Re: [PATCH v2 04/19] btrfs: remove the open-code to read disk-key

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

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On 2020/9/18 上午6:41, David Sterba wrote:
> On Thu, Sep 17, 2020 at 09:15:31PM +0800, Qu Wenruo wrote:
>> Then to me, the better solution is to make read_extent_buffer() to be
>> split into two part.
>>
>> Part 1 to handle the same page read, which should be made inline.
>> The part 1 should be small enough, as it only involves the in-page
>> offset calculation, which is also already done in current
>> generic_bin_search.
> 
> Sounds easy, the result is awful. The inlined part 1 is not that small
> and explodes for each call of read_extent_buffer. Explodes in code size,
> increases stack consumption of all callers.
> 
>> Then part 2 to handle the cross page case, and that part can be a
>> function call.
>>
>> Personally speaking, even generic_bin_search() is a hot-path, I still
>> don't believe it's worthy, as read_extent_buffer() itself is also
>> frequently called in other locations, and I never see a special handling
>> for it in any other location.
> 
> The usage pattern is different, many other location calls
> read_extent_buffer just once to read some data and process. There are
> very few functions that call it in a loop.
> 
> OTOH, bin_search jumps over the sorted array of node keys, it does not
> even have to read the actual key for comparison because it understands
> the on-disk key and just sets the pointer. Calling read_extent_buffer
> for each of them will just waste cycles copying it to the tmp variable.
> 
>> Anyway, I will use the get_eb_page_offset()/get_eb_page_index() macros
>> here first, or subpage will be completely screwed.
>>
>> And then try to use that two-part solution for read_extent_buffer().
> 
> Some numbers from a release build, patch below:
> 
> object size:
> 
>    text    data     bss     dec     hex filename
> 1099317   17972   14912 1132201  1146a9 pre/btrfs.ko
> 1165930   17972   14912 1198814  124ade post/btrfs.ko
> 
> DELTA: +66613
> 
> Stack usage meter:
> 
> send_clone                                                        +16 (128 -> 144)
> btree_readpage_end_io_hook                                        +40 (168 -> 208)
> btrfs_lookup_csum                                                  +8 (104 -> 112)
> find_free_dev_extent_start                                         +8 (144 -> 152)
> __btrfs_commit_inode_delayed_items                                 +8 (168 -> 176)
> btrfs_exclude_logged_extents                                       +8 (72 -> 80)
> btrfs_set_inode_index                                             +16 (88 -> 104)
> btrfs_shrink_device                                                +8 (160 -> 168)
> find_parent_nodes                                                  -8 (312 -> 304)
> __add_to_free_space_tree                                          +16 (112 -> 128)
> btrfs_truncate_inode_items                                         -8 (360 -> 352)
> ref_get_fields                                                    +16 (48 -> 64)
> btrfs_qgroup_trace_leaf_items                                      +8 (80 -> 88)
> did_create_dir                                                     +8 (112 -> 120)
> free_space_next_bitmap                                            +32 (56 -> 88)
> btrfs_lookup_bio_sums                                             +24 (216 -> 240)
> btrfs_read_qgroup_config                                           +8 (120 -> 128)
> btrfs_check_ref_name_override                                     +16 (152 -> 168)
> btrfs_uuid_tree_iterate                                            +8 (128 -> 136)
> log_dir_items                                                     +16 (160 -> 176)
> btrfs_ioctl_send                                                  +16 (216 -> 232)
> btrfs_get_parent                                                  +16 (80 -> 96)
> __btrfs_inc_extent_ref                                             +8 (128 -> 136)
> btrfs_unlink_subvol                                               +16 (144 -> 160)
> btrfs_del_csums                                                    +8 (184 -> 192)
> btrfs_mount                                                       -16 (184 -> 168)
> generic_bin_search                                                 +8 (104 -> 112)
> btrfs_uuid_tree_add                                               +16 (128 -> 144)
> free_space_test_bit                                                +8 (72 -> 80)
> btrfs_init_dev_stats                                              +16 (160 -> 176)
> btrfs_read_chunk_tree                                             +48 (208 -> 256)
> process_all_refs                                                  +16 (104 -> 120)
> ... and this goes on
> 
> LOST (80):
>         btrfs_ioctl_setflags                                       80
> 
> NEW (208):
>         __read_extent_buffer                                       24
>         get_order                                                   8
>         btrfs_search_path_in_tree_user                            176
> LOST/NEW DELTA:     +128
> PRE/POST DELTA:    +1944
> 
> ---
> 
> Here's the patch. I'm now quite sure we don't want to split
> read_extent_buffer and keep the bin_search optimization as is.

