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Subject: Re: [Xen-devel] [PATCH] xen/blkfront: When purging persistent grants,
 keep them in the buffer
To:     Jens Axboe <axboe@kernel.dk>,
        Sander Eikelenboom <linux@eikelenboom.it>,
        Juergen Gross <jgross@suse.com>, 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:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <63d2a50f-c22e-1b99-8354-7feca9e089e5@oracle.com>
Date:   Thu, 27 Sep 2018 17:48:58 -0400
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Sender: linux-kernel-owner@vger.kernel.org
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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