[RFC PATCH v1 16/57] perf: 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.

Refactor a BUILD_BUG_ON() so that we test against the limit; _format is
invariant to page size so testing it is no bigger than the minimum
supported size is sufficient.

Wrap global variables that are initialized with PAGE_SIZE derived values
using DEFINE_GLOBAL_PAGE_SIZE_VAR() so their initialization can be
deferred for boot-time page size builds.

Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---

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

 include/linux/perf_event.h | 2 +-
 kernel/events/core.c       | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 1a8942277ddad..b7972155f93eb 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1872,7 +1872,7 @@ _name##_show(struct device *dev,					\
 			       struct device_attribute *attr,		\
 			       char *page)				\
 {									\
-	BUILD_BUG_ON(sizeof(_format) >= PAGE_SIZE);			\
+	BUILD_BUG_ON(sizeof(_format) >= PAGE_SIZE_MIN);			\
 	return sprintf(page, _format "\n");				\
 }									\
 
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 8a6c6bbcd658a..81149663ab7d8 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -419,7 +419,7 @@ static struct kmem_cache *perf_event_cache;
 int sysctl_perf_event_paranoid __read_mostly = 2;
 
 /* Minimum for 512 kiB + 1 user control page */
-int sysctl_perf_event_mlock __read_mostly = 512 + (PAGE_SIZE / 1024); /* 'free' kiB per user */
+__DEFINE_GLOBAL_PAGE_SIZE_VAR(int, sysctl_perf_event_mlock, __read_mostly, 512 + (PAGE_SIZE / 1024)); /* 'free' kiB per user */
 
 /*
  * max perf event sample rate
-- 
2.43.0