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
> ---
> 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
>
