb, group, elr->lr_next_group,
- prefetch_ios);
+ elr->lr_next_group = ext4_mb_prefetch(sb, group, nr, &prefetch_ios);
+ ext4_mb_prefetch_fini(sb, elr->lr_next_group, nr);
+ trace_ext4_prefetch_bitmaps(sb, group, elr->lr_next_group, nr);
if (group >= elr->lr_next_group) {
ret = 1;
if (elr->lr_first_not_zeroed != ngroups &&
--
2.31.1