Re: [PATCH 11/15] btrfs-progs: Add delayed refs infrastructure

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2018年06月08日 20:47, Nikolay Borisov wrote:
> This commit pulls those portions of the kernel implementation of
> delayed refs which are necessary to have them working in user-space.
> I've done the following modifications:
> 
> 1. Replaced all kmem_cache_alloc calls to kmalloc.
> 
> 2. Removed all locking-related code, since we are single threaded in
> userspace.
> 
> 3. Removed code which deals with data refs - delayed refs in user space
> are going to be used only for cowonly trees.

That's pretty good, although still some data ref related
structures/functions are left.

> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  Makefile      |   3 +-
>  ctree.h       |   3 +
>  delayed-ref.c | 608 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  delayed-ref.h | 225 ++++++++++++++++++++++
>  extent-tree.c | 228 ++++++++++++++++++++++
>  kerncompat.h  |   8 +
>  transaction.h |   4 +
>  7 files changed, 1078 insertions(+), 1 deletion(-)
>  create mode 100644 delayed-ref.c
>  create mode 100644 delayed-ref.h
> 
> diff --git a/Makefile b/Makefile
> index 544410e6440c..9508ad4f11e6 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -116,7 +116,8 @@ objects = ctree.o disk-io.o kernel-lib/radix-tree.o extent-tree.o print-tree.o \
>  	  qgroup.o free-space-cache.o kernel-lib/list_sort.o props.o \
>  	  kernel-shared/ulist.o qgroup-verify.o backref.o string-table.o task-utils.o \
>  	  inode.o file.o find-root.o free-space-tree.o help.o send-dump.o \
> -	  fsfeatures.o kernel-lib/tables.o kernel-lib/raid56.o transaction.o
> +	  fsfeatures.o kernel-lib/tables.o kernel-lib/raid56.o transaction.o \
> +	  delayed-ref.o
>  cmds_objects = cmds-subvolume.o cmds-filesystem.o cmds-device.o cmds-scrub.o \
>  	       cmds-inspect.o cmds-balance.o cmds-send.o cmds-receive.o \
>  	       cmds-quota.o cmds-qgroup.o cmds-replace.o check/main.o \
> diff --git a/ctree.h b/ctree.h
> index b30a946658ce..d1ea45571d1e 100644
> --- a/ctree.h
> +++ b/ctree.h
> @@ -2812,4 +2812,7 @@ int btrfs_punch_hole(struct btrfs_trans_handle *trans,
>  int btrfs_read_file(struct btrfs_root *root, u64 ino, u64 start, int len,
>  		    char *dest);
>  
> +
> +/* extent-tree.c */
> +int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans, unsigned long nr);
>  #endif
> diff --git a/delayed-ref.c b/delayed-ref.c
> new file mode 100644
> index 000000000000..f3fa50239380
> --- /dev/null
> +++ b/delayed-ref.c
> @@ -0,0 +1,608 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2009 Oracle.  All rights reserved.
> + */
> +
> +#include "ctree.h"
> +#include "btrfs-list.h"
> +#include "delayed-ref.h"
> +#include "transaction.h"
> +
> +/*
> + * delayed back reference update tracking.  For subvolume trees
> + * we queue up extent allocations and backref maintenance for
> + * delayed processing.   This avoids deep call chains where we
> + * add extents in the middle of btrfs_search_slot, and it allows
> + * us to buffer up frequently modified backrefs in an rb tree instead
> + * of hammering updates on the extent allocation tree.
> + */

A little more explanation on how delayed ref works will be more appricated.

[snip]
> +struct btrfs_delayed_tree_ref {
> +	struct btrfs_delayed_ref_node node;
> +	u64 root;
> +	u64 parent;
> +	int level;
> +};
> +
> +struct btrfs_delayed_data_ref {
> +	struct btrfs_delayed_ref_node node;
> +	u64 root;
> +	u64 parent;
> +	u64 objectid;
> +	u64 offset;
> +};

Since we don't use this structure and don't support data ref yet, what
about just removing this definiation?

[snip]

> +struct btrfs_delayed_ref_head *
> +btrfs_select_ref_head(struct btrfs_trans_handle *trans);
> +
> +/*
> + * helper functions to cast a node into its container
> + */
> +static inline struct btrfs_delayed_tree_ref *
> +btrfs_delayed_node_to_tree_ref(struct btrfs_delayed_ref_node *node)
> +{
> +	return container_of(node, struct btrfs_delayed_tree_ref, node);
> +}
> +
> +static inline struct btrfs_delayed_data_ref *
> +btrfs_delayed_node_to_data_ref(struct btrfs_delayed_ref_node *node)
> +{
> +	return container_of(node, struct btrfs_delayed_data_ref, node);
> +}

So is the only user of btrfs_delayed_data_ref structure.

Thanks,
Qu