On Mon, Apr 11, 2016 at 12:31:53PM +0200, Ard Biesheuvel wrote: > On 11 April 2016 at 11:59, Chen Feng <[email protected]> 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. Will
