Re: [PATCH 1/3] writeback: introduce wbc.for_sync to cover the two sync stages

[Dave Chinner <david@fromorbit.com>]

On Sun, May 01, 2011 at 06:36:06AM +0800, Wu Fengguang wrote:
> sync(2) is performed in two stages: the WB_SYNC_NONE sync and the
> WB_SYNC_ALL sync. Tag both stages with wbc.for_sync for livelock
> prevention.
> 
> Note that writeback_inodes_sb() is called by not only sync(), they
> are treated the same because the other callers need also need livelock
> prevention.
> 
> Impacts:
> 
> - it changes the order in which pages/inodes are synced to disk. Now in
>   the WB_SYNC_NONE stage, it won't proceed to write the next inode until
>   finished with the current inode.
> 
> - this adds a new field to the writeback trace events and may possibly
>   break some scripts.
.....
> --- linux-next.orig/mm/page-writeback.c	2011-05-01 06:35:16.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2011-05-01 06:35:17.000000000 +0800
> @@ -892,12 +892,12 @@ int write_cache_pages(struct address_spa
>  			range_whole = 1;
>  		cycled = 1; /* ignore range_cyclic tests */
>  	}
> -	if (wbc->sync_mode == WB_SYNC_ALL)
> +	if (wbc->for_sync)
>  		tag = PAGECACHE_TAG_TOWRITE;
>  	else
>  		tag = PAGECACHE_TAG_DIRTY;
>  retry:
> -	if (wbc->sync_mode == WB_SYNC_ALL)
> +	if (wbc->for_sync)
>  		tag_pages_for_writeback(mapping, index, end);
>  	done_index = index;
>  	while (!done && (index <= end)) {

Doesn't that break anything that uses
filemap_write_and_wait{_range}() or filemap_fdatawrite{_range}()?
e.g. fsync, sync buffered writes, etc? i.e. everything that
currently relies on WB_SYNC_ALL for data integrity writeback is now
b0rken except for sync(1)?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com