On Mon, Feb 22, 2021 at 3:42 AM Joao Martins [off-list ref] wrote:

On 2/20/21 3:34 AM, Dan Williams wrote:

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On Tue, Dec 8, 2020 at 9:32 AM Joao Martins [off-list ref] wrote:

quoted

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.

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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.

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PFNs necessary

s/8 necessary PFNs necessary/8 PFNs necessary/

Will fix.

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to describe the subsection's 32K struct pages we are
onlining.

s/we are onlining/being mapped/

...because ZONE_DEVICE pages are never "onlined".

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On a 1G compound pagemap it let us save 4096 pages.

s/lets us save/saves/

Will fix both.

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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.

Ok, makes sense I think saying this description of needing to handle
section crossing is clearer than mentioning one of the section sizes.

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@@ -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.

Do you need to pass in a vmem_context, isn't that context local to
vmemmap_populate_compound_pages()?