Re: [PATCH 2.6.12-rc5 4/10] mm: manual page migration-rc3 -- add-sys_migrate_pages-rc3.patch

[Hirokazu Takahashi <taka@valinux.co.jp>]

Hi Ray,

> +static int
> +migrate_vma(struct task_struct *task, struct mm_struct *mm,
> +	struct vm_area_struct *vma, int *node_map)
> +{
> +	struct page *page, *page2;
> +	unsigned long vaddr;
> +	int count = 0, nr_busy;
> +	LIST_HEAD(page_list);
> +
> +	/* can't migrate mlock()'d pages */
> +	if (vma->vm_flags & VM_LOCKED)
> +		return 0;
> +
> +	/*
> +	 * gather all of the pages to be migrated from this vma into page_list
> +	 */
> +	spin_lock(&mm->page_table_lock);
> + 	for (vaddr = vma->vm_start; vaddr < vma->vm_end; vaddr += PAGE_SIZE) {
> +		page = follow_page(mm, vaddr, 0);
> +		/*
> +		 * follow_page has been known to return pages with zero mapcount
> +		 * and NULL mapping.  Skip those pages as well
> +		 */
> +		if (page && page_mapcount(page)) {
> +			if (node_map[page_to_nid(page)] >= 0) {
> +				if (steal_page_from_lru(page_zone(page), page,
> +					&page_list))
> +						count++;
> +				else
> +					BUG();
> +			}
> +		}
> +	}
> +	spin_unlock(&mm->page_table_lock);

I think you shouldn't call BUG() here because the swap code can remove
any pages from the LRU lists at any moment even though mm->page_table_lock
is held.

The preferable code would be:

		if (page && page_mapcount(page)) {
			if (node_map[page_to_nid(page)] >= 0) {
				if (steal_page_from_lru(page_zone(page), page,
					&page_list))
						count++;
				else
					continue;
			}
		}

Thanks,
Hirokazu Takahashi.
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