If my understanding correct, vmscan can find a page which lives in a already
anon_vma_unlink'ed vma. This is ok, the page is pinned, and page->mapping is
not cleared until free_hot_cold_page().
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.
However, we are doing spin_unlock(anon_vma->lock) after page_lock_anon_vma(),
and this looks unsafe to me because page_lock_anon_vma() does rcu_read_unlock()
on return.
This worked before because spin_lock() implied rcu_read_lock(), so rcu was
blocked if page_lock_anon_vma() returns !NULL. With CONFIG_PREEMPT_RCU this
is not true (yes?), so it is possible that the slab returns the memory to
the system and it is re-used when we write to anon_vma->lock.
IOW, don't we need something like this
static struct anon_vma *page_lock_anon_vma(struct page *page)
{
struct anon_vma *anon_vma;
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);
return anon_vma;
out:
rcu_read_unlock();
return NULL;
}
static inline void page_lock_anon_vma(struct anon_vma *anon_vma)
{
spin_unlock(&anon_vma->lock);
rcu_read_unlock();
}
?
Oleg.
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