[PATCH 1/7] btrfs: accessors: simplify folio bounds checks

David Sterba
2025-07-01T17:24:01Z

As we can have non-contiguous range in the eb->folios, any item can be
straddling two folios and we need to check if it can be read in one go
or in two parts. For that there's a check which is not implemented in
the simplest way:

offset in folio + size <= folio size

With a simple expression transformation:

oil + size <= unit_size
size <= unit_size - oil
sizeof() <= part

this can be simplified and reusing existing run-time or compile-time
constants.

Add likely() annotation for this expression as this is the fast path and
compiler sometimes reorders that after the followup block with the
memcpy (observed in practice with other simplifications).

Overall effect on stack consumption:

btrfs_get_8 -8 (80 -> 72)
btrfs_set_8 -8 (88 -> 80)

And .ko size (due to optimizations making use of the direct constants):

text data bss dec hex filename
1456601 115665 16088 1588354 183c82 pre/btrfs.ko
1456093 115665 16088 1587846 183a86 post/btrfs.ko

DELTA: -508

Signed-off-by: David Sterba <dsterba@suse.com>
---
fs/btrfs/accessors.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/accessors.c b/fs/btrfs/accessors.c
index 5cfb0801700e6c..b54c8abe467a06 100644
--- a/fs/btrfs/accessors.c
+++ b/fs/btrfs/accessors.c
@@ -59,7 +59,7 @@ u##bits btrfs_get_##bits(const struct extent_buffer *eb, \
u8 lebytes[sizeof(u##bits)]; \
\
ASSERT(check_setget_bounds(eb, ptr, off, size)); \
- if (INLINE_EXTENT_BUFFER_PAGES == 1 || oil + size <= unit_size) \
+ if (INLINE_EXTENT_BUFFER_PAGES == 1 || likely(sizeof(u##bits) <= part)) \
return get_unaligned_le##bits(kaddr + oil); \
\
memcpy(lebytes, kaddr + oil, part); \
@@ -82,7 +82,7 @@ void btrfs_set_##bits(const struct extent_buffer *eb, void *ptr, \
\
ASSERT(check_setget_bounds(eb, ptr, off, size)); \
if (INLINE_EXTENT_BUFFER_PAGES == 1 || \
- oil + size <= unit_size) { \
+ likely(sizeof(u##bits) <= part)) { \
put_unaligned_le##bits(val, kaddr + oil); \
return; \
} \
--
2.49.0