Linux Kernels

dm crypt: constrain crypt device's max_segment_size to PAGE_SIZE

This change “dm crypt: constrain crypt device's max_segment_size to PAGE_SIZE” (commit 586b286) in Linux kernel is authored by Mike Snitzer <snitzer [at] redhat.com> on Wed Sep 9 21:34:51 2015 -0400.

Description of "dm crypt: constrain crypt device's max_segment_size to PAGE_SIZE"

The change “dm crypt: constrain crypt device's max_segment_size to PAGE_SIZE” introduces changes as follows.

dm crypt: constrain crypt device's max_segment_size to PAGE_SIZE

Setting the dm-crypt device's max_segment_size to PAGE_SIZE is an
unfortunate constraint that is required to avoid the potential for
exceeding dm-crypt's underlying device's max_segments limits -- due to
crypt_alloc_buffer() possibly allocating pages for the encryption bio
that are not as physically contiguous as the original bio.

It is interesting to note that this problem was already fixed back in
2007 via commit 91e106259 ("dm crypt: use bio_add_page").  But Linux 4.0
commit cf2f1abfb ("dm crypt: don't allocate pages for a partial
request") regressed dm-crypt back to _not_ using bio_add_page().  But
given dm-crypt's cpu parallelization changes all depend on commit
cf2f1abfb's abandoning of the more complex io fragments processing that
dm-crypt previously had we cannot easily go back to using
bio_add_page().

So all said the cleanest way to resolve this issue is to fix dm-crypt to
properly constrain the original bios entering dm-crypt so the encryption
bios that dm-crypt generates from the original bios are always
compatible with the underlying device's max_segments queue limits.

It should be noted that technically Linux 4.3 does _not_ need this fix
because of the block core's new late bio-splitting capability.  But, it
is reasoned, there is little to be gained by having the block core split
the encrypted bio that is composed of PAGE_SIZE segments.  That said, in
the future we may revert this change.

Fixes: cf2f1abfb ("dm crypt: don't allocate pages for a partial request")
Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=104421
Suggested-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 4.0+

Linux kernel releases containing commit 586b286

The Linux kernel releases containing this commit are as follows.

Linux kernel code changes from "dm crypt: constrain crypt device's max_segment_size to PAGE_SIZE"

There are 17 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/md/dm-crypt.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)
 
diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index d60c88df5234..4b3b6f8aff0c 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -968,7 +968,8 @@ 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 should never violate the device limitations (but only because
+ * max_segment_size is being constrained to PAGE_SIZE).
  *
  * This function may be called concurrently. If we allocate from the mempool
  * concurrently, there is a possibility of deadlock. For example, if we have
@@ -2045,9 +2046,20 @@ static int crypt_iterate_devices(struct dm_target *ti,
 	return fn(ti, cc->dev, cc->start, ti->len, data);
 }
 
+static void crypt_io_hints(struct dm_target *ti, struct queue_limits *limits)
+{
+	/*
+	 * Unfortunate constraint that is required to avoid the potential
+	 * for exceeding underlying device's max_segments limits -- due to
+	 * crypt_alloc_buffer() possibly allocating pages for the encryption
+	 * bio that are not as physically contiguous as the original bio.
+	 */
+	limits->max_segment_size = PAGE_SIZE;
+}
+
 static struct target_type crypt_target = {
 	.name   = "crypt",
-	.version = {1, 14, 0},
+	.version = {1, 14, 1},
 	.module = THIS_MODULE,
 	.ctr    = crypt_ctr,
 	.dtr    = crypt_dtr,
@@ -2058,6 +2070,7 @@ static struct target_type crypt_target = {
 	.resume = crypt_resume,
 	.message = crypt_message,
 	.iterate_devices = crypt_iterate_devices,
+	.io_hints = crypt_io_hints,
 };
 
 static int __init dm_crypt_init(void)

The commit for this change in Linux stable tree is 586b286 (patch).

Last modified: 2020/01/11 09:44