dm crypt: avoid deadlock in mempools

Fix a theoretical deadlock introduced in the previous commit ("dm crypt:
don't allocate pages for a partial request").

The function crypt_alloc_buffer may be called concurrently.  If we allocate
from the mempool concurrently, there is a possibility of deadlock.  For
example, if we have mempool of 256 pages, two processes, each wanting
256, pages allocate from the mempool concurrently, it may deadlock in a
situation where both processes have allocated 128 pages and the mempool
is exhausted.

To avoid such a scenario we allocate the pages under a mutex.  In order
to not degrade performance with excessive locking, we try non-blocking
allocations without a mutex first and if that fails, we fallback to a
blocking allocations with a mutex.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 6199245..fa1dba1 100644 (file)
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -124,6 +124,7 @@ struct crypt_config {
        mempool_t *req_pool;
        mempool_t *page_pool;
        struct bio_set *bs;
+       struct mutex bio_alloc_lock;
 
        struct workqueue_struct *io_queue;
        struct workqueue_struct *crypt_queue;
@@ -949,27 +950,51 @@ static void crypt_free_buffer_pages(struct crypt_config *cc, struct bio *clone);
 /*
  * Generate a new unfragmented bio with the given size
  * This should never violate the device limitations
+ *
+ * This function may be called concurrently. If we allocate from the mempool
+ * concurrently, there is a possibility of deadlock. For example, if we have
+ * mempool of 256 pages, two processes, each wanting 256, pages allocate from
+ * the mempool concurrently, it may deadlock in a situation where both processes
+ * have allocated 128 pages and the mempool is exhausted.
+ *
+ * In order to avoid this scenario we allocate the pages under a mutex.
+ *
+ * In order to not degrade performance with excessive locking, we try
+ * non-blocking allocations without a mutex first but on failure we fallback
+ * to blocking allocations with a mutex.
  */
 static struct bio *crypt_alloc_buffer(struct dm_crypt_io *io, unsigned size)
 {
        struct crypt_config *cc = io->cc;
        struct bio *clone;
        unsigned int nr_iovecs = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
-       gfp_t gfp_mask = GFP_NOIO | __GFP_HIGHMEM;
-       unsigned i, len;
+       gfp_t gfp_mask = GFP_NOWAIT | __GFP_HIGHMEM;
+       unsigned i, len, remaining_size;
        struct page *page;
        struct bio_vec *bvec;
 
+retry:
+       if (unlikely(gfp_mask & __GFP_WAIT))
+               mutex_lock(&cc->bio_alloc_lock);
+
        clone = bio_alloc_bioset(GFP_NOIO, nr_iovecs, cc->bs);
        if (!clone)
-               return NULL;
+               goto return_clone;
 
        clone_init(io, clone);
 
+       remaining_size = size;
+
        for (i = 0; i < nr_iovecs; i++) {
                page = mempool_alloc(cc->page_pool, gfp_mask);
+               if (!page) {
+                       crypt_free_buffer_pages(cc, clone);
+                       bio_put(clone);
+                       gfp_mask |= __GFP_WAIT;
+                       goto retry;
+               }
 
-               len = (size > PAGE_SIZE) ? PAGE_SIZE : size;
+               len = (remaining_size > PAGE_SIZE) ? PAGE_SIZE : remaining_size;
 
                bvec = &clone->bi_io_vec[clone->bi_vcnt++];
                bvec->bv_page = page;
@@ -978,9 +1003,13 @@ static struct bio *crypt_alloc_buffer(struct dm_crypt_io *io, unsigned size)
 
                clone->bi_iter.bi_size += len;
 
-               size -= len;
+               remaining_size -= len;
        }
 
+return_clone:
+       if (unlikely(gfp_mask & __GFP_WAIT))
+               mutex_unlock(&cc->bio_alloc_lock);
+
        return clone;
 }
 
@@ -1679,6 +1708,8 @@ static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
                goto bad;
        }
 
+       mutex_init(&cc->bio_alloc_lock);
+
        ret = -EINVAL;
        if (sscanf(argv[2], "%llu%c", &tmpll, &dummy) != 1) {
                ti->error = "Invalid iv_offset sector";