[PATCH 3.12 176/182] aio: v4 ensure access to ctx->ring_pages is correctly serialised for migration

Jiri Slaby
2014-05-13T10:37:45Z

From: Benjamin LaHaise <bcrl@kvack.org>

3.12-stable review patch. If anyone has any objections, please let me know.

===============

commit fa8a53c39f3fdde98c9eace6a9b412143f0f6ed6 upstream.

As reported by Tang Chen, Gu Zheng and Yasuaki Isimatsu, the following issues
exist in the aio ring page migration support.

As a result, for example, we have the following problem:

thread 1 | thread 2
|
aio_migratepage() |
|-> take ctx->completion_lock |
|-> migrate_page_copy(new, old) |
| *NOW*, ctx->ring_pages[idx] == old |
|
| *NOW*, ctx->ring_pages[idx] == old
| aio_read_events_ring()
| |-> ring = kmap_atomic(ctx->ring_pages[0])
| |-> ring->head = head; *HERE, write to the old ring page*
| |-> kunmap_atomic(ring);
|
|-> ctx->ring_pages[idx] = new |
| *BUT NOW*, the content of |
| ring_pages[idx] is old. |
|-> release ctx->completion_lock |

As above, the new ring page will not be updated.

Fix this issue, as well as prevent races in aio_ring_setup() by holding
the ring_lock mutex during kioctx setup and page migration. This avoids
the overhead of taking another spinlock in aio_read_events_ring() as Tang's
and Gu's original fix did, pushing the overhead into the migration code.

Note that to handle the nesting of ring_lock inside of mmap_sem, the
migratepage operation uses mutex_trylock(). Page migration is not a 100%
critical operation in this case, so the ocassional failure can be
tolerated. This issue was reported by Sasha Levin.

Based on feedback from Linus, avoid the extra taking of ctx->completion_lock.
Instead, make page migration fully serialised by mapping->private_lock, and
have aio_free_ring() simply disconnect the kioctx from the mapping by calling
put_aio_ring_file() before touching ctx->ring_pages[]. This simplifies the
error handling logic in aio_migratepage(), and should improve robustness.

v4: always do mutex_unlock() in cases when kioctx setup fails.

Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Cc: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
fs/aio.c | 120 +++++++++++++++++++++++++++++++++++----------------------------
1 file changed, 67 insertions(+), 53 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 062a5f6a1448..12a3de0ee6da 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -52,7 +52,8 @@
struct aio_ring {
unsigned id; /* kernel internal index number */
unsigned nr; /* number of io_events */
- unsigned head;
+ unsigned head; /* Written to by userland or under ring_lock
+ * mutex by aio_read_events_ring(). */
unsigned tail;

unsigned magic;
@@ -243,6 +244,11 @@ static void aio_free_ring(struct kioctx *ctx)
{
int i;

+ /* Disconnect the kiotx from the ring file. This prevents future
+ * accesses to the kioctx from page migration.
+ */
+ put_aio_ring_file(ctx);
+
for (i = 0; i < ctx->nr_pages; i++) {
struct page *page;
pr_debug("pid(%d) [%d] page->count=%d\n", current->pid, i,
@@ -254,8 +260,6 @@ static void aio_free_ring(struct kioctx *ctx)
put_page(page);
}

- put_aio_ring_file(ctx);
-
if (ctx->ring_pages && ctx->ring_pages != ctx->internal_pages) {
kfree(ctx->ring_pages);
ctx->ring_pages = NULL;
@@ -283,29 +287,38 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
{
struct kioctx *ctx;
unsigned long flags;
+ pgoff_t idx;
int rc;

rc = 0;

- /* Make sure the old page hasn't already been changed */
+ /* mapping->private_lock here protects against the kioctx teardown. */
spin_lock(&mapping->private_lock);
ctx = mapping->private_data;
- if (ctx) {
- pgoff_t idx;
- spin_lock_irqsave(&ctx->completion_lock, flags);
- idx = old->index;
- if (idx < (pgoff_t)ctx->nr_pages) {
- if (ctx->ring_pages[idx] != old)
- rc = -EAGAIN;
- } else
- rc = -EINVAL;
- spin_unlock_irqrestore(&ctx->completion_lock, flags);
+ if (!ctx) {
+ rc = -EINVAL;
+ goto out;
+ }
+
+ /* The ring_lock mutex. The prevents aio_read_events() from writing
+ * to the ring's head, and prevents page migration from mucking in
+ * a partially initialized kiotx.
+ */
+ if (!mutex_trylock(&ctx->ring_lock)) {
+ rc = -EAGAIN;
+ goto out;
+ }
+
+ idx = old->index;
+ if (idx < (pgoff_t)ctx->nr_pages) {
+ /* Make sure the old page hasn't already been changed */
+ if (ctx->ring_pages[idx] != old)
+ rc = -EAGAIN;
} else
rc = -EINVAL;
- spin_unlock(&mapping->private_lock);

if (rc != 0)
- return rc;
+ goto out_unlock;

/* Writeback must be complete */
BUG_ON(PageWriteback(old));
@@ -314,38 +327,26 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
rc = migrate_page_move_mapping(mapping, new, old, NULL, mode, 1);
if (rc != MIGRATEPAGE_SUCCESS) {
put_page(new);
- return rc;
+ goto out_unlock;
}

