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

[John Stoffel]

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

Why this magic number of 20?  Please explain it better and replace it
was a #define that means something here.  


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

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

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