[PATCH] mm: extend max struct page size for kmsan

[Arnd Bergmann]

From: Arnd Bergmann <arnd@xxxxxxxx>

After x86 has enabled support for KMSAN, it has become possible
to have larger 'struct page' than was expected when commit
5470dea49f53 ("mm: use mm_zero_struct_page from SPARC on all 64b
architectures") was merged:

include/linux/mm.h:156:10: warning: no case matching constant switch condition '96'
        switch (sizeof(struct page)) {

Extend the maximum accordingly.

Fixes: 5470dea49f53 ("mm: use mm_zero_struct_page from SPARC on all 64b architectures")
Fixes: 4ca8cc8d1bbe ("x86: kmsan: enable KMSAN builds for x86")
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
This seems to show up extremely rarely in randconfig builds, but
enough to trigger my build machine.

I saw a related discussion at [1] about raising MAX_STRUCT_PAGE_SIZE,
but as I understand it, that needs to be addressed separately.

[1] https://lore.kernel.org/lkml/20220701142310.2188015-11-glider@xxxxxxxxxx/
---
 include/linux/mm.h | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b73ba2e5cfd2..aa39d5ddace1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -137,7 +137,7 @@ extern int mmap_rnd_compat_bits __read_mostly;
  * define their own version of this macro in <asm/pgtable.h>
  */
 #if BITS_PER_LONG == 64
-/* This function must be updated when the size of struct page grows above 80
+/* This function must be updated when the size of struct page grows above 96
  * or reduces below 56. The idea that compiler optimizes out switch()
  * statement, and only leaves move/store instructions. Also the compiler can
  * combine write statements if they are both assignments and can be reordered,
@@ -148,12 +148,18 @@ static inline void __mm_zero_struct_page(struct page *page)
 {
 	unsigned long *_pp = (void *)page;
 
-	 /* Check that struct page is either 56, 64, 72, or 80 bytes */
+	 /* Check that struct page is either 56, 64, 72, 80, 88 or 96 bytes */
 	BUILD_BUG_ON(sizeof(struct page) & 7);
 	BUILD_BUG_ON(sizeof(struct page) < 56);
-	BUILD_BUG_ON(sizeof(struct page) > 80);
+	BUILD_BUG_ON(sizeof(struct page) > 96);
 
 	switch (sizeof(struct page)) {
+	case 96:
+		_pp[11] = 0;
+		fallthrough;
+	case 88:
+		_pp[10] = 0;
+		fallthrough;
 	case 80:
 		_pp[9] = 0;
 		fallthrough;
-- 
2.39.0