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SubjectRe: [PATCH 2/8] mm: alloc_contig_freed_pages() added
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On Fri, 2011-08-19 at 16:27 +0200, Marek Szyprowski wrote:
> +unsigned long alloc_contig_freed_pages(unsigned long start, unsigned long end,
> + gfp_t flag)
> +{
> + unsigned long pfn = start, count;
> + struct page *page;
> + struct zone *zone;
> + int order;
> +
> + VM_BUG_ON(!pfn_valid(start));
> + zone = page_zone(pfn_to_page(start));

This implies that start->end are entirely contained in a single zone.
What enforces that? If some higher layer enforces that, I think we
probably need at least a VM_BUG_ON() in here and a comment about who
enforces it.

> + spin_lock_irq(&zone->lock);
> +
> + page = pfn_to_page(pfn);
> + for (;;) {
> + VM_BUG_ON(page_count(page) || !PageBuddy(page));
> + list_del(&page->lru);
> + order = page_order(page);
> + zone->free_area[order].nr_free--;
> + rmv_page_order(page);
> + __mod_zone_page_state(zone, NR_FREE_PAGES, -(1UL << order));
> + pfn += 1 << order;
> + if (pfn >= end)
> + break;
> + VM_BUG_ON(!pfn_valid(pfn));
> + page += 1 << order;
> + }

This 'struct page *'++ stuff is OK, but only for small, aligned areas.
For at least some of the sparsemem modes (non-VMEMMAP), you could walk
off of the end of the section_mem_map[] when you cross a MAX_ORDER
boundary. I'd feel a little bit more comfortable if pfn_to_page() was
being done each time, or only occasionally when you cross a section
boundary.

This may not apply to what ARM is doing today, but it shouldn't be too
difficult to fix up, or to document what's going on.

> + spin_unlock_irq(&zone->lock);
> +
> + /* After this, pages in the range can be freed one be one */
> + page = pfn_to_page(start);
> + for (count = pfn - start; count; --count, ++page)
> + prep_new_page(page, 0, flag);
> +
> + return pfn;
> +}
> +
> +void free_contig_pages(struct page *page, int nr_pages)
> +{
> + for (; nr_pages; --nr_pages, ++page)
> + __free_page(page);
> +}

The same thing about 'struct page' pointer math goes here.

-- Dave



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