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Date:   Wed, 15 Jul 2020 12:10:37 -0700
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Chao Yu <yuchao0@huawei.com>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com
Subject: Re: [f2fs-dev] [PATCH] f2fs: don't skip writeback of quota data
Message-ID: <20200715191037.GB2232118@google.com>
References: <20200709053027.351974-1-jaegeuk@kernel.org>
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 <20200710032616.GC545837@google.com>
 <01d0db54-eee1-f6cd-76c3-ebe59a7abae4@huawei.com>
 <20200710035053.GH545837@google.com>
 <77041117-f615-e6e6-591c-b02bf99e58c2@huawei.com>
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On 07/14, Chao Yu wrote:
> On 2020/7/14 1:59, Jaegeuk Kim wrote:
> > On 07/10, Chao Yu wrote:
> >> On 2020/7/10 11:50, Jaegeuk Kim wrote:
> >>> On 07/10, Chao Yu wrote:
> >>>> On 2020/7/10 11:26, Jaegeuk Kim wrote:
> >>>>> On 07/10, Chao Yu wrote:
> >>>>>> On 2020/7/10 3:05, Jaegeuk Kim wrote:
> >>>>>>> On 07/09, Chao Yu wrote:
> >>>>>>>> On 2020/7/9 13:30, Jaegeuk Kim wrote:
> >>>>>>>>> It doesn't need to bypass flushing quota data in background.
> >>>>>>>>
> >>>>>>>> The condition is used to flush quota data in batch to avoid random
> >>>>>>>> small-sized udpate, did you hit any problem here?
> >>>>>>>
> >>>>>>> I suspect this causes fault injection test being stuck by waiting for inode
> >>>>>>> writeback completion. With this patch, it has been running w/o any issue so far.
> >>>>>>> I keep an eye on this.
> >>>>>>
> >>>>>> Hmmm.. so that this patch may not fix the root cause, and it may hiding the
> >>>>>> issue deeper.
> >>>>>>
> >>>>>> How about just keeping this patch in our private branch to let fault injection
> >>>>>> test not be stuck? until we find the root cause in upstream codes.
> >>>>>
> >>>>> Well, I don't think this hides something. When the issue happens, I saw inodes
> >>>>> being stuck due to writeback while only quota has some dirty data. At that time,
> >>>>> there was no dirty data page from other inodes.
> >>>>
> >>>> Okay,
> >>>>
> >>>>>
> >>>>> More specifically, I suspect __writeback_inodes_sb_nr() gives WB_SYNC_NONE and
> >>>>> waits for wb_wait_for_completion().
> >>>>
> >>>> Did you record any callstack after the issue happened?
> >>>
> >>> I found this.
> >>>
> >>> [213389.297642]  __schedule+0x2dd/0x780^M
> >>> [213389.299224]  schedule+0x55/0xc0^M
> >>> [213389.300745]  wb_wait_for_completion+0x56/0x90^M
> >>> [213389.302469]  ? wait_woken+0x80/0x80^M
> >>> [213389.303997]  __writeback_inodes_sb_nr+0xa8/0xd0^M
> >>> [213389.305760]  writeback_inodes_sb+0x4b/0x60^M
> >>> [213389.307439]  sync_filesystem+0x2e/0xa0^M
> >>> [213389.308999]  generic_shutdown_super+0x27/0x110^M
> >>> [213389.310738]  kill_block_super+0x27/0x50^M
> >>> [213389.312327]  kill_f2fs_super+0x76/0xe0 [f2fs]^M
> >>> [213389.314014]  deactivate_locked_super+0x3b/0x80^M
> >>> [213389.315692]  deactivate_super+0x3e/0x50^M
> >>> [213389.317226]  cleanup_mnt+0x109/0x160^M
> >>> [213389.318718]  __cleanup_mnt+0x12/0x20^M
> >>> [213389.320177]  task_work_run+0x70/0xb0^M
> >>> [213389.321609]  exit_to_usermode_loop+0x131/0x160^M
> >>> [213389.323306]  do_syscall_64+0x170/0x1b0^M
> >>> [213389.324762]  entry_SYSCALL_64_after_hwframe+0x44/0xa9^M
> >>> [213389.326477] RIP: 0033:0x7fc4b5e6a35b^M
> >>
> >> Does this only happen during umount? If so, will below change help?
> >>
> >> 	if ((S_ISDIR(inode->i_mode) || IS_NOQUOTA(inode)) &&
> >> +			!is_sbi_flag_set(sbi, SBI_IS_CLOSE) &&
> >> 			wbc->sync_mode == WB_SYNC_NONE &&
> >> 			get_dirty_pages(inode) < nr_pages_to_skip(sbi, DATA) &&
> >> 			f2fs_available_free_memory(sbi, DIRTY_DENTS))
> >> 		goto skip_write;
> > 
> > Hmm, this doesn't work. The writeback was called before put_super?
> 
> Oops, still be confused about this issue. :(

Huam, I hit the problem with the patch.
I need to return back and think in other way. :(

> 
> Thanks,
> 
> > I'll try the original patch one more time.
> > 
> >>
> >>>
> >>>>
> >>>> Still I'm confused that why directory's data written could be skipped, but
> >>>> quota's data couldn't, what's the difference?
> >>>
> >>> I suspect different blocking timing from cp_error between quota and dentry.
> >>> e.g., we block dir operations right after cp_error, while quota can make
> >>
> >> No guarantee that there is no dirty dentry being created after
> >> cp_error, right?
> >>
> >> e.g.
> >>
> >> Thread A				Thread B
> >> - f2fs_create
> >> - bypass f2fs_cp_error
> >> 					- set cp_error
> >> - create dirty dentry
> >>
> >> BTW, do you know what __writeback_inodes_sb_nr is waiting for?
> >>
> >>> dirty pages in more fine granularity.
> >>>
> >>>>
> >>>>>
> >>>>>>
> >>>>>> Thanks,
> >>>>>>
> >>>>>>>
> >>>>>>> Thanks,
> >>>>>>>
> >>>>>>>>
> >>>>>>>> Thanks,
> >>>>>>>>
> >>>>>>>>>
> >>>>>>>>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> >>>>>>>>> ---
> >>>>>>>>>  fs/f2fs/data.c | 2 +-
> >>>>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>>>>>>
> >>>>>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> >>>>>>>>> index 44645f4f914b6..72e8b50e588c1 100644
> >>>>>>>>> --- a/fs/f2fs/data.c
> >>>>>>>>> +++ b/fs/f2fs/data.c
> >>>>>>>>> @@ -3148,7 +3148,7 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
> >>>>>>>>>  	if (unlikely(is_sbi_flag_set(sbi, SBI_POR_DOING)))
> >>>>>>>>>  		goto skip_write;
> >>>>>>>>>  
> >>>>>>>>> -	if ((S_ISDIR(inode->i_mode) || IS_NOQUOTA(inode)) &&
> >>>>>>>>> +	if (S_ISDIR(inode->i_mode) &&
> >>>>>>>>>  			wbc->sync_mode == WB_SYNC_NONE &&
> >>>>>>>>>  			get_dirty_pages(inode) < nr_pages_to_skip(sbi, DATA) &&
> >>>>>>>>>  			f2fs_available_free_memory(sbi, DIRTY_DENTS))
> >>>>>>>>>
> >>>>>>> .
> >>>>>>>
> >>>>> .
> >>>>>
> >>> .
> >>>
> > .
> >