Re: [PATCH v4 16/19] ext4: call ext4_mb_mark_group_bb in ext4_mb_clear_bb

From: Ojaswin Mujoo
Date: Thu Jun 08 2023 - 02:16:56 EST

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On Tue, Jun 06, 2023 at 10:16:38PM +0800, Kemeng Shi wrote:
>
>
> on 6/6/2023 5:39 PM, Ojaswin Mujoo wrote:
> > On Sat, Jun 03, 2023 at 11:03:24PM +0800, Kemeng Shi wrote:
> >> call ext4_mb_mark_group_bb in ext4_mb_clear_bb to remove repeat code
> >> to update block bitmap and group descriptor on disk.
> >>
> >> Note: ext4_mb_clear_bb will update buddy and bitmap in two critical sections
> >> instead of update in the same critical section.
> >>
> >> Original lock behavior introduced in 7a2fcbf7f857 ("ext4: don't use
> >> blocks freed but not yet committed in buddy cache init") to avoid
> >> race betwwen ext4_mb_free_blocks and ext4_mb_init_cache:
> >> ext4_mb_load_buddy_gfp
> >> ext4_lock_group
> >> mb_clear_bits(bitmap_bh, ...)
> >> mb_free_blocks/ext4_mb_free_metadata
> >> ext4_unlock_group
> >> ext4_mb_unload_buddy
> >>
> >> New lock behavior in this patch:
> >> ext4_mb_load_buddy_gfp
> >> ext4_lock_group
> >> mb_clear_bits(bitmap_bh, ...)
> >> ext4_unlock_group
> >>
> >> /* no ext4_mb_init_cache for the same group will be called as
> >> ext4_mb_load_buddy_gfp will ensure buddy page is update-to-date */
> >>
> >> ext4_lock_group
> >> mb_free_blocks/ext4_mb_free_metadata
> >> ext4_unlock_group
> >> ext4_mb_unload_buddy
> >>
> >> As buddy page for group is always update-to-date between
> >> ext4_mb_load_buddy_gfp and ext4_mb_unload_buddy. Then no
> >> ext4_mb_init_cache will be called for the same group concurrentlly when
> >> we update bitmap and buddy page betwwen buddy load and unload.
> >>
> >> Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
> >
> > Hi Kemeng,
> >
> > Sorry for the late reply I was trying to understand the codepath
> > properly. So I have a question here:
> >
> > With the changes you've made in the patch, the flow would look something
> > like:
> >
> > ext4_mb_clear_bb():
> > ext4_mb_mark_group_bb():
> > ext4_group_lock()
> > - Mark bitmap free
> > - Modify gdp
> > ext4_group_unlock()
> > ext4_handle_dirty_metadata()
> > - I understand this will add the bitmap and gdp buffers to journal's
> > dirty metadata list
> > ...
> > ext4_group_lock()
> > ext4_mb_free_metadata()
> > - Add ext4_free_data entries to sbi->s_freed_data_list. (On commit
> > ext4_journal_commit_callback() will then free the buddy for these)
> > ext4_group_unlock()
> >
> > My question is what happens if journal commits between
> > ext4_handle_dirty_metadata() and ext4_mb_free_metadata() call (Possible?). Then we might
> > never end up freeing the metadata in the buddy bitmap because the commit callback wont
> > be able to find the ext4_free_data entries in sbi->s_freed_data_list.
> >
> > Regards,
> > ojaswin
> >
> Hi Ojaswin, thanks for the reply. To my knowledge, commit should be normally done after handle
> is stopped as following:
> ext4_journal_start_sb
> start_this_handle
> read_lock(&journal->j_state_lock);
> atomic_inc(&transaction->t_updates);
> read_unlock(&journal->j_state_lock);
>
> ext4_journal_stop
> jbd2_journal_stop
> stop_this_handle
> if (atomic_dec_and_test(&transaction->t_updates))
> wake_up(&journal->j_wait_updates);
>
> jbd2_journal_commit_transaction
> jbd2_journal_wait_updates
> while (1)
> if (!atomic_read(&transaction->t_updates))
> /* break loop */
> ...
>
> if (journal->j_commit_callback)
> journal->j_commit_callback(journal, commit_transaction);
>
> So no commit of transaction should not happen between ext4_handle_dirty_metadata and
> ext4_mb_free_metadata.

