From 586b286b110e94eb31840ac5afc0c24e0881fe34 Mon Sep 17 00:00:00 2001 From: Mike Snitzer Date: Wed, 9 Sep 2015 21:34:51 -0400 Subject: 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 Signed-off-by: Mike Snitzer Cc: stable@vger.kernel.org # 4.0+ diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c index d60c88d..4b3b6f8 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) -- cgit v0.10.2