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Subject: Re: [Xen-devel] [PATCH] xen/blkfront: When purging persistent grants,
 keep them in the buffer
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        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>
 <28aa9249-7406-21c6-f509-65411828e2d7@suse.com>
 <5bd1a695-50c6-e79f-38dd-c980fc2138ad@kernel.dk>
 <00030538-e1ce-28ad-3548-8e3b07083b05@eikelenboom.it>
 <b181cc24-16fc-ac94-24f8-1955efa6565d@kernel.dk>
 <04bc976c-9991-e24b-4994-55540b06f133@oracle.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <fd0e7293-310c-a15c-dc98-2743c4504632@kernel.dk>
Date:   Thu, 27 Sep 2018 13:16:35 -0600
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On 9/27/18 1:06 PM, 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 " ?
> 
>>> 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.
> 
> If decision is to revert then I think the whole series needs to be
> reverted.

Yes, definitely.

-- 
Jens Axboe