On Fri, 19 Jul 2013 07:47:02 +0800 Jerry <uulinux@xxxxxxxxx> wrote: > 2013/7/19 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>: > > On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@xxxxxxxxx> wrote: > > > >> 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. > >> > > > > If you tell me that you have a machine which has PAGE_SIZE=2MB and this > > was the only problem which prevented Linux from running on that machine > > then I'll apply the patch ;) > > > > Hi Morton: > I just "grep -rn "#define\s\+PAGE_SHIFT" arch/", and find the > PAGE_SHIFT in some architecture is very big. > such as the following in "arch/hexagon/include/asm/page.h" > .... > #ifdef CONFIG_PAGE_SIZE_256KB > #define PAGE_SHIFT 18 > #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_256KB > #endif > > #ifdef CONFIG_PAGE_SIZE_1MB > #define PAGE_SHIFT 20 > #define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_1MB > #endif > ..... Good heavens. > In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it > won't generate any machine instruction. Just a guarantee. Well the existing code is a bit silly looking. Why can't we just do /* round applicable memory size up to nearest megabyte */ if (PAGE_SHIFT < 20) numentries = round_up(nr_kernel_pages, (1 << 20)/PAGE_SIZE); or similar? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>