Commit 4f3d1e45 authored by Ojaswin Mujoo's avatar Ojaswin Mujoo Committed by Theodore Ts'o
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ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs 



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: default avatarOjaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: default avatarRitesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/05e648ae04ec5b754207032823e9c1de9a54f87a.1685449706.git.ojaswin@linux.ibm.com


Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 3c629604

fs/ext4/mballoc.c

+0 −4
Original line number Diff line number Diff line
@@ -2734,8 +2734,6 @@ 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;
@@ -2744,8 +2742,6 @@ 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 */

fs/ext4/super.c

+4 −7
Original line number Diff line number Diff line
@@ -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 &&