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

David Sterba
2020-09-17T22:43:09Z

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.

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);