Re: [PATCH v2 09/12] ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs

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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 &&




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