> -----Original Message----- > From: Anshuman Khandual [mailto:[email protected]] > Sent: Monday, December 7, 2020 8:31 PM > To: Song Bao Hua (Barry Song) <[email protected]>; Mike Rapoport > <[email protected]>; Will Deacon <[email protected]> > Cc: [email protected]; [email protected]; > [email protected]; [email protected]; liwei (CM) > <[email protected]>; butao <[email protected]>; > [email protected]; fengbaopeng > <[email protected]> > Subject: Re: [PATCH] arm64: mm: decrease the section size to reduce the memory > reserved for the page map > > > > On 12/7/20 7:10 AM, Song Bao Hua (Barry Song) wrote: > > > > > >> -----Original Message----- > >> From: Mike Rapoport [mailto:[email protected]] > >> Sent: Saturday, December 5, 2020 12:44 AM > >> To: Will Deacon <[email protected]> > >> Cc: liwei (CM) <[email protected]>; [email protected]; fengbaopeng > >> <[email protected]>; [email protected]; > [email protected]; > >> Song Bao Hua (Barry Song) <[email protected]>; > >> [email protected]; [email protected]; butao > >> <[email protected]> > >> Subject: Re: [PATCH] arm64: mm: decrease the section size to reduce the > >> memory > >> reserved for the page map > >> > >> On Fri, Dec 04, 2020 at 11:13:47AM +0000, Will Deacon wrote: > >>> On Fri, Dec 04, 2020 at 09:44:43AM +0800, Wei Li wrote: > >>>> For the memory hole, sparse memory model that define SPARSEMEM_VMEMMAP > >>>> do not free the reserved memory for the page map, decrease the section > >>>> size can reduce the waste of reserved memory. > >>>> > >>>> Signed-off-by: Wei Li <[email protected]> > >>>> Signed-off-by: Baopeng Feng <[email protected]> > >>>> Signed-off-by: Xia Qing <[email protected]> > >>>> --- > >>>> arch/arm64/include/asm/sparsemem.h | 2 +- > >>>> 1 file changed, 1 insertion(+), 1 deletion(-) > >>>> > >>>> diff --git a/arch/arm64/include/asm/sparsemem.h > >> b/arch/arm64/include/asm/sparsemem.h > >>>> index 1f43fcc79738..8963bd3def28 100644 > >>>> --- a/arch/arm64/include/asm/sparsemem.h > >>>> +++ b/arch/arm64/include/asm/sparsemem.h > >>>> @@ -7,7 +7,7 @@ > >>>> > >>>> #ifdef CONFIG_SPARSEMEM > >>>> #define MAX_PHYSMEM_BITS CONFIG_ARM64_PA_BITS > >>>> -#define SECTION_SIZE_BITS 30 > >>>> +#define SECTION_SIZE_BITS 27 > >>> > >>> We chose '30' to avoid running out of bits in the page flags. What > >>> changed? > >> > >> I think that for 64-bit there are still plenty of free bits. I didn't > >> check now, but when I played with SPARSEMEM on m68k there were 8 bits > >> for section out of 32. > >> > >>> With this patch, I can trigger: > >>> > >>> ./include/linux/mmzone.h:1170:2: error: Allocator MAX_ORDER exceeds > >> SECTION_SIZE > >>> #error Allocator MAX_ORDER exceeds SECTION_SIZE > >>> > >>> if I bump up NR_CPUS and NODES_SHIFT. > >> > >> I don't think it's related to NR_CPUS and NODES_SHIFT. > >> This seems rather 64K pages that cause this. > >> > >> Not that is shouldn't be addressed. > > > > Right now, only 4K PAGES will define ARM64_SWAPPER_USES_SECTION_MAPS. > > Other cases will use vmemmap_populate_basepages(). > > The original patch should be only addressing the issue in 4K pages: > > https://lore.kernel.org/lkml/[email protected]/ > > > > would we do something like the below? > > #ifdef CONFIG_ARM64_4K_PAGE > > #define SECTION_SIZE_BITS 27 > > #else > > #define SECTION_SIZE_BITS 30 > > #endif > > This is bit arbitrary. Probably 27 can be further reduced for 4K page size. > Instead, we should make SECTION_SIZE_BITS explicitly depend upon MAX_ORDER. > IOW section size should be the same as the highest order page in the buddy. > CONFIG_FORCE_MAX_ZONEORDER is always defined on arm64. A quick test shows > SECTION_SIZE_BITS would be 22 on 4K pages and 29 for 64K pages. As a fall > back SECTION_SIZE_BITS can still be 30 in case CONFIG_FORCE_MAX_ZONEORDER > is not defined.
This will break the pmd mapping for vmemmap. As for each 128M(2^27), we can have 2MB pmd mapping. > > --- a/arch/arm64/include/asm/sparsemem.h > +++ b/arch/arm64/include/asm/sparsemem.h > @@ -7,7 +7,7 @@ > > #ifdef CONFIG_SPARSEMEM > #define MAX_PHYSMEM_BITS CONFIG_ARM64_PA_BITS > -#define SECTION_SIZE_BITS 30 > +#define SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT) > #endif > > #endif > > A similar approach exists on ia64 platform as well. Thanks Barry
