Re: [PATCH 1/2] f2fs: fix to commit bio cache after flushing node pages

[Chao Yu]

On 2016/9/29 4:19, Jaegeuk Kim wrote:
> On Tue, Sep 27, 2016 at 10:09:03AM +0800, Chao Yu wrote:
>> On 2016/9/27 9:39, Jaegeuk Kim wrote:
>>> On Tue, Sep 27, 2016 at 08:57:41AM +0800, Chao Yu wrote:
>>>> Hi Jaegeuk,
>>>>
>>>> On 2016/9/27 2:33, Jaegeuk Kim wrote:
>>>>> Hi Chao,
>>>>>
>>>>> On Tue, Sep 27, 2016 at 12:09:52AM +0800, Chao Yu wrote:
>>>>>> From: Chao Yu <yuchao0@xxxxxxxxxx>
>>>>>>
>>>>>> In sync_node_pages, we won't check and commit last merged pages in private
>>>>>> bio cache of f2fs, as these pages were taged as writeback, someone who is
>>>>>> waiting for writebacking of the page will be blocked until the cache was
>>>>>> committed by someone else.
>>>>>>
>>>>>> We need to commit node type bio cache to avoid potential deadlock or long
>>>>>> delay of waiting writeback.
>>>>>>
>>>>>> Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
>>>>>> ---
>>>>>>  fs/f2fs/node.c | 11 +++++++++--
>>>>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
>>>>>> index 9faddcd..f73f774 100644
>>>>>> --- a/fs/f2fs/node.c
>>>>>> +++ b/fs/f2fs/node.c
>>>>>> @@ -1416,6 +1416,7 @@ int sync_node_pages(struct f2fs_sb_info *sbi, struct writeback_control *wbc)
>>>>>>  	struct pagevec pvec;
>>>>>>  	int step = 0;
>>>>>>  	int nwritten = 0;
>>>>>> +	int ret = 0;
>>>>>>  
>>>>>>  	pagevec_init(&pvec, 0);
>>>>>>  
>>>>>> @@ -1436,7 +1437,8 @@ next_step:
>>>>>>  
>>>>>>  			if (unlikely(f2fs_cp_error(sbi))) {
>>>>>>  				pagevec_release(&pvec);
>>>>>> -				return -EIO;
>>>>>> +				ret = -EIO;
>>>>>> +				goto out;
>>>>>>  			}
>>>>>>  
>>>>>>  			/*
>>>>>> @@ -1487,6 +1489,8 @@ continue_unlock:
>>>>>>  
>>>>>>  			if (NODE_MAPPING(sbi)->a_ops->writepage(page, wbc))
>>>>>>  				unlock_page(page);
>>>>>> +			else
>>>>>> +				nwritten++;
>>>>>>  
>>>>>>  			if (--wbc->nr_to_write == 0)
>>>>>>  				break;
>>>>>> @@ -1504,7 +1508,10 @@ continue_unlock:
>>>>>>  		step++;
>>>>>>  		goto next_step;
>>>>>>  	}
>>>>>> -	return nwritten;
>>>>>> +out:
>>>>>> +	if (nwritten)
>>>>>> +		f2fs_submit_merged_bio(sbi, NODE, WRITE);
>>>>>
>>>>> IIRC, we don't need to flush this, since f2fs_submit_merged_bio_cond() would
>>>>> handle this in f2fs_wait_on_page_writeback().
>>>>
>>>> Yes, it covers all the cases in f2fs private codes, but there are still some
>>>> codes in mm or fs directory, and they didn't use f2fs_wait_on_page_writeback
>>>> when waiting page writeback. Such as do_writepages && filemap_fdatawait in
>>>> __writeback_single_inode...
>>>
>>> The do_writepages() is okay, which will call f2fs_write_node_pages().
>>> The __writeback_single_inode() won't do filemap_fdatawait() with WB_SYNC_ALL.
>>> We don't need to take care of truncation as well.
>>>
>>> Any missing one?
>>
>> Another is: while testing with first version of checkpoint error injection, I
>> encounter below dump stack:
