linux-kernel - Re: [Xen-devel] [PATCH] xen/blkfront: When purging persistent grants, keep them in the buffer

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Date:   Thu, 27 Sep 2018 15:06:14 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Jens Axboe <axboe@...nel.dk>,
        Sander Eikelenboom <linux@...elenboom.it>,
        Juergen Gross <jgross@...e.com>, konrad.wilk@...cle.com,
        roger.pau@...rix.com
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH] xen/blkfront: When purging persistent grants,
 keep them in the buffer

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@...cle.com>
>>>> Reviewed-by: Juergen Gross <jgross@...e.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.

-boris