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SubjectRe: mm, something wring in page_lock_anon_vma_read()?
On Sat, 20 May 2017, Xishi Qiu wrote:
> On 2017/5/20 6:00, Hugh Dickins wrote:
> >
> > You're ignoring the rcu_read_lock() on entry to page_lock_anon_vma_read(),
> > and the SLAB_DESTROY_BY_RCU (recently renamed SLAB_TYPESAFE_BY_RCU) nature
> > of the anon_vma_cachep kmem cache. It is not safe to muck with anon_vma->
> > root in anon_vma_free(), others could still be looking at it.
> >
> > Hugh
> >
>
> Hi Hugh,
>
> Thanks for your reply.
>
> SLAB_DESTROY_BY_RCU will let it call call_rcu() in free_slab(), but if the
> anon_vma *reuse* by someone again, access root_anon_vma is not safe, right?

That is safe, on reuse it is still a struct anon_vma; then the test for
!page_mapped(page) will show that it's no longer a reliable anon_vma for
this page, so page_lock_anon_vma_read() returns NULL.

But of course, if page->_mapcount has been corrupted or misaccounted,
it may think page_mapped(page) when actually page is not mapped,
and the anon_vma is not good for it.

>
> e.g. if I clean the root pointer before free it, then access root_anon_vma
> in page_lock_anon_vma_read() is NULL pointer access, right?

Yes, cleaning root pointer before free may result in NULL pointer access.

Hugh

>
> anon_vma_free()
> ...
> anon_vma->root = NULL;
> kmem_cache_free(anon_vma_cachep, anon_vma);
> ...
>
> Thanks,
> Xishi Qiu

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