On 2/20/21 3:34 AM, Dan Williams wrote:
> On Tue, Dec 8, 2020 at 9:32 AM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote:
>>
>> Introduce a new flag, MEMHP_REUSE_VMEMMAP, which signals that
>> struct pages are onlined with a given alignment, and should reuse the
>> tail pages vmemmap areas. On that circunstamce we reuse the PFN backing
>
> s/On that circunstamce we reuse/Reuse/
>
> Kills a "we" and switches to imperative tense. I noticed a couple
> other "we"s in the previous patches, but this crossed my threshold to
> make a comment.
>
/me nods. Will fix.
>> only the tail pages subsections, while letting the head page PFN remain
>> different. This presumes that the backing page structs are compound
>> pages, such as the case for compound pagemaps (i.e. ZONE_DEVICE with
>> PGMAP_COMPOUND set)
>>
>> On 2M compound pagemaps, it lets us save 6 pages out of the 8 necessary
>
> s/lets us save/saves/
>
Will fix.
>> PFNs necessary
>
> s/8 necessary PFNs necessary/8 PFNs necessary/
Will fix.
>
>> to describe the subsection's 32K struct pages we are
>> onlining.
>
> s/we are onlining/being mapped/
>
> ...because ZONE_DEVICE pages are never "onlined".
>
>> On a 1G compound pagemap it let us save 4096 pages.
>
> s/lets us save/saves/
>
Will fix both.
>>
>> Sections are 128M (or bigger/smaller),
>
> Huh?
>
Section size is arch-dependent if we are being hollistic.
On x86 it's 64M, 128M or 512M right?
#ifdef CONFIG_X86_32
# ifdef CONFIG_X86_PAE
# define SECTION_SIZE_BITS 29
# define MAX_PHYSMEM_BITS 36
# else
# define SECTION_SIZE_BITS 26
# define MAX_PHYSMEM_BITS 32
# endif
#else /* CONFIG_X86_32 */
# define SECTION_SIZE_BITS 27 /* matt - 128 is convenient right now */
# define MAX_PHYSMEM_BITS (pgtable_l5_enabled() ? 52 : 46)
#endif
Also, me pointing about section sizes, is because a 1GB+ page vmemmap population will
cross sections in how sparsemem populates the vmemmap. And on that case we gotta reuse the
the PTE/PMD pages across multiple invocations of vmemmap_populate_basepages(). Either
that, or looking at the previous page PTE, but that might be ineficient.
>> @@ -229,38 +235,95 @@ int __meminit vmemmap_populate_basepages(unsigned long start, unsigned long end,
>> for (; addr < end; addr += PAGE_SIZE) {
>> pgd = vmemmap_pgd_populate(addr, node);
>> if (!pgd)
>> - return -ENOMEM;
>> + return NULL;
>> p4d = vmemmap_p4d_populate(pgd, addr, node);
>> if (!p4d)
>> - return -ENOMEM;
>> + return NULL;
>> pud = vmemmap_pud_populate(p4d, addr, node);
>> if (!pud)
>> - return -ENOMEM;
>> + return NULL;
>> pmd = vmemmap_pmd_populate(pud, addr, node);
>> if (!pmd)
>> - return -ENOMEM;
>> - pte = vmemmap_pte_populate(pmd, addr, node, altmap);
>> + return NULL;
>> + pte = vmemmap_pte_populate(pmd, addr, node, altmap, block);
>> if (!pte)
>> - return -ENOMEM;
>> + return NULL;
>> vmemmap_verify(pte, node, addr, addr + PAGE_SIZE);
>> }
>>
>> + return __va(__pfn_to_phys(pte_pfn(*pte)));
>> +}
>> +
>> +int __meminit vmemmap_populate_basepages(unsigned long start, unsigned long end,
>> + int node, struct vmem_altmap *altmap)
>> +{
>> + if (!__vmemmap_populate_basepages(start, end, node, altmap, NULL))
>> + return -ENOMEM;
>> return 0;
>> }
>>
>> +static struct page * __meminit vmemmap_populate_reuse(unsigned long start,
>> + unsigned long end, int node,
>> + struct vmem_context *ctx)
>> +{
>> + unsigned long size, addr = start;
>> + unsigned long psize = PHYS_PFN(ctx->align) * sizeof(struct page);
>> +
>> + size = min(psize, end - start);
>> +
>> + for (; addr < end; addr += size) {
>> + unsigned long head = addr + PAGE_SIZE;
>> + unsigned long tail = addr;
>> + unsigned long last = addr + size;
>> + void *area;
>> +
>> + if (ctx->block_page &&
>> + IS_ALIGNED((addr - ctx->block_page), psize))
>> + ctx->block = NULL;
>> +
>> + area = ctx->block;
>> + if (!area) {
>> + if (!__vmemmap_populate_basepages(addr, head, node,
>> + ctx->altmap, NULL))
>> + return NULL;
>> +
>> + tail = head + PAGE_SIZE;
>> + area = __vmemmap_populate_basepages(head, tail, node,
>> + ctx->altmap, NULL);
>> + if (!area)
>> + return NULL;
>> +
>> + ctx->block = area;
>> + ctx->block_page = addr;
>> + }
>> +
>> + if (!__vmemmap_populate_basepages(tail, last, node,
>> + ctx->altmap, area))
>> + return NULL;
>> + }
>
> I think that compound page accounting and combined altmap accounting
> makes this difficult to read, and I think the compound page case
> deserves it's own first class loop rather than reusing
> vmemmap_populate_basepages(). With the suggestion to drop altmap
> support I'd expect a vmmemap_populate_compound that takes a compound
> page size and goes the right think with respect to mapping all the
> tail pages to the same pfn.
>
I can move this to a separate loop as suggested.
But to be able to map all tail pages in one call of vmemmap_populate_compound()
this might requires changes in sparsemem generic code that I am not so sure
they are warranted the added complexity. Otherwise I'll have to probably keep
this logic of @ctx to be able to pass the page to be reused (i.e. @block and
@block_page). That's actually the main reason that made me introduce
a struct vmem_context.
