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 12:56:00 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Sander Eikelenboom <linux@...elenboom.it>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.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 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 
> 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