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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>,
        Jens Axboe <axboe@kernel.dk>, Juergen Gross <jgross@suse.com>,
        konrad.wilk@oracle.com, roger.pau@citrix.com
Cc:     xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
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From:   Sander Eikelenboom <linux@eikelenboom.it>
Message-ID: <878eaad1-b63e-7e9b-f4c3-1ec3825d91e1@eikelenboom.it>
Date:   Fri, 28 Sep 2018 00:03:26 +0200
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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?

--
Sander