Hi all,
I would like to discuss the following topic in the LSFMM.
We have SPARSEMEM memory model and SPARSEMEM_VMEMMAP, where the only difference
between the two of them is that the latter allocates a virtual chunk to represent
the memmap array, which speeds operations like
page_to_pfn/pfn_to_page/nth_page/folio_page_idx.
page->flags layout would also be changed.
Currently we have (extracted from include/linux/page-flags-layout.h):
/*
* page->flags layout:
* No sparsemem or sparsemem vmemmap: | NODE | ZONE |
* ... | FLAGS |
* " plus space for last_cpupid: | NODE | ZONE |
* LAST_CPUPID ... | FLAGS |
* classic sparse with space for node:| SECTION | NODE | ZONE |
* ... | FLAGS |
* " plus space for last_cpupid: | SECTION | NODE | ZONE |
* LAST_CPUPID ... | FLAGS |
* classic sparse no space for node: | SECTION | ZONE | ...
* | FLAGS |
*/
The last three could be gone.
Also, by getting rid of SPARSEMEM we would also get rid of a non-trivial amount
of code.
I did some research on which arches use CONFIG_SPARSE_MEMMAP/VMEMMAP or
none(using flatmem?).
SPARSE_MEMMAP SPARSE_VMEMMAP
arc
arm X
arm64 X X
csky
hexagon
loongarch X X
m68k
microblaze
mips X
nios2
openrisc
parisc
powerpc X X
riscv X X
s390 X X
sh X
sparc X X
um
x86 X X
xtensa
arm, mips, parisc and sh operate with SPARSE_MEMMAP but are lacking code for
SPARSE_VMEMMAP.
I think these archs should be the first thing to focus on, to see if we can
make them work on SPARSE_VMEMMAP.
If we can, and when all arches can run on SPARSE_VMEMMAP, we can think of killing
SPARSE_MEMMAP.
This is not for free, of course.
There is a certain memory overhead because we have to allocate the virtual
memmap for each section.
Taking MIPS as an example:
- SECTION_SIZE_BITS 28 (256MB)
PAGE_SIZE_4KB: 4MB per section
PAGE_SIZE_16KB: 1MB per section
PAGE_SIZE_64KB: 256KB per section
- SECTION_SIZE_BITS 29 (512MB)
PAGE_SIZE_64KB: 512KB per section
e.g: When the section size is 256MB and we operate on 4KB page size, we spend
4MB for the virtual memmap array per section.
(65536 pages per section * sizeof(struct page)) = 4MB
Ideally, we can discuss:
1) whether it makes sense to definitely switch over to SPARSE_VMEMMAP
2) if yes, how can this be arranged. Implementing all the code and giving
a grace period?
3) if not, why? and what can be done so we can proceed.
Thanks
--
Oscar Salvador
SUSE Labs
