Rodrigo Baroni wrote:
Hi all.
Does anybody knows why this instruction ? - in mm/rmap.c:
anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
This is because the mapping pointer is normally word aligned,
but is "marked" with PAGE_MAPPING_ANON, simply by adding this
amount to the pointer.
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;
Which means that we can recognize whether a pointer is pointing
to an anon_vma or an address_space by looking at the last bit
of the pointer.
A struct is always 32 bit aligned on a 32 bit architecture
(unless packed), so this dirty trick works.
if (!page_mapped(page))
goto out;
anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
spin_lock(&anon_vma->lock);
However, to find the real pointer to the anon_vma, we have to
substract the magic marker again.
--
What is important? What you want to be true, or what is true?
--
Kernelnewbies: Help each other learn about the Linux kernel.
Archive: http://mail.nl.linux.org/kernelnewbies/
FAQ: http://kernelnewbies.org/faq/
