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minmax: avoid overly complicated constant expressions in VM code

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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>
This commit is contained in:
Linus Torvalds 2024-07-26 15:32:27 -07:00
parent e8432ac802
commit 3a7e02c040
2 changed files with 9 additions and 2 deletions
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7
include/linux/minmax.h
View File

@ -270,4 +270,11 @@ static inline bool in_range32(u32 val, u32 start, u32 len)
#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 */

4
include/linux/pageblock-flags.h
View File

@ -41,13 +41,13 @@ 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_PAGE_ORDER)
#define pageblock_order MIN_T(unsigned int, HUGETLB_PAGE_ORDER, MAX_PAGE_ORDER)
#endif /* CONFIG_HUGETLB_PAGE_SIZE_VARIABLE */
#elif defined(CONFIG_TRANSPARENT_HUGEPAGE)
#define pageblock_order min_t(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)
#define pageblock_order MIN_T(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)
#else /* CONFIG_TRANSPARENT_HUGEPAGE */