- /* We can potentially race against kioctx teardown here. Use the
- * address_space's private data lock to protect the mapping's
- * private_data.
+ /* Take completion_lock to prevent other writes to the ring buffer
+ * while the old page is copied to the new. This prevents new
+ * events from being lost.
*/
- spin_lock(&mapping->private_lock);
- ctx = mapping->private_data;
- if (ctx) {
- pgoff_t idx;
- spin_lock_irqsave(&ctx->completion_lock, flags);
- migrate_page_copy(new, old);
- idx = old->index;
- if (idx < (pgoff_t)ctx->nr_pages) {
- /* And only do the move if things haven't changed */
- if (ctx->ring_pages[idx] == old)
- ctx->ring_pages[idx] = new;
- else
- rc = -EAGAIN;
- } else
- rc = -EINVAL;
- spin_unlock_irqrestore(&ctx->completion_lock, flags);
- } else
- rc = -EBUSY;
- spin_unlock(&mapping->private_lock);
+ spin_lock_irqsave(&ctx->completion_lock, flags);
+ migrate_page_copy(new, old);
+ BUG_ON(ctx->ring_pages[idx] != old);
+ ctx->ring_pages[idx] = new;
+ spin_unlock_irqrestore(&ctx->completion_lock, flags);

- if (rc == MIGRATEPAGE_SUCCESS)
- put_page(old);
- else
- put_page(new);
+ /* The old page is no longer accessible. */
+ put_page(old);

+out_unlock:
+ mutex_unlock(&ctx->ring_lock);
+out:
+ spin_unlock(&mapping->private_lock);
return rc;
}
#endif
@@ -380,7 +381,7 @@ static int aio_setup_ring(struct kioctx *ctx)
file = aio_private_file(ctx, nr_pages);
if (IS_ERR(file)) {
ctx->aio_ring_file = NULL;
- return -EAGAIN;
+ return -ENOMEM;
}

ctx->aio_ring_file = file;
@@ -415,7 +416,7 @@ static int aio_setup_ring(struct kioctx *ctx)

if (unlikely(i != nr_pages)) {
aio_free_ring(ctx);
- return -EAGAIN;
+ return -ENOMEM;
}

ctx->mmap_size = nr_pages * PAGE_SIZE;
@@ -429,7 +430,7 @@ static int aio_setup_ring(struct kioctx *ctx)
if (IS_ERR((void *)ctx->mmap_base)) {
ctx->mmap_size = 0;
aio_free_ring(ctx);
- return -EAGAIN;
+ return -ENOMEM;
}

pr_debug("mmap address: 0x%08lx\n", ctx->mmap_base);
@@ -556,6 +557,10 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
rcu_read_unlock();
spin_unlock(&mm->ioctx_lock);

+ /* While kioctx setup is in progress,
+ * we are protected from page migration
+ * changes ring_pages by ->ring_lock.
+ */
ring = kmap_atomic(ctx->ring_pages[0]);
ring->id = ctx->id;
kunmap_atomic(ring);
@@ -640,24 +645,28 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)

ctx->max_reqs = nr_events;

- if (percpu_ref_init(&ctx->users, free_ioctx_users))
- goto err;
-
- if (percpu_ref_init(&ctx->reqs, free_ioctx_reqs))
- goto err;
-
spin_lock_init(&ctx->ctx_lock);
spin_lock_init(&ctx->completion_lock);
mutex_init(&ctx->ring_lock);
+ /* Protect against page migration throughout kiotx setup by keeping
+ * the ring_lock mutex held until setup is complete. */
+ mutex_lock(&ctx->ring_lock);
init_waitqueue_head(&ctx->wait);

INIT_LIST_HEAD(&ctx->active_reqs);

+ if (percpu_ref_init(&ctx->users, free_ioctx_users))
+ goto err;
+
+ if (percpu_ref_init(&ctx->reqs, free_ioctx_reqs))
+ goto err;
+
ctx->cpu = alloc_percpu(struct kioctx_cpu);
if (!ctx->cpu)
goto err;

- if (aio_setup_ring(ctx) < 0)
+ err = aio_setup_ring(ctx);
+ if (err < 0)
goto err;

atomic_set(&ctx->reqs_available, ctx->nr_events - 1);
@@ -683,6 +692,9 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
if (err)
goto err_cleanup;

+ /* Release the ring_lock mutex now that all setup is complete. */
+ mutex_unlock(&ctx->ring_lock);
+
pr_debug("allocated ioctx %p[%ld]: mm=%p mask=0x%x\n",
ctx, ctx->user_id, mm, ctx->nr_events);
return ctx;
@@ -692,6 +704,7 @@ err_cleanup:
err_ctx:
aio_free_ring(ctx);
err:
+ mutex_unlock(&ctx->ring_lock);
free_percpu(ctx->cpu);
free_percpu(ctx->reqs.pcpu_count);
free_percpu(ctx->users.pcpu_count);
@@ -1024,6 +1037,7 @@ static long aio_read_events_ring(struct kioctx *ctx,

mutex_lock(&ctx->ring_lock);

+ /* Access to ->ring_pages here is protected by ctx->ring_lock. */
ring = kmap_atomic(ctx->ring_pages[0]);
head = ring->head;
tail = ring->tail;
--
1.9.3