[Devel] [PATCH vz7] fs: avoid writeback busy-loop if redirty

[Maxim Patlasov]

I think BUG_ON is better than WARN_ON in that particular place because: 
1) we won't hit it unless someone (in future) will introduce a bug 
returning error (instead of OK) after redirty_page_for_writepage; 2) if 
we hit WARN_ON, it will be very difficult to investigate root cause 
(much harder than having a crash dump after WARN_ON).


On 12/15/2016 03:57 AM, Konstantin Khorenko wrote:
> BUG_ON() again.
>
> Ok, let's assume it's WARN_ON() there. :)
>
> -- 
> Best regards,
>
> Konstantin Khorenko,
> Virtuozzo Linux Kernel Team
>
> On 12/14/2016 09:48 PM, Maxim Patlasov wrote:
>> Alexey,
>>
>> Can you please review this one-liner below. I think it can
>> be responsible for inexplicable delays and CPU peaks in bugs like
>> https://jira.sw.ru/browse/PSBM-55919. Thanks, Maxim
>>
>> wb_writeback() bails out only if either nr_pages comes to zero or
>> "progress" (as reported by writeback_sb_inodes or __writeback_inodes_wb)
>> is zero.
>>
>> If a page is temporarily unavailable (is under fuse-writeback),
>> fuse_writepages_fill calls redirty_page_for_writepage, unlock the page
>> and returns OK. This OK results in progress=1. So wb_writeback
>> loops over writeback_sb_inodes/__writeback_inodes_wb again and again
>> until all nr_pages are exhausted.
>>
>> The patch ensures that progress=0 if fuse_writepages_fill did nothing.
>>
>> Signed-off-by: Maxim Patlasov <mpatlasov at virtuozzo.com>
>> ---
>>  fs/fs-writeback.c |    3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
>> index 5c9a082..483bbb9 100644
>> --- a/fs/fs-writeback.c
>> +++ b/fs/fs-writeback.c
>> @@ -774,6 +774,9 @@ static long writeback_sb_inodes(struct 
>> super_block *sb,
>>              if (work->nr_pages <= 0)
>>                  break;
>>          }
>> +
>> +        BUG_ON(wbc.pages_skipped > write_chunk - wbc.nr_to_write);
>> +        wrote -= wbc.pages_skipped;
>>      }
>>      return wrote;
>>  }
>>
>> .
>>