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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>, 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>
From:   Sander Eikelenboom <linux@eikelenboom.it>
Message-ID: <00030538-e1ce-28ad-3548-8e3b07083b05@eikelenboom.it>
Date:   Thu, 27 Sep 2018 20:52:33 +0200
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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.

--
Sander