Fine, I'll change the patch to use get_eb_page_offset/index() in
generic_bin_search().

> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index afac70ef0cc5..77c1df5771bf 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -5584,7 +5584,7 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num)
>  	return ret;
>  }
>  
> -static bool report_eb_range(const struct extent_buffer *eb, unsigned long start,
> +bool report_eb_range(const struct extent_buffer *eb, unsigned long start,
>  			    unsigned long len)
>  {
>  	btrfs_warn(eb->fs_info,
> @@ -5595,45 +5595,17 @@ static bool report_eb_range(const struct extent_buffer *eb, unsigned long start,
>  	return true;
>  }
>  
> -/*
> - * Check if the [start, start + len) range is valid before reading/writing
> - * the eb.
> - * NOTE: @start and @len are offset inside the eb, not logical address.
> - *
> - * Caller should not touch the dst/src memory if this function returns error.
> - */
> -static inline int check_eb_range(const struct extent_buffer *eb,
> -				 unsigned long start, unsigned long len)
> -{
> -	unsigned long offset;
> -
> -	/* start, start + len should not go beyond eb->len nor overflow */
> -	if (unlikely(check_add_overflow(start, len, &offset) || offset > eb->len))
> -		return report_eb_range(eb, start, len);
> -
> -	return false;
> -}
> -
> -void read_extent_buffer(const struct extent_buffer *eb, void *dstv,
> +void __read_extent_buffer(const struct extent_buffer *eb, void *dstv,
>  			unsigned long start, unsigned long len)
>  {
> -	size_t cur;
> -	size_t offset;
> -	struct page *page;
> -	char *kaddr;
> +	unsigned long offset = offset_in_page(start);
>  	char *dst = (char *)dstv;
>  	unsigned long i = start >> PAGE_SHIFT;
>  
> -	if (check_eb_range(eb, start, len))
> -		return;
> -
> -	offset = offset_in_page(start);
> -
>  	while (len > 0) {
> -		page = eb->pages[i];
> +		const char *kaddr = page_address(eb->pages[i]);
> +		size_t cur = min(len, (PAGE_SIZE - offset));
>  
> -		cur = min(len, (PAGE_SIZE - offset));
> -		kaddr = page_address(page);
>  		memcpy(dst, kaddr + offset, cur);
>  
>  		dst += cur;
> diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
> index 3bbc25b816ea..7ea53794f927 100644
> --- a/fs/btrfs/extent_io.h
> +++ b/fs/btrfs/extent_io.h
> @@ -241,9 +241,57 @@ static inline int extent_buffer_uptodate(const struct extent_buffer *eb)
>  
>  int memcmp_extent_buffer(const struct extent_buffer *eb, const void *ptrv,
>  			 unsigned long start, unsigned long len);
> +/* NEW */
> +
> +bool report_eb_range(const struct extent_buffer *eb, unsigned long start,
> +			    unsigned long len);
> +void __read_extent_buffer(const struct extent_buffer *eb, void *dst,
> +			unsigned long start,
> +			unsigned long len);
> +/*
> + * Check if the [start, start + len) range is valid before reading/writing
> + * the eb.
> + * NOTE: @start and @len are offset inside the eb, not logical address.
> + *
> + * Caller should not touch the dst/src memory if this function returns error.
> + */
> +static inline int check_eb_range(const struct extent_buffer *eb,
> +				 unsigned long start, unsigned long len)
> +{
> +	unsigned long offset;
> +
> +	/* start, start + len should not go beyond eb->len nor overflow */
> +	if (unlikely(check_add_overflow(start, len, &offset) || offset > eb->len))
> +		return report_eb_range(eb, start, len);
> +
> +	return false;
> +}
> +
> +static inline void read_extent_buffer(const struct extent_buffer *eb, void *dstv,
> +				      unsigned long start, unsigned long len)
> +{
> +	const unsigned long oip = offset_in_page(start);
> +
> +	if (check_eb_range(eb, start, len))
> +		return;
> +
> +	if (likely(oip + len <= PAGE_SIZE)) {
> +		const unsigned long idx = start >> PAGE_SHIFT;
> +		const char *kaddr = page_address(eb->pages[idx]);
> +
> +		memcpy(dstv, kaddr + oip, len);
> +		return;
> +	}
> +
> +	__read_extent_buffer(eb, dstv, start, len);
> +}
> +
> +/* END */
> +/*
>  void read_extent_buffer(const struct extent_buffer *eb, void *dst,
>  			unsigned long start,
>  			unsigned long len);
> +*/
>  int read_extent_buffer_to_user_nofault(const struct extent_buffer *eb,
>  				       void __user *dst, unsigned long start,
>  				       unsigned long len);
> 


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