Date:   Thu, 27 May 2021 14:18:37 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Wang Jianchao <jianchao.wan9@...il.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        lishujin@...ishou.com,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH V2 7/7] ext4: get discard out of jbd2 commit kthread
 contex

On May 26, 2021, at 2:44 AM, Wang Jianchao <jianchao.wan9@...il.com> wrote:
> 
> Right now, discard is issued and waited to be completed in jbd2
> commit kthread context after the logs are committed. When large
> amount of files are deleted and discard is flooding, jbd2 commit
> kthread can be blocked for long time. Then all of the metadata
> operations can be blocked to wait the log space.
> 
> One case is the page fault path with read mm->mmap_sem held, which
> wants to update the file time but has to wait for the log space.
> When other threads in the task wants to do mmap, then write mmap_sem
> is blocked. Finally all of the following read mmap_sem requirements
> are blocked, even the ps command which need to read the /proc/pid/
> -cmdline. Our monitor service which needs to read /proc/pid/cmdline
> used to be blocked for 5 mins.
> 
> This patch frees the blocks back to buddy after commit and then do
> discard in a async kworker context in fstrim fashion, namely,
> - mark blocks to be discarded as used if they have not been allocated
> - do discard
> - mark them free
> After this, jbd2 commit kthread won't be blocked any more by discard
> and we won't get NOSPC even if the discard is slow or throttled.

I definitely agree that sharing the existing fstrim functionality makes
the most sense here.  Some comments inline on the implementation.

> Link: https://marc.info/?l=linux-kernel&m=162143690731901&w=2
> Suggested-by: Theodore Ts'o <tytso@....edu>
> Signed-off-by: Wang Jianchao <wangjianchao@...ishou.com>
> ---
> fs/ext4/ext4.h    |   2 +
> fs/ext4/mballoc.c | 162 +++++++++++++++++++++++++++++++++---------------------
> fs/ext4/mballoc.h |   3 -
> 3 files changed, 101 insertions(+), 66 deletions(-)
> 
> @@ -3024,30 +3039,77 @@ static inline int ext4_issue_discard(struct super_block *sb,
> 		return sb_issue_discard(sb, discard_block, count, GFP_NOFS, 0);
> }
> 
> -static void ext4_free_data_in_buddy(struct super_block *sb,
> -				    struct ext4_free_data *entry)
> +static void ext4_discard_work(struct work_struct *work)
> {
> +	struct ext4_sb_info *sbi = container_of(work,
> +			struct ext4_sb_info, s_discard_work);
> +	struct super_block *sb = sbi->s_sb;
> +	ext4_group_t ngroups = ext4_get_groups_count(sb);
> +	struct ext4_group_info *grp;
> +	struct ext4_free_data *fd, *nfd;
> 	struct ext4_buddy e4b;
> -	struct ext4_group_info *db;
> -	int err, count = 0, count2 = 0;
> +	int i, err;
> +
> +	for (i = 0; i < ngroups; i++) {
> +		grp = ext4_get_group_info(sb, i);
> +		if (RB_EMPTY_ROOT(&grp->bb_discard_root))
> +			continue;

For large filesystems there may be millions of block groups, so it
seems inefficient to scan all of the groups each time the work queue
is run.  Having a list of block group numbers, or bitmap/rbtree/xarray
of the group numbers that need to be trimmed may be more efficient?

Most of the complexity in the rest of the patch goes away if the trim
tracking is only done on a whole-group basis (min/max or just a single
bit per group).

