Re: [PATCH 1/2] Generic Virtual Memmap suport for SPARSEMEM

[Dave Hansen <hansendc@us.ibm.com>]

First of all, nice set of patches.

On Sat, 2007-03-31 at 23:10 -0800, Christoph Lameter wrote:
> --- linux-2.6.21-rc5-mm2.orig/include/asm-generic/memory_model.h	2007-03-31 22:47:14.000000000 -0700
> +++ linux-2.6.21-rc5-mm2/include/asm-generic/memory_model.h	2007-03-31 22:59:35.000000000 -0700
> @@ -47,6 +47,13 @@
>  })
> 
>  #elif defined(CONFIG_SPARSEMEM)
> +#ifdef CONFIG_SPARSE_VIRTUAL
> +/*
> + * We have a virtual memmap that makes lookups very simple
> + */
> +#define __pfn_to_page(pfn)	(vmemmap + (pfn))
> +#define __page_to_pfn(page)	((page) - vmemmap)
> +#else
>  /*
>   * Note: section's mem_map is encorded to reflect its start_pfn.
>   * section[i].section_mem_map == mem_map's address - start_pfn;
> @@ -62,6 +69,7 @@
>  	struct mem_section *__sec = __pfn_to_section(__pfn);	\
>  	__section_mem_map_addr(__sec) + __pfn;		\
>  })
> +#endif
>  #endif /* CONFIG_FLATMEM/DISCONTIGMEM/SPARSEMEM */

Any chance this can be done without embedding this inside another
#ifdef?  I really hate untangling the mess when an #endif goes
missing.  

Any reason this can't just be another #elif?

>  #ifdef CONFIG_OUT_OF_LINE_PFN_TO_PAGE
> Index: linux-2.6.21-rc5-mm2/mm/sparse.c
> ===================================================================
> --- linux-2.6.21-rc5-mm2.orig/mm/sparse.c	2007-03-31 22:47:14.000000000 -0700
> +++ linux-2.6.21-rc5-mm2/mm/sparse.c	2007-03-31 22:59:35.000000000 -0700
> @@ -9,6 +9,7 @@
>  #include <linux/spinlock.h>
>  #include <linux/vmalloc.h>
>  #include <asm/dma.h>
> +#include <asm/pgalloc.h>
> 
>  /*
>   * Permanent SPARSEMEM data:
> @@ -101,7 +102,7 @@ static inline int sparse_index_init(unsi
> 
>  /*
>   * Although written for the SPARSEMEM_EXTREME case, this happens
> - * to also work for the flat array case becase
> + * to also work for the flat array case because
>   * NR_SECTION_ROOTS==NR_MEM_SECTIONS.
>   */
>  int __section_nr(struct mem_section* ms)
> @@ -211,6 +212,90 @@ static int sparse_init_one_section(struc
>  	return 1;
>  }
> 
> +#ifdef CONFIG_SPARSE_VIRTUAL
> +
> +void *vmemmap_alloc_block(unsigned long size, int node)
> +{
> +	if (slab_is_available()) {
> +		struct page *page =
> +			alloc_pages_node(node, GFP_KERNEL,
> +				get_order(size));
> +
> +		BUG_ON(!page);
> +		return page_address(page);
> +	} else
> +		return __alloc_bootmem_node(NODE_DATA(node), size, size,
> +					__pa(MAX_DMA_ADDRESS));
> +}

Hmmmmmmm.  Can we combine this with sparse_index_alloc()?  Also, why not
just use the slab for this?

Let's get rid of the _block() part, too.  I'm not sure it does any good.
At least make it _bytes() so that we know what the units are.  Also, if
you're just going to round up internally and _not_ use the slab, can you
just make the argument in pages, or even order?

Can you think of any times when we'd want that BUG_ON() to be a
WARN_ON(), instead?  I can see preferring having my mem_map[] on the
wrong node than hitting a BUG().

