mm/rmap.c: anon_vma

["Rodrigo Baroni" <baroni@xxxxxxxxxx>]

Hi all.

 Does anybody knows why this instruction ? - in mm/rmap.c:

anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);

 Above the full function with the instruction:

(line number here: 189)

static struct anon_vma *page_lock_anon_vma(struct page *page)
{
	struct anon_vma *anon_vma = NULL;
	unsigned long anon_mapping;

	rcu_read_lock();
	anon_mapping = (unsigned long) page->mapping;
	if (!(anon_mapping & PAGE_MAPPING_ANON))
		goto out;
	if (!page_mapped(page))
		goto out;

	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
	spin_lock(&anon_vma->lock);
out:
	rcu_read_unlock();
	return anon_vma;
}

Thanks in advance
Baroni

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