Re: [PATCH RFC 3/9] sparse-vmemmap: Reuse vmemmap areas for a given page size

[Joao Martins <joao.m.martins@oracle.com>]

On 2/20/21 3:34 AM, Dan Williams wrote:
> On Tue, Dec 8, 2020 at 9:32 AM Joao Martins <joao.m.martins@oracle.com> 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.
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