> +#ifndef ARCH_POPULATES_VIRTUAL_MEMMAP
> +/*
> + * Virtual memmap populate functionality for architectures that support
> + * PMDs for huge pages like i386, x86_64 etc.
> + */

How about:

/*
 * Virtual memmap support for architectures that use Linux pagetables
 * natively in hardware, and support mapping huge pages with PMD
 * entries.
 */

It wouldn't make sense to map the vmemmap area with Linux pagetables on
an arch that didn't use them in hardware, right?  So, perhaps this
doesn't quite belong in mm/sparse.c.  Perhaps we need
arch/x86/sparse.c. ;)

> +static void vmemmap_pop_pmd(pud_t *pud, unsigned long addr,
> +				unsigned long end, int node)
> +{
> +	pmd_t *pmd;
> +
> +	end = pmd_addr_end(addr, end);
> +
> +	for (pmd = pmd_offset(pud, addr); addr < end;
> +			pmd++, addr += PMD_SIZE) {
> +  		if (pmd_none(*pmd)) {
> +  			void *block;
> +			pte_t pte;
> +
> +			block = vmemmap_alloc_block(PMD_SIZE, node);
> +			pte = pfn_pte(__pa(block) >> PAGE_SHIFT,
> +						PAGE_KERNEL);
> +			pte_mkdirty(pte);
> +			pte_mkwrite(pte);
> +			pte_mkyoung(pte);
> +			mk_pte_huge(pte);
> +			set_pmd(pmd, __pmd(pte_val(pte)));
> +		}
> +	}
> +}

Nitpick: I think this would look quite a bit neater with a little less
indentation.

How about making the loop start with

	if (!pmd_none(*pmd))
		continue;

It should bring the rest of the code in a bit and make that long line
more readable.

> +static void vmemmap_pop_pud(pgd_t *pgd, unsigned long addr,
> +					unsigned long end, int node)
> +{
> +	pud_t *pud;
> +
> +	end = pud_addr_end(addr, end);
> +	for (pud = pud_offset(pgd, addr); addr < end;
> +				pud++, addr += PUD_SIZE) {
> +
> +		if (pud_none(*pud))
> +			pud_populate(&init_mm, pud,
> +				vmemmap_alloc_block(PAGE_SIZE, node));
> +
> +		vmemmap_pop_pmd(pud, addr, end, node);
> +	}
> +}
> +
> +static void vmemmap_populate(struct page *start_page, unsigned long nr,
> +								int node)
> +{
> +	pgd_t *pgd;
> +	unsigned long addr = (unsigned long)(start_page);
> +	unsigned long end = pgd_addr_end(addr,
> +			(unsigned long)((start_page + nr)));

There appear to be a few extra parentheses on these lines.

> +	for (pgd = pgd_offset_k(addr); addr < end;
> +				pgd++, addr += PGDIR_SIZE) {
> +
> +		if (pgd_none(*pgd))
> +			pgd_populate(&init_mm, pgd,
> +				vmemmap_alloc_block(PAGE_SIZE, node));
> +		vmemmap_pop_pud(pgd, addr, end, node);
> +	}
> +}
> +#endif
> +#endif /* CONFIG_SPARSE_VIRTUAL */

We don't really need these #ifdefs embedded inside of each other,
either, right?  Kconfig should take care of enforcing the dependency.

>   static struct page *sparse_early_mem_map_alloc(unsigned long pnum)
>  {
>  	struct page *map;
> @@ -221,8 +306,13 @@ static struct page *sparse_early_mem_map
>  	if (map)
>  		return map;
> 
> +#ifdef CONFIG_SPARSE_VIRTUAL
> +	map = pfn_to_page(pnum * PAGES_PER_SECTION);
> +	vmemmap_populate(map, PAGES_PER_SECTION, nid);
> +#else
>  	map = alloc_bootmem_node(NODE_DATA(nid),
>  			sizeof(struct page) * PAGES_PER_SECTION);
> +#endif

We really worked hard to keep #ifdefs out of the code flow in that file
and keep it as clean as possible.  Could we hide this behind a helper?  

         map = alloc_remap(nid, sizeof(struct page) * PAGES_PER_SECTION);
         if (map)
                 return map;

+        map = alloc_vmemmap(map, PAGES_PER_SECTION, nid);
+        if (map)
+                return map;
+
         map = alloc_bootmem_node(NODE_DATA(nid),
                         sizeof(struct page) * PAGES_PER_SECTION);
         if (map)
                 return map;

Then, do whatever magic you want in alloc_vmemmap().

-- Dave

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