Patch "minmax: avoid overly complicated constant expressions in VM code" has been added to the 6.1-stable tree

[<gregkh@linuxfoundation.org>]

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 6.1-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-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From linux-staging+bounces-34579-greg=kroah.com@lists.linux.dev Wed Sep 24 22:28:45 2025
From: Eliav Farber <farbere@amazon.com>
Date: Wed, 24 Sep 2025 20:23:06 +0000
Subject: minmax: avoid overly complicated constant expressions in VM code
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Cc: Linus Torvalds <torvalds@linux-foundation.org>, Lorenzo Stoakes <lorenzo.stoakes@oracle.com>, David Laight <David.Laight@aculab.com>
Message-ID: <20250924202320.32333-6-farbere@amazon.com>

From: Linus Torvalds <torvalds@linux-foundation.org>

[ 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 <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/all/36aa2cad-1db1-4abf-8dd2-fb20484aabc3@lucifer.local/
Cc: David Laight <David.Laight@aculab.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Eliav Farber <farbere@amazon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 include/linux/minmax.h          |    7 +++++++
 include/linux/pageblock-flags.h |    2 +-
 2 files changed, 8 insertions(+), 1 deletion(-)

--- 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 */
--- a/include/linux/pageblock-flags.h
+++ b/include/linux/pageblock-flags.h
@@ -41,7 +41,7 @@ extern unsigned int pageblock_order;
  * Huge pages are a constant size, but don't exceed the maximum allocation
  * granularity.
  */
-#define pageblock_order		min_t(unsigned int, HUGETLB_PAGE_ORDER, MAX_ORDER - 1)
+#define pageblock_order		MIN_T(unsigned int, HUGETLB_PAGE_ORDER, MAX_ORDER - 1)
 
 #endif /* CONFIG_HUGETLB_PAGE_SIZE_VARIABLE */
 


Patches currently in stable-queue which might be from farbere@amazon.com are

queue-6.1/minmax-add-a-few-more-min_t-max_t-users.patch
queue-6.1/minmax-fix-indentation-of-__cmp_once-and-__clamp_once.patch
queue-6.1/minmax-add-in_range-macro.patch
queue-6.1/minmax-deduplicate-__unconst_integer_typeof.patch
queue-6.1/minmax-simplify-and-clarify-min_t-max_t-implementation.patch
queue-6.1/minmax-avoid-overly-complicated-constant-expressions-in-vm-code.patch
queue-6.1/minmax-introduce-min-max-_array.patch