On Mon, Apr 11, 2016 at 11:40:13AM +0100, 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. With SPARSEMEM_VMEMMAP, SECTION_SIZE_BITS no longer affect the page flags since we no longer need to encode the section number in page->flags. -- Catalin -- 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>