dm crypt: avoid unnecessary wait when splitting bio

Don't wait between submitting crypt requests for a bio unless
we are short of memory.

There are two situations when we must split an encrypted bio:
  1) there are no free pages;
  2) the new bio would violate underlying device restrictions
(e.g. max hw segments).

In case (2) we do not need to wait.

Add output variable to crypt_alloc_buffer() to distinguish between
these cases.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index f6018f5..682ef9e 100644 (file)
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -457,9 +457,11 @@ static void dm_crypt_bio_destructor(struct bio *bio)
 /*
  * Generate a new unfragmented bio with the given size
  * This should never violate the device limitations
- * May return a smaller bio when running out of pages
+ * May return a smaller bio when running out of pages, indicated by
+ * *out_of_pages set to 1.
  */
-static struct bio *crypt_alloc_buffer(struct dm_crypt_io *io, unsigned size)
+static struct bio *crypt_alloc_buffer(struct dm_crypt_io *io, unsigned size,
+                                     unsigned *out_of_pages)
 {
        struct crypt_config *cc = io->target->private;
        struct bio *clone;
@@ -473,11 +475,14 @@ static struct bio *crypt_alloc_buffer(struct dm_crypt_io *io, unsigned size)
                return NULL;
 
        clone_init(io, clone);
+       *out_of_pages = 0;
 
        for (i = 0; i < nr_iovecs; i++) {
                page = mempool_alloc(cc->page_pool, gfp_mask);
-               if (!page)
+               if (!page) {
+                       *out_of_pages = 1;
                        break;
+               }
 
                /*
                 * if additional pages cannot be allocated without waiting,
@@ -696,6 +701,7 @@ static void kcryptd_crypt_write_convert(struct dm_crypt_io *io)
        struct crypt_config *cc = io->target->private;
        struct bio *clone;
        int crypt_finished;
+       unsigned out_of_pages = 0;
        unsigned remaining = io->base_bio->bi_size;
        int r;
 
@@ -710,7 +716,7 @@ static void kcryptd_crypt_write_convert(struct dm_crypt_io *io)
         * so repeat the whole process until all the data can be handled.
         */
        while (remaining) {
-               clone = crypt_alloc_buffer(io, remaining);
+               clone = crypt_alloc_buffer(io, remaining, &out_of_pages);
                if (unlikely(!clone)) {
                        io->error = -ENOMEM;
                        break;
@@ -737,11 +743,15 @@ static void kcryptd_crypt_write_convert(struct dm_crypt_io *io)
                                break;
                }
 
-               /* out of memory -> run queues */
-               if (unlikely(remaining)) {
-                       wait_event(cc->writeq, !atomic_read(&io->ctx.pending));
+               /*
+                * Out of memory -> run queues
+                * But don't wait if split was due to the io size restriction
+                */
+               if (unlikely(out_of_pages))
                        congestion_wait(WRITE, HZ/100);
-               }
+
+               if (unlikely(remaining))
+                       wait_event(cc->writeq, !atomic_read(&io->ctx.pending));
        }
 
        crypt_dec_pending(io);