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Subject: Re: [Xen-devel] [PATCH] xen/blkfront: When purging persistent grants,
 keep them in the buffer
To:     Sander Eikelenboom <linux@eikelenboom.it>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Jens Axboe <axboe@kernel.dk>, konrad.wilk@oracle.com,
        roger.pau@citrix.com
Cc:     xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
References: <20180922195549.27953-1-boris.ostrovsky@oracle.com>
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From:   Juergen Gross <jgross@suse.com>
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Date:   Fri, 28 Sep 2018 08:44:39 +0200
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On 28/09/2018 00:03, Sander Eikelenboom wrote:
> On 27/09/18 23:48, Boris Ostrovsky wrote:
>> On 9/27/18 5:37 PM, Jens Axboe wrote:
>>> On 9/27/18 2:33 PM, Sander Eikelenboom wrote:
>>>> On 27/09/18 21:06, Boris Ostrovsky wrote:
>>>>> On 9/27/18 2:56 PM, Jens Axboe wrote:
>>>>>> On 9/27/18 12:52 PM, Sander Eikelenboom wrote:
>>>>>>> On 27/09/18 16:26, Jens Axboe wrote:
>>>>>>>> On 9/27/18 1:12 AM, Juergen Gross wrote:
>>>>>>>>> On 22/09/18 21:55, Boris Ostrovsky wrote:
>>>>>>>>>> Commit a46b53672b2c ("xen/blkfront: cleanup stale persistent grants")
>>>>>>>>>> added support for purging persistent grants when they are not in use. As
>>>>>>>>>> part of the purge, the grants were removed from the grant buffer, This
>>>>>>>>>> eventually causes the buffer to become empty, with BUG_ON triggered in
>>>>>>>>>> get_free_grant(). This can be observed even on an idle system, within
>>>>>>>>>> 20-30 minutes.
>>>>>>>>>>
>>>>>>>>>> We should keep the grants in the buffer when purging, and only free the
>>>>>>>>>> grant ref.
>>>>>>>>>>
>>>>>>>>>> Fixes: a46b53672b2c ("xen/blkfront: cleanup stale persistent grants")
>>>>>>>>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>>>>>>>> Reviewed-by: Juergen Gross <jgross@suse.com>
>>>>>>>> Since Konrad is out, I'm going to queue this up for 4.19.
>>>>>>>>
>>>>>>> Hi Boris/Juergen.
>>>>>>>
>>>>>>> Last week i tested a linux-4.19-rc4 kernel with xen-next and this patch from Boris pulled on top. 
>>>>>>> Unfortunately it made a VM hang (probably because it's rootFS is shuffled from under it's feet 
>>>>> What do you mean by "rootFS is shuffled from under it's feet " ?
>>>> Assumption that block-front getting borked and either a kernel crash or rootfs becoming mounted readonly. Didn't (try) to check though.
>>>>
>>>>>>> and it gave these in dom0 dmesg:
>>>>>>>
>>>>>>> [ 9251.696090] xen-blkback: requesting a grant already in use
>>>>>>> [ 9251.705861] xen-blkback: trying to add a gref that's already in the tree
>>>>>>> [ 9251.715781] xen-blkback: requesting a grant already in use
>>>>>>> [ 9251.725756] xen-blkback: trying to add a gref that's already in the tree
>>>>>>> [ 9251.735698] xen-blkback: requesting a grant already in use
>>>>>>> [ 9251.745573] xen-blkback: trying to add a gref that's already in the tree
>>>>>>>
>>>>>>> The VM was a HVM with 4 vcpu's and 2 phy disks:
>>>>>>> xen-blkback: backend/vbd/14/768: using 4 queues, protocol 1 (x86_64-abi) persistent grants
>>>>>>> xen-blkback: backend/vbd/14/832: using 4 queues, protocol 1 (x86_64-abi) persistent grants
>>>>>>>
>>>>>>>
>>>>>>> Currently i have been running 4.19-rc5 with xen-next on top and commit
>>>>>>> a46b53672b2c reverted, for a couple of days. That seems to run stable
>>>>>>> for me (since it's a small box so i'm not hit by what a46b53672b2c
>>>>>>> tried to fix.
>>>>>>>
>>>>>>> If you can come up with a debug patch i can give that a spin tomorrow
>>>>>>> evening or in the weekend, so we are hopefully still in time for the
>>>>>>> 4.19 release.
>>>>>> At this late in the game, might make more sense to simply revert the
>>>>>> buggy commit.  Especially since what is currently out there doesn't fix
>>>>>> the issue for you.
>>>> Don't know if Boris or Juergen have a hunch about the issue, if not
>>>> perhaps a revert is the best.
>>> Anyone? Unless I hear otherwise, I'll revert the series tomorrow.
>>
>> Juergen may have something to say by tomorrow, but from my perspective,
>> given that we are coming up on rc6 --- yes.
>>
>> I looked at the patches again and didn't see anything obvious.
>>
>> -boris
> 
> Could also be that what i hit is a latent bug, 
> that is not caused by these patches but merely got uncovered by them.
> 
> xl dmesg also shows quite some:
>     (XEN) [2018-09-24 03:15:46.847] grant_table.c:1755:d14v0 Expanding d14 grant table from 19 to 20 frames
>     (XEN) [2018-09-24 03:15:46.849] grant_table.c:1755:d14v0 Expanding d14 grant table from 20 to 21 frames
> (and has done that for ages on my box not leading to any direct problems to my knowledge)
> 
> I don't know if there could be related and something around the (persistent) grants for block devices could be leaking under some conditions?

I could reproduce the issue Boris has seen and I have found the fault
in his patch. Just testing a fix.


Juergen