Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [Xen-devel] [PATCH] xen/blkfront: When purging persistent grants,
 keep them in the buffer
To:     Sander Eikelenboom <linux@eikelenboom.it>,
        Juergen Gross <jgross@suse.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.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>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <b181cc24-16fc-ac94-24f8-1955efa6565d@kernel.dk>
Date:   Thu, 27 Sep 2018 12:56:00 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.0
MIME-Version: 1.0
In-Reply-To: <00030538-e1ce-28ad-3548-8e3b07083b05@eikelenboom.it>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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 
> 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.

-- 
Jens Axboe