This is a note to let you know that I've just added the patch titled
minmax: avoid overly complicated constant expressions in VM code
to the 5.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
minmax-avoid-overly-complicated-constant-expressions-in-vm-code.patch
and it can be found in the queue-5.10 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From [email protected] Fri Oct 17 11:09:55 2025
From: Eliav Farber <[email protected]>
Date: Fri, 17 Oct 2025 09:05:05 +0000
Subject: minmax: avoid overly complicated constant expressions in VM code
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Cc: Linus Torvalds <[email protected]>, Lorenzo Stoakes
<[email protected]>, David Laight <[email protected]>
Message-ID: <[email protected]>
From: Linus Torvalds <[email protected]>
[ Upstream commit 3a7e02c040b130b5545e4b115aada7bacd80a2b6 ]
The minmax infrastructure is overkill for simple constants, and can
cause huge expansions because those simple constants are then used by
other things.
For example, 'pageblock_order' is a core VM constant, but because it was
implemented using 'min_t()' and all the type-checking that involves, it
actually expanded to something like 2.5kB of preprocessor noise.
And when that simple constant was then used inside other expansions:
#define pageblock_nr_pages (1UL << pageblock_order)
#define pageblock_start_pfn(pfn) ALIGN_DOWN((pfn), pageblock_nr_pages)
and we then use that inside a 'max()' macro:
case ISOLATE_SUCCESS:
update_cached = false;
last_migrated_pfn = max(cc->zone->zone_start_pfn,
pageblock_start_pfn(cc->migrate_pfn - 1));
the end result was that one statement expanding to 253kB in size.
There are probably other cases of this, but this one case certainly
stood out.
I've added 'MIN_T()' and 'MAX_T()' macros for this kind of "core simple
constant with specific type" use. These macros skip the type checking,
and as such need to be very sparingly used only for obvious cases that
have active issues like this.
Reported-by: Lorenzo Stoakes <[email protected]>
Link:
https://lore.kernel.org/all/[email protected]/
Cc: David Laight <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Eliav Farber <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
include/linux/minmax.h | 7 +++++++
1 file changed, 7 insertions(+)
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -270,4 +270,11 @@ static inline bool in_range32(u32 val, u
#define swap(a, b) \
do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
+/*
+ * Use these carefully: no type checking, and uses the arguments
+ * multiple times. Use for obvious constants only.
+ */
+#define MIN_T(type,a,b) __cmp(min,(type)(a),(type)(b))
+#define MAX_T(type,a,b) __cmp(max,(type)(a),(type)(b))
+
#endif /* _LINUX_MINMAX_H */
Patches currently in stable-queue which might be from [email protected] are
queue-5.10/minmax-allow-comparisons-of-int-against-unsigned-char-short.patch
queue-5.10/minmax-add-a-few-more-min_t-max_t-users.patch
queue-5.10/minmax-improve-macro-expansion-and-type-checking.patch
queue-5.10/minmax-fix-indentation-of-__cmp_once-and-__clamp_once.patch
queue-5.10/minmax.h-simplify-the-variants-of-clamp.patch
queue-5.10/minmax-add-in_range-macro.patch
queue-5.10/minmax.h-move-all-the-clamp-definitions-after-the-min-max-ones.patch
queue-5.10/minmax-allow-min-max-clamp-if-the-arguments-have-the-same-signedness.patch
queue-5.10/minmax-don-t-use-max-in-situations-that-want-a-c-constant-expression.patch
queue-5.10/minmax.h-remove-some-defines-that-are-only-expanded-once.patch
queue-5.10/minmax.h-use-build_bug_on_msg-for-the-lo-hi-test-in-clamp.patch
queue-5.10/minmax-simplify-min-max-clamp-implementation.patch
queue-5.10/minmax-deduplicate-__unconst_integer_typeof.patch
queue-5.10/minmax-simplify-and-clarify-min_t-max_t-implementation.patch
queue-5.10/minmax.h-add-whitespace-around-operators-and-after-commas.patch
queue-5.10/minmax-sanity-check-constant-bounds-when-clamping.patch
queue-5.10/minmax-avoid-overly-complicated-constant-expressions-in-vm-code.patch
queue-5.10/minmax-make-generic-min-and-max-macros-available-everywhere.patch
queue-5.10/minmax-fix-up-min3-and-max3-too.patch
queue-5.10/minmax.h-reduce-the-define-expansion-of-min-max-and-clamp.patch
queue-5.10/minmax-fix-header-inclusions.patch
queue-5.10/minmax-introduce-min-max-_array.patch
queue-5.10/btrfs-remove-duplicated-in_range-macro.patch
queue-5.10/overflow-tracing-define-the-is_signed_type-macro-once.patch
queue-5.10/minmax-relax-check-to-allow-comparison-between-unsigned-arguments-and-signed-constants.patch
queue-5.10/minmax-clamp-more-efficiently-by-avoiding-extra-comparison.patch
queue-5.10/minmax.h-update-some-comments.patch