Cheers, Andreas

> -	mb_debug(sb, "gonna free %u blocks in group %u (0x%p):",
> -		 entry->efd_count, entry->efd_group, entry);
> +		err = ext4_mb_load_buddy(sb, i, &e4b);
> +		if (err) {
> +			ext4_warning(sb, "Error %d loading buddy information for %u",
> +					err, i);
> +			continue;
> +		}
> 
> -	err = ext4_mb_load_buddy(sb, entry->efd_group, &e4b);
> -	/* we expect to find existing buddy because it's pinned */
> -	BUG_ON(err != 0);
> +		ext4_lock_group(sb, i);
> +		rbtree_postorder_for_each_entry_safe(fd, nfd,
> +				&grp->bb_discard_root, efd_node) {
> +			rb_erase(&fd->efd_node, &grp->bb_discard_root);
> +			/*
> +			 * If filesystem is umounting, give up the discard
> +			 */
> +			if (sb->s_flags & SB_ACTIVE)
> +				ext4_try_to_trim_range(sb, &e4b, fd->efd_start_cluster,
> +						fd->efd_start_cluster + fd->efd_count - 1, 1);
> +			kmem_cache_free(ext4_free_data_cachep, fd);
> +		}
> +		ext4_unlock_group(sb, i);
> +		ext4_mb_unload_buddy(&e4b);
> +	}
> +}
> 
> -	atomic_sub(entry->efd_count, &EXT4_SB(sb)->s_mb_free_pending);
> +static int ext4_free_data_in_buddy(struct super_block *sb,
> +		ext4_group_t group, tid_t commit_tid)
> +{
> +	struct ext4_buddy e4b;
> +	struct ext4_group_info *db;
> +	struct ext4_free_data *fd, *nfd;
> +	int cnt, nr_fd;
> 
> +	/* we expect to find existing buddy because it's pinned */
> +	BUG_ON(ext4_mb_load_buddy(sb, group, &e4b));
> 	db = e4b.bd_info;
> -	/* there are blocks to put in buddy to make them really free */
> -	count += entry->efd_count;
> -	count2++;
> -	ext4_lock_group(sb, entry->efd_group);
> -	/* Take it out of per group rb tree */
> -	rb_erase(&entry->efd_node, &(db->bb_free_root));
> -	mb_free_blocks(NULL, &e4b, entry->efd_start_cluster, entry->efd_count);
> +	cnt = 0;
> +	nr_fd = 0;
> +	ext4_lock_group(sb, group);
> +	rbtree_postorder_for_each_entry_safe(fd, nfd,
> +			&db->bb_free_root, efd_node) {
> +		if (fd->efd_tid != commit_tid)
> +			continue;
> +
> +		mb_debug(sb, "gonna free %u blocks in group %u (0x%p):",
> +			 fd->efd_count, fd->efd_group, fd);
> +		atomic_sub(fd->efd_count, &EXT4_SB(sb)->s_mb_free_pending);
> +		cnt += fd->efd_count;
> +		nr_fd++;
> +		rb_erase(&fd->efd_node, &db->bb_free_root);
> +		mb_free_blocks(NULL, &e4b, fd->efd_start_cluster, fd->efd_count);
> +		if (test_opt(sb, DISCARD))
> +			ext4_insert_free_data(&db->bb_discard_root, fd);
> +		else
> +			kmem_cache_free(ext4_free_data_cachep, fd);
> +	}
> 
> 	/*
> 	 * Clear the trimmed flag for the group so that the next
> @@ -3055,22 +3117,22 @@ static void ext4_free_data_in_buddy(struct super_block *sb,
> 	 * If the volume is mounted with -o discard, online discard
> 	 * is supported and the free blocks will be trimmed online.
> 	 */
> -	if (!test_opt(sb, DISCARD))
> +	if (!test_opt(sb, DISCARD) && cnt)
> 		EXT4_MB_GRP_CLEAR_TRIMMED(db);
> 
> -	if (!db->bb_free_root.rb_node) {
> +	if (RB_EMPTY_ROOT(&db->bb_free_root) && cnt) {
> 		/* No more items in the per group rb tree
> 		 * balance refcounts from ext4_mb_free_metadata()
> 		 */
> 		put_page(e4b.bd_buddy_page);
> 		put_page(e4b.bd_bitmap_page);
> 	}
> -	ext4_unlock_group(sb, entry->efd_group);
