Re: [PATCH 2/6] writeback: wb_start_writeback() should use WB_SYNC_ALL for WB_REASON_SYNC

[Jens Axboe <axboe@fb.com>]

On 03/22/2016 04:04 PM, Dave Chinner wrote:
> On Tue, Mar 22, 2016 at 03:40:28PM -0600, Jens Axboe wrote:
>> On 03/22/2016 03:34 PM, Dave Chinner wrote:
>>> On Tue, Mar 22, 2016 at 11:55:16AM -0600, Jens Axboe wrote:
>>>> If you call sync, the initial call to wakeup_flusher_threads() ends up
>>>> calling wb_start_writeback() with reason=WB_REASON_SYNC, but
>>>> wb_start_writeback() always uses WB_SYNC_NONE as the writeback mode.
>>>> Ensure that we use WB_SYNC_ALL for a sync operation.
>>>
>>> This seems wrong to me. We want background write to happen as
>>> quickly as possible and /not block/ when we first kick sync.
>>
>> It's not going to block. wakeup_flusher_threads() async queues
>> writeback work through wb_start_writeback().
>
> The flusher threads block, not the initial wakeup. e.g. they will
> now block waiting for data writeback to complete before writing the
> inode. i.e. this code in __writeback_single_inode() is now triggered
> by the background flusher:
>
>          /*
>           * Make sure to wait on the data before writing out the metadata.
>           * This is important for filesystems that modify metadata on data
>           * I/O completion. We don't do it for sync(2) writeback because it has a
>           * separate, external IO completion path and ->sync_fs for guaranteeing
>           * inode metadata is written back correctly.
>           */
>          if (wbc->sync_mode == WB_SYNC_ALL && !wbc->for_sync) {
>                  int err = filemap_fdatawait(mapping);
>                  if (ret == 0)
>                          ret = err;
>          }

Yeah, that's not ideal, for this case we'd really like something that 
WB_SYNC_ALL_NOWAIT...

> It also changes the writeback chunk size in write_cache_pages(), so
> instead of doing a bit of writeback from all dirty inodes, it tries
> to write everything from each inode in turn (see
> writeback_chunk_size()) which will further exacerbate the wait
> above.

But that part is fine, if it wasn't for the waiting.

>>> The latter blocking passes of sync use WB_SYNC_ALL to ensure that we
>>> block waiting for all remaining IO to be issued and waited on, but
>>> the background writeback doesn't need to do this.
>>
>> That's fine, they can get to wait on the previously issued IO, which
>> was properly submitted with WB_SYNC_ALL.
>>
>> Maybe I'm missing your point?
>
> Making the background flusher block and wait for data makes it
> completely ineffective in speeding up sync() processing...

Agree, we should not wait on the pages individually, we want them 
submitted and then waited on. And I suppose it's no differently than 
handling the normal buffered write from an application, which then gets 
waited on with fsync() or similar. So I can drop this patch, it'll work 
fine without it.

-- 
Jens Axboe