Re: [PATCH] writeback: Don't wait for completion in writeback_inodes_sb_nr

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jul 21, 2011 at 12:00:12AM +0200, Jan Kara wrote:
> > Removing this call breaks sys_syncfs and similar semantics on filesystem
> > just pushing metadata into buffers in ->write_inode or ->sync_fs and
> > then expecting the caller to write them out.  This list of filesystem
> > includes ext2 and in general most filesystems without journaling or
> > similar technics.
>   Note that for sys_syncfs() the __sync_blockdev() has just moved from
> __sync_filesystem() to sync_filesystem(). So nothing should change.

Sorry, I missed that.

> > I'm perfectly fine with pushing the sync_blockdev call into the
> > filesystem for these, but we'll need a way to handle them.
>   Yes, calling sync_blockdev() from sync_fs() might actually make sence
> (e.g. ext3 and ext4 don't need it) but I didn't want to go that far in
> this patch.

The more fine-grained we can split the patches, the better.  It'll help
to explain what we're doing to thise trying to figure out a few years
down the road.

> > At which point we could fold this code into a blkdev_sync_fs method for
> > now.  Long term we'll need to support multiple BDIs per SB anyway, at
> > which point the code can go away again.
>   Ah, I had to think a bit before I understood what you mean :). It's kind
> of elegant but also slightly subtle (it's not immediately obvious how
> blockdevs are synced during sync when you look at the code). Umm, and you
> don't have any guarantee in which order superblocks are on the sb list so
> you could sync block devices before some filesystems are finished. So I
> don't think using blkdev_sync_fs() is a good idea after all.

This required calling sync_blkdev from ->sync_fs of those filesystems
that actually need it as a pre-requisite of course.