[RFC PATCH v1 18/57] trace: Remove PAGE_SIZE compile-time constant assumption

[Ryan Roberts]

To prepare for supporting boot-time page size selection, refactor code
to remove assumptions about PAGE_SIZE being compile-time constant. Code
intended to be equivalent when compile-time page size is active.

Convert BUILD_BUG_ON() BUG_ON() since the argument depends on PAGE_SIZE
and its not trivial to test against a page size limit.

Redefine FTRACE_KSTACK_ENTRIES so that "struct ftrace_stacks" is always
sized at 32K for 64-bit and 16K for 32-bit. It was previously defined in
terms of PAGE_SIZE (and worked out at the quoted sizes for a 4K page
size). But for 64K pages, the size expanded to 512K. Given the ftrace
stacks should be invariant to page size, this seemed like a waste. As a
side effect, it removes the PAGE_SIZE compile-time constant assumption
from this code.

Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>
---

***NOTE***
Any confused maintainers may want to read the cover note here for context:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/

 kernel/trace/fgraph.c | 2 +-
 kernel/trace/trace.c  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index d7d4fb403f6f0..47aa5c8d8090e 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -534,7 +534,7 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func,
 	if (!current->ret_stack)
 		return -EBUSY;
 
-	BUILD_BUG_ON(SHADOW_STACK_SIZE % sizeof(long));
+	BUG_ON(SHADOW_STACK_SIZE % sizeof(long));
 
 	/* Set val to "reserved" with the delta to the new fgraph frame */
 	val = (FGRAPH_TYPE_RESERVED << FGRAPH_TYPE_SHIFT) | FGRAPH_FRAME_OFFSET;
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c3b2c7dfadef1..0f2ec3d30579f 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2887,7 +2887,7 @@ trace_function(struct trace_array *tr, unsigned long ip, unsigned long
 /* Allow 4 levels of nesting: normal, softirq, irq, NMI */
 #define FTRACE_KSTACK_NESTING	4
 
-#define FTRACE_KSTACK_ENTRIES	(PAGE_SIZE / FTRACE_KSTACK_NESTING)
+#define FTRACE_KSTACK_ENTRIES	(SZ_4K / FTRACE_KSTACK_NESTING)
 
 struct ftrace_stack {
 	unsigned long		calls[FTRACE_KSTACK_ENTRIES];
-- 
2.43.0