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

Qu Wenruo
2020-09-17T23:26:20Z

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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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