> -	kmem_cache_free(ext4_free_data_cachep, entry);
> +	ext4_unlock_group(sb, group);
> 	ext4_mb_unload_buddy(&e4b);
> 
> -	mb_debug(sb, "freed %d blocks in %d structures\n", count,
> -		 count2);
> +	mb_debug(sb, "freed %d blocks in %d structures\n", cnt, nr_fd);
> +
> +	return nr_fd;
> }
> 
> /*
> @@ -3081,52 +3143,21 @@ void ext4_process_freed_data(struct super_block *sb, tid_t commit_tid)
> {
> 	struct ext4_sb_info *sbi = EXT4_SB(sb);
> 	ext4_group_t ngroups = ext4_get_groups_count(sb);
> -	struct ext4_free_data *fd, *nfd;
> 	struct ext4_group_info *grp;
> -	struct bio *discard_bio = NULL;
> -	struct list_head freed_data_list;
> -	int err, i;
> +	int i, nr;
> 
> 	if (!atomic_read(&sbi->s_mb_free_pending))
> 		return;
> 
> -	INIT_LIST_HEAD(&freed_data_list);
> -	for (i = 0; i < ngroups; i++) {
> +	for (i = 0, nr = 0; i < ngroups; i++) {
> 		grp = ext4_get_group_info(sb, i);
> -		ext4_lock_group(sb, i);
> -		rbtree_postorder_for_each_entry_safe(fd, nfd,
> -				&grp->bb_free_root, efd_node) {
> -			if (fd->efd_tid != commit_tid)
> -				continue;
> -			INIT_LIST_HEAD(&fd->efd_list);
> -			list_add_tail(&fd->efd_list, &freed_data_list);
> -		}
> -		ext4_unlock_group(sb, i);
> -	}
> -
> -	if (test_opt(sb, DISCARD)) {
> -		list_for_each_entry(fd, &freed_data_list, efd_list) {
> -			err = ext4_issue_discard(sb, fd->efd_group,
> -						 fd->efd_start_cluster,
> -						 fd->efd_count,
> -						 &discard_bio);
> -			if (err && err != -EOPNOTSUPP) {
> -				ext4_msg(sb, KERN_WARNING, "discard request in"
> -					 " group:%d block:%d count:%d failed"
> -					 " with %d", fd->efd_group,
> -					 fd->efd_start_cluster, fd->efd_count, err);
> -			} else if (err == -EOPNOTSUPP)
> -				break;
> -		}
> -
> -		if (discard_bio) {
> -			submit_bio_wait(discard_bio);
> -			bio_put(discard_bio);
> -		}
> +		if (RB_EMPTY_ROOT(&grp->bb_free_root))
> +			continue;
> +		nr += ext4_free_data_in_buddy(sb, i, commit_tid);
> 	}
> 
> -	list_for_each_entry_safe(fd, nfd, &freed_data_list, efd_list)
> -		ext4_free_data_in_buddy(sb, fd);
> +	if (nr && test_opt(sb, DISCARD))
> +		queue_work(ext4_discard_wq, &sbi->s_discard_work);
> }
> 
> int __init ext4_init_mballoc(void)
> @@ -3146,8 +3177,13 @@ int __init ext4_init_mballoc(void)
> 	if (ext4_free_data_cachep == NULL)
> 		goto out_ac_free;
> 
> -	return 0;
> +	ext4_discard_wq = alloc_workqueue("ext4discard", WQ_UNBOUND, 0);
> +	if (!ext4_discard_wq)
> +		goto out_free_data;
> 
> +	return 0;
> +out_free_data:
> +	kmem_cache_destroy(ext4_free_data_cachep);
> out_ac_free:
> 	kmem_cache_destroy(ext4_ac_cachep);
> out_pa_free:
> diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
> index e75b474..d76286b 100644
> --- a/fs/ext4/mballoc.h
> +++ b/fs/ext4/mballoc.h
> @@ -79,9 +79,6 @@
> #define MB_DEFAULT_MAX_INODE_PREALLOC	512
> 
> struct ext4_free_data {
> -	/* this links the free block information from sb_info */
> -	struct list_head		efd_list;
> -
> 	/* this links the free block information from group_info */
> 	struct rb_node			efd_node;
> 
> --
> 1.8.3.1


Cheers, Andreas






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