dm-crypt: limit the size of encryption requests

Commit Message

Mikulas Patocka July 3, 2024, 12:46 p.m. UTC

There was a performance regression reported where dm-crypt would perform
worse on new kernels than on old kernels. The reason is that the old
kernels split the bios to NVMe request size (that is usually 65536 or
131072 bytes) and the new kernels pass the big bios through dm-crypt and
split them underneath.

If a big 1MiB bio is passed to dm-crypt, dm-crypt processes it on a single 
core without parallelization and this is what causes the performance 
degradation.

This commit introduces new tunable variables
/sys/module/dm_crypt/parameters/max_read_size and
/sys/module/dm_crypt/parameters/max_write_size that specify the maximum
bio size for dm-crypt. Bios larger than this value are split, so that
they can be encrypted in parallel by multiple cores. If these variables
are '0', a default 131072 is used.

Splitting bios may cause performance regressions in other workloads - if
this happens, the user should increase the value in max_read_size and
max_write_size variables.

max_read_size:
128k	2399MiB/s
256k	2368MiB/s
512k	1986MiB/s
1024	1790MiB/s

max_write_size:
128k	1712MiB/s
256k	1651MiB/s
512k	1537MiB/s
1024k	1332MiB/s

Note that if you run dm-crypt inside a virtual machine, you may need to do 
"echo numa >/sys/module/workqueue/parameters/default_affinity_scope" to 
improve performance.

Signed-off-by: Mikulas Patocka <[e-mail]>
Tested-by: Laurence Oberman <[e-mail]>

---
 Documentation/admin-guide/device-mapper/dm-crypt.rst |   11 ++++++
 drivers/md/dm-crypt.c                                |   32 +++++++++++++++++--
 2 files changed, 40 insertions(+), 3 deletions(-)

Patch

Index: linux-2.6/Documentation/admin-guide/device-mapper/dm-crypt.rst
===================================================================
--- linux-2.6.orig/Documentation/admin-guide/device-mapper/dm-crypt.rst	2024-06-29 21:26:11.000000000 +0200
+++ linux-2.6/Documentation/admin-guide/device-mapper/dm-crypt.rst	2024-06-29 21:26:11.000000000 +0200
@@ -160,6 +160,17 @@  iv_large_sectors
    The <iv_offset> must be multiple of <sector_size> (in 512 bytes units)
    if this flag is specified.
 
+
+Module parameters::
+max_read_size
+max_write_size
+- Maximum size of read or write requests. When a request larger than this size
+  is received, dm-crypt will split the request. The splitting improves
+  concurrency (the split requests could be encrypted in parallel by multiple
+  cores), but it also causes overhead. The user should tune this parameter to
+  fit the actual workload.
+
+
 Example scripts
 ===============
 LUKS (Linux Unified Key Setup) is now the preferred way to set up disk
Index: linux-2.6/drivers/md/dm-crypt.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-crypt.c	2024-06-29 21:26:11.000000000 +0200
+++ linux-2.6/drivers/md/dm-crypt.c	2024-07-01 17:42:49.000000000 +0200
@@ -241,6 +241,31 @@  static unsigned int dm_crypt_clients_n;
 static volatile unsigned long dm_crypt_pages_per_client;
 #define DM_CRYPT_MEMORY_PERCENT			2
 #define DM_CRYPT_MIN_PAGES_PER_CLIENT		(BIO_MAX_VECS * 16)
+#define DM_CRYPT_DEFAULT_MAX_READ_SIZE		131072
+#define DM_CRYPT_DEFAULT_MAX_WRITE_SIZE		131072
+
+static unsigned int max_read_size = 0;
+module_param(max_read_size, uint, 0644);
+MODULE_PARM_DESC(max_read_size, "Maximum size of a read request");
+static unsigned int max_write_size = 0;
+module_param(max_write_size, uint, 0644);
+MODULE_PARM_DESC(max_write_size, "Maximum size of a write request");
+static unsigned get_max_request_size(struct crypt_config *cc, bool wrt)
+{
+	unsigned val, sector_align;
+	val = !wrt ? READ_ONCE(max_read_size) : READ_ONCE(max_write_size);
+	if (likely(!val))
+		val = !wrt ? DM_CRYPT_DEFAULT_MAX_READ_SIZE : DM_CRYPT_DEFAULT_MAX_WRITE_SIZE;
+	if (wrt || cc->on_disk_tag_size) {
+		if (unlikely(val > BIO_MAX_VECS << PAGE_SHIFT))
+			val = BIO_MAX_VECS << PAGE_SHIFT;
+	}
+	sector_align = max(bdev_logical_block_size(cc->dev->bdev), (unsigned)cc->sector_size);
+	val = round_down(val, sector_align);
+	if (unlikely(!val))
+		val = sector_align;
+	return val >> SECTOR_SHIFT;
+}
 
 static void crypt_endio(struct bio *clone);
 static void kcryptd_queue_crypt(struct dm_crypt_io *io);
@@ -3474,6 +3499,7 @@  static int crypt_map(struct dm_target *t
 {
 	struct dm_crypt_io *io;
 	struct crypt_config *cc = ti->private;
+	unsigned max_sectors;
 
 	/*
 	 * If bio is REQ_PREFLUSH or REQ_OP_DISCARD, just bypass crypt queues.
@@ -3492,9 +3518,9 @@  static int crypt_map(struct dm_target *t
 	/*
 	 * Check if bio is too large, split as needed.
 	 */
-	if (unlikely(bio->bi_iter.bi_size > (BIO_MAX_VECS << PAGE_SHIFT)) &&
-	    (bio_data_dir(bio) == WRITE || cc->on_disk_tag_size))
-		dm_accept_partial_bio(bio, ((BIO_MAX_VECS << PAGE_SHIFT) >> SECTOR_SHIFT));
+	max_sectors = get_max_request_size(cc, bio_data_dir(bio) == WRITE);
+	if (unlikely(bio_sectors(bio) > max_sectors))
+		dm_accept_partial_bio(bio, max_sectors);
 
 	/*
 	 * Ensure that bio is a multiple of internal sector encryption size