Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:5 as permitted sender) client-ip=2620:137:e000::3:5;
Date:   Sun,  8 Oct 2023 23:42:26 -0700
In-Reply-To: <20231009064230.2952396-1-surenb@google.com>
Mime-Version: 1.0
References: <20231009064230.2952396-1-surenb@google.com>
Message-ID: <20231009064230.2952396-2-surenb@google.com>
Subject: [PATCH v3 1/3] mm/rmap: support move to different root anon_vma in folio_move_anon_rmap()
From:   Suren Baghdasaryan <surenb@google.com>
To:     akpm@linux-foundation.org
Cc:     viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org,
        aarcange@redhat.com, lokeshgidra@google.com, peterx@redhat.com,
        david@redhat.com, hughd@google.com, mhocko@suse.com,
        axelrasmussen@google.com, rppt@kernel.org, willy@infradead.org,
        Liam.Howlett@oracle.com, jannh@google.com, zhangpeng362@huawei.com,
        bgeffon@google.com, kaleshsingh@google.com, ngeoffray@google.com,
        jdduke@google.com, surenb@google.com, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org, kernel-team@android.com
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

From: Andrea Arcangeli <aarcange@redhat.com>

For now, folio_move_anon_rmap() was only used to move a folio to a
different anon_vma after fork(), whereby the root anon_vma stayed
unchanged. For that, it was sufficient to hold the folio lock when
calling folio_move_anon_rmap().

However, we want to make use of folio_move_anon_rmap() to move folios
between VMAs that have a different root anon_vma. As folio_referenced()
performs an RMAP walk without holding the folio lock but only holding the
anon_vma in read mode, holding the folio lock is insufficient.

When moving to an anon_vma with a different root anon_vma, we'll have to
hold both, the folio lock and the anon_vma lock in write mode.
Consequently, whenever we succeeded in folio_lock_anon_vma_read() to
read-lock the anon_vma, we have to re-check if the mapping was changed
in the meantime. If that was the case, we have to retry.

Note that folio_move_anon_rmap() must only be called if the anon page is
exclusive to a process, and must not be called on KSM folios.

This is a preparation for UFFDIO_MOVE, which will hold the folio lock,
the anon_vma lock in write mode, and the mmap_lock in read mode.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/rmap.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/mm/rmap.c b/mm/rmap.c
index c1f11c9dbe61..f9ddc50269d2 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
 	struct anon_vma *root_anon_vma;
 	unsigned long anon_mapping;
 
+retry:
 	rcu_read_lock();
+retry_under_rcu:
 	anon_mapping = (unsigned long)READ_ONCE(folio->mapping);
 	if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
 		goto out;
@@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
 	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
 	root_anon_vma = READ_ONCE(anon_vma->root);
 	if (down_read_trylock(&root_anon_vma->rwsem)) {
+		/*
+		 * folio_move_anon_rmap() might have changed the anon_vma as we
+		 * might not hold the folio lock here.
+		 */
+		if (unlikely((unsigned long)READ_ONCE(folio->mapping) !=
+			     anon_mapping)) {
+			up_read(&root_anon_vma->rwsem);
+			goto retry_under_rcu;
+		}
+
 		/*
 		 * If the folio is still mapped, then this anon_vma is still
 		 * its anon_vma, and holding the mutex ensures that it will
@@ -586,6 +598,18 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
 	rcu_read_unlock();
 	anon_vma_lock_read(anon_vma);
 
+	/*
+	 * folio_move_anon_rmap() might have changed the anon_vma as we might
+	 * not hold the folio lock here.
+	 */
+	if (unlikely((unsigned long)READ_ONCE(folio->mapping) !=
+		     anon_mapping)) {
+		anon_vma_unlock_read(anon_vma);
+		put_anon_vma(anon_vma);
+		anon_vma = NULL;
+		goto retry;
+	}
+
 	if (atomic_dec_and_test(&anon_vma->refcount)) {
 		/*
 		 * Oops, we held the last refcount, release the lock
-- 
2.42.0.609.gbb76f46606-goog