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: > > On 2016/4/11 16:00, Ard Biesheuvel wrote: > >> On 11 April 2016 at 09:55, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote: > >>> On 2016/4/11 15:35, Ard Biesheuvel wrote: > >>>> On 11 April 2016 at 04:49, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote: > >>>>> 0 1.5G 2G 3.5G 4G > >>>>> | | | | | > >>>>> +--------------+------+---------------+--------------+ > >>>>> | MEM | hole | MEM | IO (regs) | > >>>>> +--------------+------+---------------+--------------+ > >>> The hole in 1.5G ~ 2G is also allocated mem-map array. And also with the 3.5G ~ 4G. > >>> > >> > >> No, it is not. It may be covered by a section, but that does not mean > >> sparsemem vmemmap will actually allocate backing for it. The > >> granularity used by sparsemem vmemmap on a 4k pages kernel is 128 MB, > >> due to the fact that the backing is performed at PMD granularity. > >> > >> Please, could you share the contents of the vmemmap section in > >> /sys/kernel/debug/kernel_page_tables of your system running with > >> sparsemem vmemmap enabled? You will need to set CONFIG_ARM64_PTDUMP=y > > > > 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 ]--- [...] > > 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. IIRC, SECTION_SIZE_BITS was chosen to be the maximum sane value we were thinking of at the time, assuming that 1GB RAM alignment to be fairly normal. For the !SPARSEMEM_VMEMMAP case, we should probably be fine with 29 but, as Will said, we need to be careful with the page flags. At a quick look, we have 25 page flags, 2 bits per zone, NUMA nodes and (48 - section_size_bits) for the section width. We also need to take into account 4 more bits for 52-bit PA support (ARMv8.2). So, without NUMA nodes, we are currently at 49 bits used in page->flags. For the SPARSEMEM_VMEMMAP case, we can decrease the SECTION_SIZE_BITS in the MAX_ORDER limit. An alternative would be to free the vmemmap holes later (but still keep the vmemmap mapping alias). Yet another option would be to change the sparse_mem_map_populate() logic get the actual section end rather than always assuming PAGES_PER_SECTION. But I don't think any of these are worth if we can safely reduce SECTION_SIZE_BITS. -- 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>