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. > . > -- 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>