Date:	Fri, 29 Apr 2016 13:50:02 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Jens Axboe <axboe@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
	Btrfs BTRFS <linux-btrfs@...r.kernel.org>,
	Shaun Tancheff <shaun.tancheff@...gate.com>,
	Alan Cox <alan@...ux.intel.com>, Neil Brown <neilb@...e.de>,
	Liu Bo <bo.li.liu@...cle.com>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v3 3/3] block: avoid to call .bi_end_io() recursively

On Fri, Apr 29, 2016 at 12:59 AM, Mikulas Patocka <mpatocka@...hat.com> wrote:
>
>
> On Fri, 29 Apr 2016, Ming Lei wrote:
>
>> On Thu, Apr 28, 2016 at 11:58 PM, Mikulas Patocka <mpatocka@...hat.com> wrote:
>> >
>> >
>> > On Thu, 28 Apr 2016, Ming Lei wrote:
>> >
>> >> Hi Mikulas,
>> >>
>> >> On Thu, Apr 28, 2016 at 11:29 PM, Mikulas Patocka <mpatocka@...hat.com> wrote:
>> >> >
>> >> >
>> >> > On Thu, 28 Apr 2016, Ming Lei wrote:
>> >> >
>> >> >> There were reports about heavy stack use by recursive calling
>> >> >> .bi_end_io()([1][2][3]). For example, more than 16K stack is
>> >> >> consumed in a single bio complete path[3], and in [2] stack
>> >> >> overflow can be triggered if 20 nested dm-crypt is used.
>> >> >>
>> >> >> Also patches[1] [2] [3] were posted for addressing the issue,
>> >> >> but never be merged. And the idea in these patches is basically
>> >> >> similar, all serializes the recursive calling of .bi_end_io() by
>> >> >> percpu list.
>> >> >>
>> >> >> This patch still takes the same idea, but uses bio_list to
>> >> >> implement it, which turns out more simple and the code becomes
>> >> >> more readable meantime.
>> >> >>
>> >> >> One corner case which wasn't covered before is that
>> >> >> bi_endio() may be scheduled to run in process context(such
>> >> >> as btrfs), and this patch just bypasses the optimizing for
>> >> >> that case because one new context should have enough stack space,
>> >> >> and this approach isn't capable of optimizing it too because
>> >> >> there isn't easy way to get a per-task linked list head.
>> >> >
>> >> > Hi
>> >> >
>> >> > You could use preempt_disable() and then you could use per-cpu list even
>> >> > in the process context.
>> >>
>> >> Image why the .bi_end_io() is scheduled to process context, and the only
>> >> workable/simple way I thought of is to use per-task list because it may sleep.
>> >
>> > The bi_end_io callback should not sleep, even if it is called from the
>> > process context.
>>
>> If it shouldn't sleep, why is it scheduled to run in process context by paying
>> extra context switch cost?
>
> Some device mapper (and other) drivers use a worker thread to process
> bios. So the bio may be finished from the worker thread. It would be
> advantageous to prevent stack overflow even in this case.

If the .bi_end_io wouldn't sleep, it can be put back into interrupt context
for the sake of performance when this patch is merged. The cost of context
switch in high IOPS case isn't cheap.

It isn't easy to avoid the recursive calling in process context except you
can add something 'task_struct' or introduce 'alloca()' in kernel. Or do you
have better ideas?

>
>> And you can find that btrfs_subio_endio_read() does sleep for checksum stuff.
>
> I'm not an expert on btrfs. What happens if it is called from an
> interrupt? Do you have an actual stracktrace when this function is called

What do you expect if sleepable function is called in softirq or
hardirq handler? :-)

> from bio_endio and when it sleeps?

The problem is observed in xfstests generic/323 by this patch v1. Sometimes the
test hangs, and sometimes kernel oops is triggered. and the issue is
fixed by introducing 'if (!in_interrupt())' block for handling running
.bi_end_io() from
process context.

If the block of 'if (!in_interrupt())' is removed and
preempt_disable()/preempt_enable() is added around bio->bi_end_io(),
the following kernel warning can be seen easily:

[   51.086303] BUG: sleeping function called from invalid context at
mm/slab.h:388
[   51.087442] in_atomic(): 1, irqs_disabled(): 0, pid: 633, name: kworker/u8:4
[   51.088575] CPU: 3 PID: 633 Comm: kworker/u8:4 Not tainted 4.6.0-rc3+ #2017
[   51.088578] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009),
BIOS rel-1.9.0-0-g01a84be-prebuilt.qemu-project.org 04/01/2014
[   51.088637] Workqueue: btrfs-endio btrfs_endio_helper [btrfs]
[   51.088640]  0000000000000000 ffff88007bbebc00 ffffffff813d92d3
ffff88007ba6ce00
[   51.088643]  0000000000000184 ffff88007bbebc18 ffffffff810a38bb
ffffffff81a35310
[   51.088645]  ffff88007bbebc40 ffffffff810a3949 0000000002400040
0000000002400040
[   51.088648] Call Trace:
[   51.088651]  [<ffffffff813d92d3>] dump_stack+0x63/0x90
[   51.088655]  [<ffffffff810a38bb>] ___might_sleep+0xdb/0x120
[   51.088657]  [<ffffffff810a3949>] __might_sleep+0x49/0x80
[   51.088659]  [<ffffffff811e98e7>] kmem_cache_alloc+0x1a7/0x210
[   51.088670]  [<ffffffffc0102741>] ? alloc_extent_state+0x21/0xe0 [btrfs]
[   51.088680]  [<ffffffffc0102741>] alloc_extent_state+0x21/0xe0 [btrfs]
[   51.088689]  [<ffffffffc01046ce>] __clear_extent_bit+0x2ae/0x3d0 [btrfs]
[   51.088698]  [<ffffffffc0104e6a>] clear_extent_bit+0x2a/0x30 [btrfs]
[   51.088708]  [<ffffffffc00e96d0>] btrfs_endio_direct_read+0x70/0xf0 [btrfs]
[   51.088711]  [<ffffffff8139f1e7>] bio_endio+0xf7/0x140
[   51.088718]  [<ffffffffc00dbb9c>] end_workqueue_fn+0x3c/0x40 [btrfs]
[   51.088728]  [<ffffffffc01184d7>] normal_work_helper+0xc7/0x310 [btrfs]
[   51.088737]  [<ffffffffc01187f2>] btrfs_endio_helper+0x12/0x20 [btrfs]
[   51.088740]  [<ffffffff81097b77>] process_one_work+0x157/0x420
[   51.088742]  [<ffffffff810985ab>] worker_thread+0x12b/0x4d0
[   51.088744]  [<ffffffff817171a8>] ? __schedule+0x368/0x950
[   51.088746]  [<ffffffff81098480>] ? rescuer_thread+0x380/0x380
[   51.088748]  [<ffffffff8109de84>] kthread+0xd4/0xf0
[   51.088750]  [<ffffffff8171b7a2>] ret_from_fork+0x22/0x40
[   51.088752]  [<ffffffff8109ddb0>] ? kthread_park+0x60/0x60


Not mention wait_for_completion() in both __btrfs_correct_data_nocsum()
and __btrfs_subio_endio_read(), which are called by btrfs_subio_endio_read()
in btrfs_endio_direct_read().


Thanks,
Ming

>
>> Thanks,
>> Ming
>
> Mikulas
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