Messages in this thread | | | Date | Tue, 19 May 2015 13:59:50 -0600 | From | Jens Axboe <> | Subject | Re: [Regression] Guest fs corruption with 'block: loop: improve performance via blk-mq' |
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On 05/18/2015 05:38 PM, santosh shilimkar wrote: > On 5/18/2015 4:25 PM, Ming Lei wrote: >> On Tue, May 19, 2015 at 7:13 AM, santosh shilimkar >> <santosh.shilimkar@oracle.com> wrote: >>> On 5/18/2015 11:07 AM, santosh shilimkar wrote: >>>> >>>> On 5/17/2015 6:26 PM, Ming Lei wrote: >>>>> >>>>> Hi Santosh, >>>>> >>>>> Thanks for your report! >>>>> >>>>> On Sun, May 17, 2015 at 4:13 AM, santosh shilimkar >>>>> <santosh.shilimkar@oracle.com> wrote: >>>>>> >>>>>> Hi Ming Lei, Jens, >>>>>> >>>>>> While doing few tests with recent kernels with Xen Server, >>>>>> we saw guests(DOMU) disk image getting corrupted while booting it. >>>>>> Strangely the issue is seen so far only with disk image over ocfs2 >>>>>> volume. If the same image kept on the EXT3/4 drive, no corruption >>>>>> is observed. The issue is easily reproducible. You see the flurry >>>>>> of errors while guest is mounting the file systems. >>>>>> >>>>>> After doing some debug and bisects, we zeroed down the issue with >>>>>> commit "b5dd2f6 block: loop: improve performance via blk-mq". With >>>>>> that commit reverted the corruption goes away. >>>>>> >>>>>> Some more details on the test setup: >>>>>> 1. OVM(XEN) Server kernel(DOM0) upgraded to more recent kernel >>>>>> which includes commit b5dd2f6. Boot the Server. >>>>>> 2. On DOM0 file system create a ocfs2 volume >>>>>> 3. Keep the Guest(VM) disk image on ocfs2 volume. >>>>>> 4. Boot guest image. (xm create vm.cfg) >>>>> >>>>> >>>>> I am not familiar with xen, so is the image accessed via >>>>> loop block inside of guest VM? Is he loop block created >>>>> in DOM0 or guest VM? >>>>> >>>> Guest. The Guest disk image is represented as a file by loop >>>> device. >>>> >>>>>> 5. Observe the VM boot console log. VM itself use the EXT3 fs. >>>>>> You will see errors like below and after this boot, that file >>>>>> system/disk-image gets corrupted and mostly won't boot next time. >>>>> >>>>> >>>>> OK, that means the image is corrupted by VM booting. >>>>> >>>> Right >>>> >>>> [...] >>>> >>>>>> >>>>>> From the debug of the actual data on the disk vs what is read by >>>>>> the guest VM, we suspect the *reads* are actually not going all >>>>>> the way to disk and possibly returning the wrong data. Because >>>>>> the actual data on ocfs2 volume at those locations seems >>>>>> to be non-zero where as the guest seems to be read it as zero. >>>>> >>>>> >>>>> Two big changes in the patchset are: 1) use blk-mq request based IO; >>>>> 2) submit I/O concurrently(write vs. write is still serialized) >>>>> >>>>> Could you apply the patch in below link to see if it can fix the >>>>> issue? >>>>> BTW, this patch only removes concurrent submission. >>>>> >>>>> http://marc.info/?t=143093223200004&r=1&w=2 >>>>> >>>> What kernel is this patch generated against ? It doesn't apply against >>>> v4.0. Does this need the AIO/DIO conversion patches as well. Do you >>>> have the dependent patch-set I can't apply it against v4.0. >>>> >>> Anyways, I created patch(end of the email) against v4.0, based on >>> your patch >>> and tested it. The corruption is no more seen so it does fix >>> the issue after backing out concurrent submission changes from >>> commit b5dd2f6. Let me know whats you plan with it since linus >>> tip as well as v4.0 needs this fix. >> >> If your issue is caused by concurrent IO submittion, it might be one >> issue of ocfs2. As you see, there isn't such problem for ext3/ext4. >> > As we speak, I got to know about another regression with XFS as well > and am quite confident based on symptom that its similar issue. > I will get a confirmation on the same by tomorrow whether the patch > fixes it or not. > >> And the single thread patch is introduced for aio/dio support, which >> shouldn't have been a fix patch. >> > > Well before the loop blk-mq conversion commit b5dd2f6, the loop driver > was single threaded and as you see that issue seen with that > commit. Now with this experiment, it also proves that those work-queue > split changes are problematic. So am not sure why do you say that it > shouldn't be a fix patch.
There should be no issue with having concurrent submissions. If something relies on serialization of some sort, then that is broken and should be fixed up. That's not a problem with the loop driver. That's why it's not a fix.
-- Jens Axboe
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