Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in mmu_take_all_locks()

Peter Zijlstra
2010-04-01T16:45:22Z

On Thu, 2010-04-01 at 18:18 +0200, Andrea Arcangeli wrote:
> On Thu, Apr 01, 2010 at 06:12:34PM +0200, Peter Zijlstra wrote:
> > One thing we can do there is to mutex_trylock() if we get the lock, see
> > if we've got the right object, if the trylock fails we can do the
> > refcount thing and sleep. That would allow the fast-path to remain a
> > single atomic.
>
> But then how do you know which anon_vma_unlink has to decrease the
> refcount and which not? That info should need to be stored in the
> kernel stack, can't be stored in the vma. I guess it's feasible but
> passing that info around sounds more tricky than the trylock itself
> (adding params to those functions with int &refcount).

I was thinking of something like:

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) ACCESS_ONCE(page->mapping);
if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
goto out;
if (!page_mapped(page))
goto out;

anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
if (!mutex_trylock(&anon_vma->lock)) {
if (atomic_inc_unless_zero(&anon_vma->ref)) {
rcu_read_unlock();
mutex_lock(&anon_vma->lock);
atomic_dec(&anon_vma->ref); /* ensure the lock pins it */
} else
anon_vma = NULL;
}
rcu_read_unlock();

return anon_vma;
}

void page_unlock_anon_vma(struct anon_vma *anon_vma)
{
mutex_unlock(&anon_vma->lock);
}

Then anybody reaching ref==0 would only need to sync against the lock
before freeing.