btrfs: remove redundant check in up check_setget_bounds

There are two separate checks in the bounds checker, the first one being
a special case of the second. As this function is performance critical
due to checking access to any eb member, reducing the size can slightly
improve performance.

On a release build on x86_64 the helper is completely inlined so the
function call overhead is also gone.

There was a report of 5% performance drop on metadata heavy workload,
that disappeared after disabling asserts. The most significant part of
that is the bounds checker.

https://lore.kernel.org/linux-btrfs/20200724164147.39925-1-josef@toxicpanda.com/

After the analysis, the optimized code removes the worst overhead which
is the function call and the performance was restored.

https://lore.kernel.org/linux-btrfs/20200730110943.GE3703@twin.jikos.cz/

1. baseline, asserts on, setget check on

run time:		46s
run time with perf:	48s

2. asserts on, comment out setget check

run time:		44s
run time with perf:	47s

So this is confirms the 5% difference

3. asserts on, optimized seget check

run time:		44s
run time with perf:	47s

The optimizations are reducing the number of ifs to 1 and inlining the
hot path. Low-level stuff, gets the performance back. Patch below.

4. asserts off, no setget check

run time:		44s
run time with perf:	45s

This verifies that asserts other than the setget check have negligible
impact on performance and it's not harmful to keep them on.

Analysis where the performance is lost:

* check_setget_bounds is short function, but it's still a function call,
  changing the flow of instructions and given how many times it's
  called the overhead adds up

* there are two conditions, one to check if the range is
  completely outside (member_offset > eb->len) or partially inside
  (member_offset + size > eb->len)
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/struct-funcs.c

@@ -12,15 +12,10 @@ static bool check_setget_bounds(const struct extent_buffer *eb,
 {
 	const unsigned long member_offset = (unsigned long)ptr + off;
 
-	if (member_offset > eb->len) {
+	if (unlikely(member_offset + size > eb->len)) {
 		btrfs_warn(eb->fs_info,
-	"bad eb member start: ptr 0x%lx start %llu member offset %lu size %d",
-			(unsigned long)ptr, eb->start, member_offset, size);
-		return false;
-	}
-	if (member_offset + size > eb->len) {
-		btrfs_warn(eb->fs_info,
-	"bad eb member end: ptr 0x%lx start %llu member offset %lu size %d",
+		"bad eb member %s: ptr 0x%lx start %llu member offset %lu size %d",
+			(member_offset > eb->len ? "start" : "end"),
 			(unsigned long)ptr, eb->start, member_offset, size);
 		return false;
 	}