[PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry]

When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
calculating here will generate an unexpected result. In addition, if
PAGE_SHIFT > 20, The memory size represented by numentries was already
integral multiple of 1MB.

Signed-off-by: Jerry <uulinux@xxxxxxxxx>
---
 mm/page_alloc.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b100255..cd41797 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5745,9 +5745,11 @@ void *__init alloc_large_system_hash(const char *tablename,
 	if (!numentries) {
 		/* round applicable memory size up to nearest megabyte */
 		numentries = nr_kernel_pages;
-		numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
-		numentries >>= 20 - PAGE_SHIFT;
-		numentries <<= 20 - PAGE_SHIFT;
+		if (20 > PAGE_SHIFT) {
+			numentries += (1UL << (20 - PAGE_SHIFT)) - 1;
+			numentries >>= 20 - PAGE_SHIFT;
+			numentries <<= 20 - PAGE_SHIFT;
+		}
 
 		/* limit to 1 bucket per 2^scale bytes of low memory */
 		if (scale > PAGE_SHIFT)
-- 
1.8.1.5

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