Optimize the page_lock_anon_vma() fast path to be one LOCKed op, instead of two. Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> --- mm/rmap.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 61 insertions(+), 4 deletions(-) Index: linux-2.6/mm/rmap.c =================================================================== --- linux-2.6.orig/mm/rmap.c +++ linux-2.6/mm/rmap.c @@ -351,20 +351,69 @@ 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; + + 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->lock)) { + /* + * 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; + } + + /* 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; + } - if (anon_vma) - anon_vma_lock(anon_vma); + 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); } /* @@ -1422,6 +1471,14 @@ int try_to_munlock(struct page *page) void __put_anon_vma(struct anon_vma *anon_vma) { + /* + * Synchronize against page_lock_anon_vma() such that + * we can safely hold the lock without the anon_vma getting + * freed. + */ + anon_vma_lock(anon_vma); + anon_vma_unlock(anon_vma); + if (anon_vma->root != anon_vma) put_anon_vma(anon_vma->root); anon_vma_free(anon_vma); -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html