On 02/24, Hugh Dickins wrote:
>
> On Sat, 24 Feb 2007, Oleg Nesterov wrote:
>
> > So page_lock_anon_vma() works correctly due to SLAB_DESTROY_BY_RCU even if
> > anon_vma_unlink() has already freed anon_vma. In that case we should see
> > list_empty(&anon_vma->head), we are safe.
>
> (It doesn't affect your argument, but we won't necessarily see list_empty
> there: the anon_vma slot may already have got reused for a different
> bundle of vmas completely; but its lock remains a lock and its list
> remains a list of vmas, and the worst that happens is that
> page_referenced_anon or try_to_unmap_anon wanders through an irrelevant
> bundle of vmas, looking for a page that cannot be found there.)
Yes, but in that case we are safe, right? We hold the lock, anon_vma can't be
freed. But thanks for clarification! Somehow I missed that not only unlock()
is unsafe (in theory). If anon_vma's memory was re-used for something else, we
can't assume that we will see list_empty(&anon_vma->head).
> > static inline void page_lock_anon_vma(struct anon_vma *anon_vma)
>
> It might be wiser to call that one page_unlock_anon_vma ;)
Congratulations, you passed the test! Paul didn't :)
> (I'm slightly disgruntled that page_lock_anon_vma takes a struct page *,
> but page_unlock_anon_vma no struct page *. But it would be silly to do
> it differently, or mess with the naming: besides, it's a static function
> and the prototype guards against error anyway.)
OK. I thought about "unlock_anon_vma", but symmetry is good indeed.
Oleg.
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