On 11 April 2016 at 12:48, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote: > > > On 2016/4/11 17:59, Chen Feng wrote: >> Hi Ard, >> >> On 2016/4/11 16:00, Ard Biesheuvel wrote: >>> On 11 April 2016 at 09:55, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote: >>>> Hi Ard, >>>> >>>> On 2016/4/11 15:35, Ard Biesheuvel wrote: >>>>> On 11 April 2016 at 04:49, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote: >>>>>> Hi will, >>>>>> Thanks for review. >>>>>> >>>>>> On 2016/4/7 22:21, Will Deacon wrote: >>>>>>> On Tue, Apr 05, 2016 at 04:22:51PM +0800, Chen Feng wrote: >>>>>>>> We can reduce the memory allocated at mem-map >>>>>>>> by flatmem. >>>>>>>> >>>>>>>> currently, the default memory-model in arm64 is >>>>>>>> sparse memory. The mem-map array is not freed in >>>>>>>> this scene. If the physical address is too long, >>>>>>>> it will reserved too much memory for the mem-map >>>>>>>> array. >>>>>>> >>>>>>> Can you elaborate a bit more on this, please? We use the vmemmap, so any >>>>>>> spaces between memory banks only burns up virtual space. What exactly is >>>>>>> the problem you're seeing that makes you want to use flatmem (which is >>>>>>> probably unsuitable for the majority of arm64 machines). >>>>>>> >>>>>> The root cause we want to use flat-mem is the mam_map alloced in sparse-mem >>>>>> is not freed. >>>>>> >>>>>> take a look at here: >>>>>> arm64/mm/init.c >>>>>> void __init mem_init(void) >>>>>> { >>>>>> #ifndef CONFIG_SPARSEMEM_VMEMMAP >>>>>> free_unused_memmap(); >>>>>> #endif >>>>>> } >>>>>> >>>>>> Memory layout (3GB) >>>>>> >>>>>> 0 1.5G 2G 3.5G 4G >>>>>> | | | | | >>>>>> +--------------+------+---------------+--------------+ >>>>>> | MEM | hole | MEM | IO (regs) | >>>>>> +--------------+------+---------------+--------------+ >>>>>> >>>>>> >>>>>> Memory layout (4GB) >>>>>> >>>>>> 0 3.5G 4G 4.5G >>>>>> | | | | >>>>>> +-------------------------------------+--------------+-------+ >>>>>> | MEM | IO (regs) | MEM | >>>>>> +-------------------------------------+--------------+-------+ >>>>>> >>>>>> Currently, the sparse memory section is 1GB. >>>>>> >>>>>> 3GB ddr: the 1.5 ~2G and 3.5 ~ 4G are holes. >>>>>> 3GB ddr: the 3.5 ~ 4G and 4.5 ~ 5G are holes. >>>>>> >>>>>> This will alloc 1G/4K * (struct page) memory for mem_map array. >>>>>> >>>>> >>>>> No, this is incorrect. Sparsemem vmemmap only allocates struct pages >>>>> for memory regions that are actually populated. >>>>> >>>>> For instance, on the Foundation model with 4 GB of memory, you may see >>>>> something like this in the boot log >>>>> >>>>> [ 0.000000] vmemmap : 0xffffffbdc0000000 - 0xffffffbfc0000000 >>>>> ( 8 GB maximum) >>>>> [ 0.000000] 0xffffffbdc0000000 - 0xffffffbde2000000 >>>>> ( 544 MB actual) >>>>> >>>>> but in reality, only the following regions have been allocated >>>>> >>>>> ---[ vmemmap start ]--- >>>>> 0xffffffbdc0000000-0xffffffbdc2000000 32M RW NX SHD AF >>>>> BLK UXN MEM/NORMAL >>>>> 0xffffffbde0000000-0xffffffbde2000000 32M RW NX SHD AF >>>>> BLK UXN MEM/NORMAL >>>>> ---[ vmemmap end ]--- >>>>> >>>>> so only 64 MB is used to back 4 GB of RAM with struct pages, which is >>>>> minimal. Moving to flatmem will not reduce the memory footprint at >>>>> all. >>>> >>>> Yes,but the populate is section, which is 1GB. Take a look at the above >>>> memory layout. >>>> >>>> The section 1G ~ 2G is a section. But 1.5G ~ 2G is a hole. >>>> >>>> The section 3G ~ 4G is a section. But 3.5G ~ 4G is a hole. >>>>>> 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. >> So the 4GB has 5 sections, which used 5 * 14MB memory. >> >> > Sorry, 1G memory is 16GB > 5 sections is 5 * 16 = 80MB > > 1G / 4K * (struct page) 64B = 16MB > > I don't know why the vmemap dump in pg-tables is 70MB. > It may be the PTDUMP code that emits the vmemmap start marker incorrectly. Could you please double check? > I add hack code in vmemmap_populate sparse_mem_map_populate. > > here is the log: > sparse_mem_map_populate 188 start ffffffbdc0000000 end ffffffbdc1000000 PAGES_PER_SECTION 40000 nid 0 > vmemmap_populate 549 size 200000 total 200000 addr ffffffbdc0000000 > vmemmap_populate 549 size 200000 total 400000 addr ffffffbdc0200000 > vmemmap_populate 549 size 200000 total 600000 addr ffffffbdc0400000 > vmemmap_populate 549 size 200000 total 800000 addr ffffffbdc0600000 > vmemmap_populate 549 size 200000 total a00000 addr ffffffbdc0800000 > vmemmap_populate 549 size 200000 total c00000 addr ffffffbdc0a00000 > vmemmap_populate 549 size 200000 total e00000 addr ffffffbdc0c00000 > vmemmap_populate 549 size 200000 total 1000000 addr ffffffbdc0e00000 > sparse_mem_map_populate 188 start ffffffbdc1000000 end ffffffbdc2000000 PAGES_PER_SECTION 40000 nid 0 > ... > sparse_mem_map_populate 188 start ffffffbdc2000000 end ffffffbdc3000000 PAGES_PER_SECTION 40000 nid 0 > sparse_mem_map_populate 188 start ffffffbdc3000000 end ffffffbdc4000000 PAGES_PER_SECTION 40000 nid 0 > sparse_mem_map_populate 188 start ffffffbdc4000000 end ffffffbdc5000000 PAGES_PER_SECTION 40000 nid 0 > > > With 4GB memory, it allocated 2MB * 8 * 5 = 80MB. >> 0 3.5G 4G 4.5G >> | | | | >> +-------------------------------------+--------------+-------+ >> | MEM | IO (regs) | MEM | >> +-------------------------------------+--------------+-------+ > > 4GB memory ,5 sections. 80MB mem_map allocated. > I suppose using #define SECTION_SIZE_BITS 29 in arch/arm64/include/asm/sparsemem.h would get rid of the overhead completely in this particular case. Could you confirm, please? @Will: is the rationale for the default value of 30 for SECTION_SIZE_BITS documented anywhere? Compared to other architectures, it seems on the high side, but I did notice that 64k granule kernels require at least 28 in order not to trigger the following assertion include/linux/mmzone.h:1029:2: error: #error Allocator MAX_ORDER exceeds SECTION_SIZE -- 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>