> 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.
Because 2^20 = 1MB.
The intention of previous code is "/* round applicable memory size up
to nearest megabyte */".
>
> 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
>
> Jerry> --
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