Messages in this thread | | | Date | Thu, 09 Oct 2014 13:58:07 -0400 | From | Jeff Mahoney <> | Subject | Re: [PATCH] block: copy bi_vcnt in __bio_clone_fast |
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On 10/9/14, 12:13 PM, Ming Lei wrote: > On Thu, Oct 9, 2014 at 11:25 PM, Ming Lei <ming.lei@canonical.com> > wrote: >> On Thu, Oct 9, 2014 at 10:26 PM, Jeff Mahoney <jeffm@suse.com> >> wrote: >>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 >>> >>> On 10/9/14, 9:53 AM, Jeff Moyer wrote: >>>> Jeff Mahoney <jeffm@suse.com> writes: >>>> >>>>> Commit 05f1dd53152173 (block: add queue flag for disabling >>>>> SG merging) uses bi_vcnt to assign bio->bi_phys_segments if >>>>> sg merging is disabled. When using device mapper on top of >>>>> a blk-mq device (virtio_blk in my test), we'd end up >>>>> overflowing the scatterlist in __blk_bios_map_sg. >>>>> >>>>> __bio_clone_fast copies bi_iter and bi_io_vec but not >>>>> bi_vcnt, so blk_recount_segments would report >>>>> bi_phys_segments as 0. Since rq->nr_phys_segments is 0 as >>>>> well, the checks to ensure that we don't exceed the queue's >>>>> segment limit end up allowing more bios (and segments) to >>>>> attach the a request until we finally map it. That also >>>>> means we pass the BUG_ON at the beginning of >>>>> virtio_queue_rq, ultimately causing memory corruption and >>>>> a crash. >>>>> >>>>> If we copy bi_vcnt in __bio_clone_fast, the bios and >>>>> requests properly report the number of segments and >>>>> everything works as expected. >>>>> >>>>> Originally reported at >>>>> http://bugzilla.opensuse.org/show_bug.cgi?id=888259 >>>> >>>> Hi, Jeff, >>>> >>>> Did you manage to reproduce this problem with commit 0738854 >>>> (blk-merge: fix blk_recount_segments) applied? Or perhaps >>>> with commit 200612e (dm table: propagate >>>> QUEUE_FLAG_NO_SG_MERGE)? >>> >>> Yep. I was able to reproduce it with 3.17. I did try 0738854 >>> when I was still using 3.16 since it looked like a good >>> candidate. Neither of those patches affect the problem here. >>> bio->bi_phys_segments never gets a value set in the fast clone >>> case and that translates to req->nr_phys_segments never getting >>> properly accumulated. That might not be a problem except that >>> the NO_SG_MERGE behavior bypasses the iteration that would come >>> up with the correct value. In either case, >> >> This patch may get incorrect rq->nr_phys_segments because bio >> cloning is often used in case of I/O splitting, so could you test >> if the attached patch fixes your problem?
Ah. Right.
> Please ignore last patch and test the attached patch since we still > should use no sg merge to recalculate req's segments for cloned > bio.
Yep, this fix works as expected.
Thanks,
- -Jeff
- -- Jeff Mahoney SUSE Labs -----BEGIN PGP SIGNATURE----- Version: GnuPG/MacGPG2 v2.0.19 (Darwin)
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