Hi Catalin, Thanks for your reply. On 2016/4/12 22:59, Catalin Marinas 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: >>> 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. > Yes, currently,it's safely to reduce the SECTION_SIZE_BITS to match this issue very well. As I mentioned before, if the memory layout is not like this scene. There will be not suitable to reduce the SECTION_SIZE_BITS. We have 4G memory, and 64GB phys address. There will be a lot of holes in the memory layout. And the *holes size are not always the same*. So,it's the reason I want to enable flat-mem in ARM64-ARCH. Why not makes the flat-mem an optional setting for arm64？ -- 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>