Hi Kemeng,

Okay makes sense. Thanks for the explanation :)

Regards,
ojaswin

>
> >> ---
> >> fs/ext4/mballoc.c | 90 ++++++++++++-----------------------------------
> >> 1 file changed, 23 insertions(+), 67 deletions(-)
> >>
> >> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> >> index 46b37f5c9223..e4f1b34448e3 100644
> >> --- a/fs/ext4/mballoc.c
> >> +++ b/fs/ext4/mballoc.c
> >> @@ -6135,19 +6135,21 @@ static void ext4_mb_clear_bb(handle_t *handle, struct inode *inode,
> >> ext4_fsblk_t block, unsigned long count,
> >> int flags)
> >> {
> >> - struct buffer_head *bitmap_bh = NULL;
> >> + struct ext4_mark_context mc = {
> >> + .handle = handle,
> >> + .sb = inode->i_sb,
> >> + .state = 0,
> >> + };
> >> struct super_block *sb = inode->i_sb;
> >> - struct ext4_group_desc *gdp;
> >> struct ext4_group_info *grp;
> >> unsigned int overflow;
> >> ext4_grpblk_t bit;
> >> - struct buffer_head *gd_bh;
> >> ext4_group_t block_group;
> >> struct ext4_sb_info *sbi;
> >> struct ext4_buddy e4b;
> >> unsigned int count_clusters;
> >> int err = 0;
> >> - int ret;
> >> + int mark_flags = 0;
> >>
> >> sbi = EXT4_SB(sb);
> >>
> >> @@ -6179,18 +6181,6 @@ static void ext4_mb_clear_bb(handle_t *handle, struct inode *inode,
> >> /* The range changed so it's no longer validated */
> >> flags &= ~EXT4_FREE_BLOCKS_VALIDATED;
> >> }
> >> - count_clusters = EXT4_NUM_B2C(sbi, count);
> >> - bitmap_bh = ext4_read_block_bitmap(sb, block_group);
> >> - if (IS_ERR(bitmap_bh)) {
> >> - err = PTR_ERR(bitmap_bh);
> >> - bitmap_bh = NULL;
> >> - goto error_return;
> >> - }
> >> - gdp = ext4_get_group_desc(sb, block_group, &gd_bh);
> >> - if (!gdp) {
> >> - err = -EIO;
> >> - goto error_return;
> >> - }
> >>
> >> if (!(flags & EXT4_FREE_BLOCKS_VALIDATED) &&
> >> !ext4_inode_block_valid(inode, block, count)) {
> >> @@ -6200,28 +6190,7 @@ static void ext4_mb_clear_bb(handle_t *handle, struct inode *inode,
> >> goto error_return;
> >> }
> >>
> >> - BUFFER_TRACE(bitmap_bh, "getting write access");
> >> - err = ext4_journal_get_write_access(handle, sb, bitmap_bh,
> >> - EXT4_JTR_NONE);
> >> - if (err)
> >> - goto error_return;
> >> -
> >> - /*
> >> - * We are about to modify some metadata. Call the journal APIs
> >> - * to unshare ->b_data if a currently-committing transaction is
> >> - * using it
> >> - */
> >> - BUFFER_TRACE(gd_bh, "get_write_access");
> >> - err = ext4_journal_get_write_access(handle, sb, gd_bh, EXT4_JTR_NONE);
> >> - if (err)
> >> - goto error_return;
> >> -#ifdef AGGRESSIVE_CHECK
> >> - {
> >> - int i;
> >> - for (i = 0; i < count_clusters; i++)
> >> - BUG_ON(!mb_test_bit(bit + i, bitmap_bh->b_data));
> >> - }
> >> -#endif
> >> + count_clusters = EXT4_NUM_B2C(sbi, count);
> >> trace_ext4_mballoc_free(sb, inode, block_group, bit, count_clusters);
> >>
> >> /* __GFP_NOFAIL: retry infinitely, ignore TIF_MEMDIE and memcg limit. */
> >> @@ -6230,6 +6199,22 @@ static void ext4_mb_clear_bb(handle_t *handle, struct inode *inode,
> >> if (err)
> >> goto error_return;
> >>
> >> +#ifdef AGGRESSIVE_CHECK
