Re: [PATCH 2/2] f2fs: support data compression

[Chao Yu]

On 2019/10/31 1:02, Eric Biggers wrote:
> On Wed, Oct 30, 2019 at 04:43:52PM +0800, Chao Yu wrote:
>>>>>>  static void bio_post_read_processing(struct bio_post_read_ctx *ctx)
>>>>>>  {
>>>>>> -	/*
>>>>>> -	 * We use different work queues for decryption and for verity because
>>>>>> -	 * verity may require reading metadata pages that need decryption, and
>>>>>> -	 * we shouldn't recurse to the same workqueue.
>>>>>> -	 */
>>>>>
>>>>> Why is it okay (i.e., no deadlocks) to no longer use different work queues for
>>>>> decryption and for verity?  See the comment above which is being deleted.
>>>>
>>>> Could you explain more about how deadlock happen? or share me a link address if
>>>> you have described that case somewhere?
>>>>
>>>
>>> The verity work can read pages from the file which require decryption.  I'm
>>> concerned that it could deadlock if the work is scheduled on the same workqueue.
>>
>> I assume you've tried one workqueue, and suffered deadlock..
>>
>>> Granted, I'm not an expert in Linux workqueues, so if you've investigated this
>>> and determined that it's safe, can you explain why?
>>
>> I'm not familiar with workqueue...  I guess it may not safe that if the work is
>> scheduled to the same cpu in where verity was waiting for data? if the work is
>> scheduled to other cpu, it may be safe.
>>
>> I can check that before splitting the workqueue for verity and decrypt/decompress.
>>
> 
> Yes this is a real problem, try 'kvm-xfstests -c f2fs/encrypt generic/579'.
> The worker thread gets deadlocked in f2fs_read_merkle_tree_page() waiting for
> the Merkle tree page to be decrypted.  This is with the v2 compression patch;
> it works fine on current mainline.

Oh, alright...

Let me split them, thanks very much for all the comments and test anyway.

