diff options
| -rw-r--r-- | mm/rmap.c | 86 |
1 files changed, 82 insertions, 4 deletions
@@ -86,6 +86,29 @@ static inline struct anon_vma *anon_vma_alloc(void) static inline void anon_vma_free(struct anon_vma *anon_vma) { VM_BUG_ON(atomic_read(&anon_vma->refcount)); + + /* + * Synchronize against page_lock_anon_vma() such that + * we can safely hold the lock without the anon_vma getting + * freed. + * + * Relies on the full mb implied by the atomic_dec_and_test() from + * put_anon_vma() against the acquire barrier implied by + * mutex_trylock() from page_lock_anon_vma(). This orders: + * + * page_lock_anon_vma() VS put_anon_vma() + * mutex_trylock() atomic_dec_and_test() + * LOCK MB + * atomic_read() mutex_is_locked() + * + * LOCK should suffice since the actual taking of the lock must + * happen _before_ what follows. + */ + if (mutex_is_locked(&anon_vma->root->mutex)) { + anon_vma_lock(anon_vma); + anon_vma_unlock(anon_vma); + } + kmem_cache_free(anon_vma_cachep, anon_vma); } @@ -372,20 +395,75 @@ out: return anon_vma; } +/* + * Similar to page_get_anon_vma() except it locks the anon_vma. + * + * Its a little more complex as it tries to keep the fast path to a single + * atomic op -- the trylock. If we fail the trylock, we fall back to getting a + * reference like with page_get_anon_vma() and then block on the mutex. + */ struct anon_vma *page_lock_anon_vma(struct page *page) { - struct anon_vma *anon_vma = page_get_anon_vma(page); + struct anon_vma *anon_vma = NULL; + unsigned long anon_mapping; - if (anon_vma) - anon_vma_lock(anon_vma); + 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->root->mutex)) { + /* + * If we observe a !0 refcount, then holding the lock ensures + * the anon_vma will not go away, see __put_anon_vma(). + */ + if (!atomic_read(&anon_vma->refcount)) { + anon_vma_unlock(anon_vma); + anon_vma = NULL; + } + goto out; + } + + /* trylock failed, we got to sleep */ + if (!atomic_inc_not_zero(&anon_vma->refcount)) { + anon_vma = NULL; + goto out; + } + if (!page_mapped(page)) { + put_anon_vma(anon_vma); + anon_vma = NULL; + goto out; + } + + /* we pinned the anon_vma, its safe to sleep */ + rcu_read_unlock(); + anon_vma_lock(anon_vma); + + if (atomic_dec_and_test(&anon_vma->refcount)) { + /* + * Oops, we held the last refcount, release the lock + * and bail -- can't simply use put_anon_vma() because + * we'll deadlock on the anon_vma_lock() recursion. + */ + anon_vma_unlock(anon_vma); + __put_anon_vma(anon_vma); + anon_vma = NULL; + } + + return anon_vma; + +out: + rcu_read_unlock(); return anon_vma; } void page_unlock_anon_vma(struct anon_vma *anon_vma) { anon_vma_unlock(anon_vma); - put_anon_vma(anon_vma); } /* |