> >> + mark_flags |= EXT4_MB_BITMAP_MARKED_CHECK;
> >> +#endif
> >> + err = ext4_mb_mark_group_bb(&mc, block_group, bit, count_clusters,
> >> + mark_flags);
> >> +
> >> +
> >> + if (err && mc.changed == 0) {
> >> + ext4_mb_unload_buddy(&e4b);
> >> + goto error_return;
> >> + }
> >> +
> >> +#ifdef AGGRESSIVE_CHECK
> >> + BUG_ON(mc.changed != count_clusters);
> >> +#endif
> >> +
> >> /*
> >> * We need to make sure we don't reuse the freed block until after the
> >> * transaction is committed. We make an exception if the inode is to be
> >> @@ -6252,13 +6237,8 @@ static void ext4_mb_clear_bb(handle_t *handle, struct inode *inode,
> >> new_entry->efd_tid = handle->h_transaction->t_tid;
> >>
> >> ext4_lock_group(sb, block_group);
> >> - mb_clear_bits(bitmap_bh->b_data, bit, count_clusters);
> >> ext4_mb_free_metadata(handle, &e4b, new_entry);
> >> } else {
> >> - /* need to update group_info->bb_free and bitmap
> >> - * with group lock held. generate_buddy look at
> >> - * them with group lock_held
> >> - */
> >> if (test_opt(sb, DISCARD)) {
> >> err = ext4_issue_discard(sb, block_group, bit,
> >> count_clusters, NULL);
> >> @@ -6271,23 +6251,11 @@ static void ext4_mb_clear_bb(handle_t *handle, struct inode *inode,
> >> EXT4_MB_GRP_CLEAR_TRIMMED(e4b.bd_info);
> >>
> >> ext4_lock_group(sb, block_group);
> >> - mb_clear_bits(bitmap_bh->b_data, bit, count_clusters);
> >> mb_free_blocks(inode, &e4b, bit, count_clusters);
> >> }
> >>
> >> - ret = ext4_free_group_clusters(sb, gdp) + count_clusters;
> >> - ext4_free_group_clusters_set(sb, gdp, ret);
> >> - ext4_block_bitmap_csum_set(sb, gdp, bitmap_bh);
> >> - ext4_group_desc_csum_set(sb, block_group, gdp);
> >> ext4_unlock_group(sb, block_group);
> >>
> >> - if (sbi->s_log_groups_per_flex) {
> >> - ext4_group_t flex_group = ext4_flex_group(sbi, block_group);
> >> - atomic64_add(count_clusters,
> >> - &sbi_array_rcu_deref(sbi, s_flex_groups,
> >> - flex_group)->free_clusters);
> >> - }
> >> -
> >> /*
> >> * on a bigalloc file system, defer the s_freeclusters_counter
> >> * update to the caller (ext4_remove_space and friends) so they
> >> @@ -6302,26 +6270,14 @@ static void ext4_mb_clear_bb(handle_t *handle, struct inode *inode,
> >>
> >> ext4_mb_unload_buddy(&e4b);
> >>
> >> - /* We dirtied the bitmap block */
> >> - BUFFER_TRACE(bitmap_bh, "dirtied bitmap block");
> >> - err = ext4_handle_dirty_metadata(handle, NULL, bitmap_bh);
> >> -
> >> - /* And the group descriptor block */
> >> - BUFFER_TRACE(gd_bh, "dirtied group descriptor block");
> >> - ret = ext4_handle_dirty_metadata(handle, NULL, gd_bh);
> >> - if (!err)
> >> - err = ret;
> >> -
> >> if (overflow && !err) {
> >> block += count;
> >> count = overflow;
> >> - put_bh(bitmap_bh);
> >> /* The range changed so it's no longer validated */
> >> flags &= ~EXT4_FREE_BLOCKS_VALIDATED;
> >> goto do_more;
> >> }
> >> error_return:
> >> - brelse(bitmap_bh);
> >> ext4_std_error(sb, err);
> >> return;
> >> }
> >> --
> >> 2.30.0
> >>
> >
>
> --
> Best wishes
> Kemeng Shi
>

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