From mboxrd@z Thu Jan 1 00:00:00 1970 From: Neil Brown Subject: Re: Weird Issue with raid 5+0 Date: Mon, 8 Mar 2010 16:50:21 +1100 Message-ID: <20100308165021.6529fe6d@notabene.brown> References: <31e44a111002202033m4a9dfba9yf8aef62b8b39933a@mail.gmail.com> <20100221164805.5bdc2d60@notabene.brown> <31e44a111002202326x407c814dsaa60e51a8a0ff049@mail.gmail.com> <20100221191640.39b68b01@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20100221191640.39b68b01@notabene.brown> Sender: linux-raid-owner@vger.kernel.org Cc: chris , linux-raid@vger.kernel.org List-Id: linux-raid.ids On Sun, 21 Feb 2010 19:16:40 +1100 Neil Brown wrote: > On Sun, 21 Feb 2010 02:26:42 -0500 > chris wrote: > > > That is exactly what I didn't want to hear :( I am running > > 2.6.26-2-xen-amd64. Are you sure its a kernel problem and nothing to > > do with my chunk/block sizes? If this is a bug what versions are > > affected, I'll build a new domU kernel and see if I can get it working > > there. > > > > - chris > > I'm absolutely sure it is a kernel bug. And I think I now know what the bug is. A patch was recently posted to dm-devel which I think addresses exactly this problem. I reproduce it below. NeilBrown ------------------- 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 --- 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