If the lower device exposes a merge_bvec_fn, dm_set_device_limits() restricts max_sectors to PAGE_SIZE "just to be safe". This is not sufficient, however. If someone uses bio_add_page() to add 8 disjunct 512 byte partial pages to a bio, it would succeed, but could still cross a border of whatever restrictions are below us (e.g. raid10 stripe boundary). An attempted bio_split() would not succeed, because bi_vcnt is 8. One example that triggered this frequently is the xen io layer. raid10_make_request bug: can't convert block across chunks or bigger than 64k 209265151 1 Signed-off-by: Lars <lars.ellenberg@xxxxxxxxxx> --- Neil: you may want to double check linear.c and raid*.c for similar logic, even though it is unlikely that someone puts md raid6 on top of something exposing a merge_bvec_fn. This is not the first time this has been patched, btw. See https://bugzilla.redhat.com/show_bug.cgi?id=440093 and the patch by Mikulas: https://bugzilla.redhat.com/attachment.cgi?id=342638&action=diff --- drivers/md/dm-table.c | 12 ++++++++++-- 1 files changed, 10 insertions(+), 2 deletions(-) diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c index 4b22feb..c686ff4 100644 --- a/drivers/md/dm-table.c +++ b/drivers/md/dm-table.c @@ -515,14 +515,22 @@ int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev, /* * Check if merge fn is supported. - * If not we'll force DM to use PAGE_SIZE or + * If not we'll force DM to use single bio_vec of PAGE_SIZE or * smaller I/O, just to be safe. */ - if (q->merge_bvec_fn && !ti->type->merge) + if (q->merge_bvec_fn && !ti->type->merge) { limits->max_sectors = min_not_zero(limits->max_sectors, (unsigned int) (PAGE_SIZE >> 9)); + /* Restricting max_sectors is not enough. + * If someone uses bio_add_page to add 8 disjunct 512 byte + * partial pages to a bio, it would succeed, + * but could still cross a border of whatever restrictions + * are below us (e.g. raid0 stripe boundary). An attempted + * bio_split() would not succeed, because bi_vcnt is 8. */ + limits->max_segments = 1; + } return 0; } EXPORT_SYMBOL_GPL(dm_set_device_limits); -- 1.6.3.3 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
