Hi Will, On 2016/4/11 18:40, Will Deacon wrote: > On Mon, Apr 11, 2016 at 12:31:53PM +0200, Ard Biesheuvel wrote: >> On 11 April 2016 at 11:59, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote: >>> Please see the pg-tables below. >>> >>> >>> With sparse and vmemmap enable. >>> >>> ---[ vmemmap start ]--- >>> 0xffffffbdc0200000-0xffffffbdc4800000 70M RW NX SHD AF UXN MEM/NORMAL >>> ---[ vmemmap end ]--- >>> >> >> OK, I see what you mean now. Sorry for taking so long to catch up. >> >>> The board is 4GB, and the memap is 70MB >>> 1G memory --- 14MB mem_map array. >> >> No, this is incorrect. 1 GB corresponds with 16 MB worth of struct >> pages assuming sizeof(struct page) == 64 >> >> So you are losing 6 MB to rounding here, which I agree is significant. >> I wonder if it makes sense to use a lower value for SECTION_SIZE_BITS >> on 4k pages kernels, but perhaps we're better off asking the opinion >> of the other cc'ees. > > You need to be really careful making SECTION_SIZE_BITS smaller because > it has a direct correlation on the use of page->flags and you can end up > running out of bits fairly easily. Yes, making SECTION_SIZE_BITS smaller can solve the current situation. But if the phys-addr is 64GB, but only 4GB ddr is the valid address. And the holes are not always 512MB. But, can you tell us why *smaller SIZE makes running out of bits fairly easily*? And how about the flat-mem model? > > Will > > . > -- 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>