Hello,
On 5/30/23 20:33, Ojaswin Mujoo wrote:
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.
Thanks for the series.
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.
Seems ext4_mb_prefetch_fini can't be called by ext4_mb_regular_allocator
if nr is 0.
https://elixir.bootlin.com/linux/v6.4-rc5/source/fs/ext4/mballoc.c#L2816
Am I miss something?
Thanks,
Guoqing
Signed-off-by: Ojaswin Mujoo<ojaswin@xxxxxxxxxxxxx>
Reviewed-by: Ritesh Harjani (IBM)<ritesh.list@xxxxxxxxx>
Reviewed-by: Jan Kara<jack@xxxxxxx>
---
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 79455c7e645b..6775d73dfc68 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2735,8 +2735,6 @@ 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;
@@ -2745,8 +2743,6 @@ 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 2da5476fa48b..27c1dabacd43 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3692,16 +3692,13 @@ 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 &&
