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Re: [qemu-s390x] [Qemu-devel] [PATCH v4 1/6] vfio-ccw: make it safe to a

From: Halil Pasic
Subject: Re: [qemu-s390x] [Qemu-devel] [PATCH v4 1/6] vfio-ccw: make it safe to access channel programs
Date: Wed, 10 Apr 2019 01:34:34 +0200

On Mon, 8 Apr 2019 19:07:47 +0200
Cornelia Huck <address@hidden> wrote:

> On Mon, 8 Apr 2019 13:02:12 -0400
> Farhan Ali <address@hidden> wrote:
> > On 03/01/2019 04:38 AM, Cornelia Huck wrote:
> > > When we get a solicited interrupt, the start function may have
> > > been cleared by a csch, but we still have a channel program
> > > structure allocated. Make it safe to call the cp accessors in
> > > any case, so we can call them unconditionally.
> > > 
> > > While at it, also make sure that functions called from other parts
> > > of the code return gracefully if the channel program structure
> > > has not been initialized (even though that is a bug in the caller).
> > > 
> > > Reviewed-by: Eric Farman<address@hidden>
> > > Signed-off-by: Cornelia Huck<address@hidden>
> > > ---  
> > 
> > Hi Connie,
> > 
> > My series of fixes for vfio-ccw depends on this patch as I would like to 
> > call cp_free unconditionally :) (I had developed my code on top of your 
> > patches).
> > 
> > Could we pick this patch up as well when/if you pick up my patch series? 
> > I am in the process of sending out a v2.
> > 
> > Regarding this patch we could merge it as a stand alone patch, separate 
> > from this series. And also the patch LGTM
> > 
> > Reviewed-by: Farhan Ali <address@hidden>
> Actually, I wanted to ask how people felt about merging this whole
> series for the next release :) It would be one thing less on my plate...

Sorry I was not able to spend any significant amount of time on this

Gave the combined set (this + Farhans fio-ccw fixes for kernel
stacktraces v2) it a bit of smoke testing after some minor adjustments
to make it compile:

--- a/drivers/s390/cio/vfio_ccw_ops.c
+++ b/drivers/s390/cio/vfio_ccw_ops.c
@@ -13,6 +13,7 @@
 #include <linux/vfio.h>
 #include <linux/mdev.h>
 #include <linux/nospec.h>
+#include <linux/slab.h>
 #include "vfio_ccw_private.h"

I'm just running fio on a pass-through DASD and on some virto-blk disks
in parallel. My QEMU is today's vfio-ccw-caps from your repo.

I see stuff like this:
qemu-git: vfio-ccw: wirte I/O region failed with errno=16[1811/7332/0 iops] 
[eta 26m:34s]
[Thread 0x3ff75890910 (LWP 43803) exited]/7932KB/0KB /s] [1930/7932/0 iops] 
[eta 26m:33s]
[Thread 0x3ff6b7b7910 (LWP 43800) exited]/8030KB/0KB /s] [2031/8030/0 iops] 
[eta 26m:32s]
dasd-eckd 0.0.1234: An error occurred in the DASD device driver, reason=14 
INFO: task kworker/u6:1:26 blocked for more than 122 seconds.ps] [eta 
23m:26s]eta 23m:25s]
      Not tainted 5.1.0-rc3-00217-g6ab18dc #598
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u6:1    D    0    26      2 0x00000000
Workqueue: writeback wb_workfn (flush-94:0)
Call Trace:
([<0000000000ae23f2>] __schedule+0x4fa/0xc98)
 [<0000000000ae2bda>] schedule+0x4a/0xb0 
 [<00000000001b30ec>] io_schedule+0x2c/0x50 
 [<000000000071cc9c>] blk_mq_get_tag+0x1bc/0x310 
 [<000000000071571c>] blk_mq_get_request+0x1a4/0x4a8 
 [<0000000000719d38>] blk_mq_make_request+0x100/0x728 
 [<000000000070aa0a>] generic_make_request+0x26a/0x478 
 [<000000000070ac76>] submit_bio+0x5e/0x178 
 [<00000000004cfa2c>] ext4_io_submit+0x74/0x88 
 [<00000000004cfd32>] ext4_bio_write_page+0x2d2/0x4c8 
 [<00000000004aa5b4>] mpage_submit_page+0x74/0xa8 
 [<00000000004aa676>] mpage_process_page_bufs+0x8e/0x1b8 
 [<00000000004aa9bc>] mpage_prepare_extent_to_map+0x21c/0x390 
 [<00000000004b063c>] ext4_writepages+0x4bc/0x11a0 
 [<000000000032ef7a>] do_writepages+0x3a/0xf0 
 [<0000000000416226>] __writeback_single_inode+0x86/0x7a0 
 [<0000000000417154>] writeback_sb_inodes+0x2cc/0x550 
 [<0000000000417476>] __writeback_inodes_wb+0x9e/0xe8 
 [<00000000004179e0>] wb_writeback+0x468/0x598 
 [<0000000000418780>] wb_workfn+0x3b8/0x710 
 [<0000000000199322>] process_one_work+0x25a/0x668 
 [<000000000019977a>] worker_thread+0x4a/0x428 
 [<00000000001a1ae8>] kthread+0x150/0x170 
 [<0000000000aeadda>] kernel_thread_starter+0x6/0xc 
 [<0000000000aeadd4>] kernel_thread_starter+0x0/0xc 
4 locks held by kworker/u6:1/26:
 #0: 00000000792cf224 ((wq_completion)writeback){+.+.}, at: 
 #1: 000000009888c0e5 ((work_completion)(&(&wb->dwork)->work)){+.+.}, at: 
 #2: 000000002bfb76f0 (&type->s_umount_key#29){++++}, at: 
 #3: 00000000ff47fe1d (&sbi->s_journal_flag_rwsem){.+.+}, at: 

Since I haven't had the time to keep up lately, I will just trust Eric
and Farhan on whether this should be merged or not. From a quick look at
the code, and a quick stroll through my remaining memories, I think, there
are a couple of things, that I myself would try to solve differently. But
that is not a valid reason to hold this up.

I would like to spare the hustle of revisiting my old comments for everyone.
From the stability and utility perspective I'm pretty convinced we are
better off than without the patches in question.

If it is good enough for Eric and Farhan, I have no objections against merging.


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