linux-kernel - Re: [Xen-devel] [PATCH] xen-blkback: free requests on disconnection

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Date:	Mon, 7 Sep 2015 12:10:36 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Bob Liu <bob.liu@...cle.com>
CC:	<xen-devel@...ts.xenproject.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	<linux-kernel@...r.kernel.org>,
	Roger Pau Monne <roger.pau@...rix.com>
Subject: Re: [Xen-devel] [PATCH] xen-blkback: free requests on disconnection

On 07/09/15 07:07, Bob Liu wrote:
> Hi Julien,

Hi Bob,

> On 09/04/2015 09:51 PM, Julien Grall wrote:
>> Hi Roger,
>>
>> On 04/09/15 11:08, Roger Pau Monne wrote:
>>> Request allocation has been moved to connect_ring, which is called every
>>> time blkback connects to the frontend (this can happen multiple times during
>>> a blkback instance life cycle). On the other hand, request freeing has not
>>> been moved, so it's only called when destroying the backend instance. Due to
>>> this mismatch, blkback can allocate the request pool multiple times, without
>>> freeing it.
>>>
>>> In order to fix it, move the freeing of requests to xen_blkif_disconnect to
>>> restore the symmetry between request allocation and freeing.
>>>
>>> Reported-by: Julien Grall <julien.grall@...rix.com>
>>> Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
>>> Cc: Julien Grall <julien.grall@...rix.com>
>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
>>> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>> Cc: David Vrabel <david.vrabel@...rix.com>
>>> Cc: xen-devel@...ts.xenproject.org
>>
>> The patch is fixing my problem when using UEFI in the guest. Thank you!
>>
> 
> Could you please explain the problem you met a bit more?
> So that I can know back port this patch if met similar issue.

This is related to commit 86839c56dee28c315a4c19b7bfee450ccd84cd25
"xen/block: add multi-page ring support" (Roger, it may be worth to
indicate the offending commit in you commit message).

When starting a guest using UEFI. After the domain is destroyed I get
the following warning from blkback:


------------[ cut here ]------------
WARNING: CPU: 2 PID: 95 at
/home/julien/works/linux/drivers/block/xen-blkback/xenbus.c:274
xen_blkif_deferred_free+0x1f4/0x1f8()
Modules linked in:
CPU: 2 PID: 95 Comm: kworker/2:1 Tainted: G        W       4.2.0 #85
Hardware name: APM X-Gene Mustang board (DT)
Workqueue: events xen_blkif_deferred_free
Call trace:
[<ffff8000000890a8>] dump_backtrace+0x0/0x124
[<ffff8000000891dc>] show_stack+0x10/0x1c
[<ffff8000007653bc>] dump_stack+0x78/0x98
[<ffff800000097e88>] warn_slowpath_common+0x9c/0xd4
[<ffff800000097f80>] warn_slowpath_null+0x14/0x20
[<ffff800000557a0c>] xen_blkif_deferred_free+0x1f0/0x1f8
[<ffff8000000ad020>] process_one_work+0x160/0x3b4
[<ffff8000000ad3b4>] worker_thread+0x140/0x494
[<ffff8000000b2e34>] kthread+0xd8/0xf0
---[ end trace 6f859b7883c88cdd ]---

This is because the allocation of the requests are done during the
connection but the free is done when the domain is destroyed. Therefore
if the domain is re-initializing the connection (because UEFI or PV Grub
is used), the request won't be free and kept until the end.

So I think this should be backported in Linux 4.2 where the patch has
been introduced.

Regards,

-- 
Julien Grall
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