>>
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> mount           D ffff8801c1bf7960     0 97685  97397 0x00080000
>>  ffff8801c1bf7960 ffff8801c1bf7930 ffff880175900000 ffff8801c1bf7980
>>  ffff8801c1bf8000 0000000000000000 7fffffffffffffff ffff88021f7be340
>>  ffffffff817c8880 ffff8801c1bf7978 ffffffff817c80a5 ffff880214f58fc0
>> Call Trace:
>>  [<ffffffff817c8880>] ? bit_wait+0x50/0x50
>>  [<ffffffff817c80a5>] schedule+0x35/0x80
>>  [<ffffffff817cb152>] schedule_timeout+0x292/0x3d0
>>  [<ffffffff81022ab5>] ? xen_clocksource_get_cycles+0x15/0x20
>>  [<ffffffff810eeb5c>] ? ktime_get+0x3c/0xb0
>>  [<ffffffff817c8880>] ? bit_wait+0x50/0x50
>>  [<ffffffff817c7906>] io_schedule_timeout+0xa6/0x110
>>  [<ffffffff817c889b>] bit_wait_io+0x1b/0x60
>>  [<ffffffff817c84e4>] __wait_on_bit+0x64/0x90
>>  [<ffffffff8117dcd4>] wait_on_page_bit+0xc4/0xd0
>>  [<ffffffff810bc4d0>] ? autoremove_wake_function+0x40/0x40
>>  [<ffffffff81190a29>] truncate_inode_pages_range+0x409/0x840
>>  [<ffffffff811a406d>] ? pcpu_free_area+0x13d/0x1a0
>>  [<ffffffff810bc025>] ? wake_up_bit+0x25/0x30
>>  [<ffffffff81190ecc>] truncate_inode_pages_final+0x4c/0x60
>>  [<ffffffffa025e9e8>] f2fs_evict_inode+0x48/0x390 [f2fs]
>>  [<ffffffff812212f7>] evict+0xc7/0x1a0
>>  [<ffffffff81221f77>] iput+0x197/0x200
>>  [<ffffffffa0268242>] f2fs_fill_super+0xab2/0x1130 [f2fs]
>>  [<ffffffff81209454>] mount_bdev+0x184/0x1c0
>>  [<ffffffffa0267790>] ? f2fs_commit_super+0x100/0x100 [f2fs]
>>  [<ffffffffa02646a5>] f2fs_mount+0x15/0x20 [f2fs]
>>  [<ffffffff81209e19>] mount_fs+0x39/0x160
>>  [<ffffffff81225e47>] vfs_kern_mount+0x67/0x110
>>  [<ffffffff812283bb>] do_mount+0x1bb/0xc80
>>  [<ffffffff81229163>] SyS_mount+0x83/0xd0
>>  [<ffffffff8100391e>] do_syscall_64+0x6e/0x170
>>  [<ffffffff817cc325>] entry_SYSCALL64_slow_path+0x25/0x25
>>
>> Any thoughts?
> 
> I think this should not happen normally, since f2fs_stop_checkpoint() calls
> f2fs_flush_merged_bios().

In write_end_io, f2fs_stop_checkpoint will not call f2fs_flush_merged_bios.

One other problem here is it can cause latency during waiting writeback:

In fsync()
1. fsync_node_pages a/b/c pages is submitted, and d/e/f pages is still in bio
cache as we didn't commit the bio in the end.
2. wait_on_node_pages_writeback will wait a/b/c pages for writebacking, then
submit bio with d/e/f pages and the wait until they have writebacked to device.

Here we need to submit cached bios at the end of
fsync_node_pages/sync_node_pages to let them being merged in block layer as much
as possible, and also to avoid suffering more delay time due to double submit&wait.

Thanks,

> 
> Thanks,
> 
>>
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>>> +	return ret;
>>>>>>  }
>>>>>>  
>>>>>>  int wait_on_node_pages_writeback(struct f2fs_sb_info *sbi, nid_t ino)
>>>>>> -- 
>>>>>> 2.7.2
>>>>>
>>>>> .
>>>>>
>>>
>>> .
>>>
> 
> .
>