Thanks,

> 
> INFO: task kworker/u5:0:61 blocked for more than 30 seconds.
>       Not tainted 5.4.0-rc1-00119-g464e31ba60d0 #13
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> kworker/u5:0    D    0    61      2 0x80004000
> Workqueue: f2fs_post_read_wq f2fs_post_read_work
> Call Trace:
>  context_switch kernel/sched/core.c:3384 [inline]
>  __schedule+0x299/0x6c0 kernel/sched/core.c:4069
>  schedule+0x44/0xd0 kernel/sched/core.c:4136
>  io_schedule+0x11/0x40 kernel/sched/core.c:5780
>  wait_on_page_bit_common mm/filemap.c:1174 [inline]
>  wait_on_page_bit mm/filemap.c:1223 [inline]
>  wait_on_page_locked include/linux/pagemap.h:527 [inline]
>  wait_on_page_locked include/linux/pagemap.h:524 [inline]
>  wait_on_page_read mm/filemap.c:2767 [inline]
>  do_read_cache_page+0x407/0x660 mm/filemap.c:2810
>  read_cache_page+0xd/0x10 mm/filemap.c:2894
>  f2fs_read_merkle_tree_page+0x2e/0x30 include/linux/pagemap.h:396
>  verify_page+0x110/0x560 fs/verity/verify.c:120
>  fsverity_verify_bio+0xe6/0x1a0 fs/verity/verify.c:239
>  verity_work fs/f2fs/data.c:142 [inline]
>  f2fs_post_read_work+0x36/0x50 fs/f2fs/data.c:160
>  process_one_work+0x225/0x550 kernel/workqueue.c:2269
>  worker_thread+0x4b/0x3c0 kernel/workqueue.c:2415
>  kthread+0x125/0x140 kernel/kthread.c:255
>  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
> INFO: task kworker/u5:1:1140 blocked for more than 30 seconds.
>       Not tainted 5.4.0-rc1-00119-g464e31ba60d0 #13
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> kworker/u5:1    D    0  1140      2 0x80004000
> Workqueue: f2fs_post_read_wq f2fs_post_read_work
> Call Trace:
>  context_switch kernel/sched/core.c:3384 [inline]
>  __schedule+0x299/0x6c0 kernel/sched/core.c:4069
>  schedule+0x44/0xd0 kernel/sched/core.c:4136
>  io_schedule+0x11/0x40 kernel/sched/core.c:5780
>  wait_on_page_bit_common mm/filemap.c:1174 [inline]
>  wait_on_page_bit mm/filemap.c:1223 [inline]
>  wait_on_page_locked include/linux/pagemap.h:527 [inline]
>  wait_on_page_locked include/linux/pagemap.h:524 [inline]
>  wait_on_page_read mm/filemap.c:2767 [inline]
>  do_read_cache_page+0x407/0x660 mm/filemap.c:2810
>  read_cache_page+0xd/0x10 mm/filemap.c:2894
>  f2fs_read_merkle_tree_page+0x2e/0x30 include/linux/pagemap.h:396
>  verify_page+0x110/0x560 fs/verity/verify.c:120
>  fsverity_verify_bio+0xe6/0x1a0 fs/verity/verify.c:239
>  verity_work fs/f2fs/data.c:142 [inline]
>  f2fs_post_read_work+0x36/0x50 fs/f2fs/data.c:160
>  process_one_work+0x225/0x550 kernel/workqueue.c:2269
>  worker_thread+0x4b/0x3c0 kernel/workqueue.c:2415
>  kthread+0x125/0x140 kernel/kthread.c:255
>  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
> 
> Showing all locks held in the system:
> 1 lock held by khungtaskd/21:
>  #0: ffffffff82250520 (rcu_read_lock){....}, at: rcu_lock_acquire.constprop.0+0x0/0x30 include/trace/events/lock.h:13
> 2 locks held by kworker/u5:0/61:
>  #0: ffff88807b78eb28 ((wq_completion)f2fs_post_read_wq){+.+.}, at: set_work_data kernel/workqueue.c:619 [inline]
>  #0: ffff88807b78eb28 ((wq_completion)f2fs_post_read_wq){+.+.}, at: set_work_pool_and_clear_pending kernel/workqueue.c:647 [inline]
>  #0: ffff88807b78eb28 ((wq_completion)f2fs_post_read_wq){+.+.}, at: process_one_work+0x1ad/0x550 kernel/workqueue.c:2240
>  #1: ffffc90000253e50 ((work_completion)(&ctx->work)){+.+.}, at: set_work_data kernel/workqueue.c:619 [inline]
>  #1: ffffc90000253e50 ((work_completion)(&ctx->work)){+.+.}, at: set_work_pool_and_clear_pending kernel/workqueue.c:647 [inline]
>  #1: ffffc90000253e50 ((work_completion)(&ctx->work)){+.+.}, at: process_one_work+0x1ad/0x550 kernel/workqueue.c:2240
> 2 locks held by kworker/u5:1/1140:
>  #0: ffff88807b78eb28 ((wq_completion)f2fs_post_read_wq){+.+.}, at: set_work_data kernel/workqueue.c:619 [inline]
>  #0: ffff88807b78eb28 ((wq_completion)f2fs_post_read_wq){+.+.}, at: set_work_pool_and_clear_pending kernel/workqueue.c:647 [inline]
>  #0: ffff88807b78eb28 ((wq_completion)f2fs_post_read_wq){+.+.}, at: process_one_work+0x1ad/0x550 kernel/workqueue.c:2240
>  #1: ffffc9000174be50 ((work_completion)(&ctx->work)){+.+.}, at: set_work_data kernel/workqueue.c:619 [inline]
>  #1: ffffc9000174be50 ((work_completion)(&ctx->work)){+.+.}, at: set_work_pool_and_clear_pending kernel/workqueue.c:647 [inline]
>  #1: ffffc9000174be50 ((work_completion)(&ctx->work)){+.+.}, at: process_one_work+0x1ad/0x550 kernel/workqueue.c:2240
> .
>