Re: [rfc][patch] mm: direct io less aggressive syncs and invalidates

[Jeff Moyer <jmoyer@redhat.com>]

Nick Piggin <npiggin@suse.de> writes:

> Direct IO can invalidate and sync a lot of pagecache pages in the mapping. A
> 4K direct IO will actually try to sync and/or invalidate the pagecache of the
> entire file, for example (which might be many GB or TB large).
>
> Improve this by doing range syncs. Also, memory no longer has to be unmapped
> to catch the dirty bits for syncing, as dirty bits would remain coherent due to
> dirty mmap accounting.
>
> This should fix the immediate DM deadlocks when doing direct IO reads to
> block device with a mounted filesystem, if only by papering over the problem
> somewhat rather than addressing the fsync starvation cases. Not that the
> patch itself is a hack, but for this particular problem it is not really
> the correct solution IMO. But anyway, this might be more appropriate to go
> into stable kernels if this DM deadlock is biting users.
>
> Yes, I still need to put more time into finishing my pagecache tag based
> sync solution. Sorry :(
>
>
> ---
> Index: linux-2.6/mm/filemap.c
> ===================================================================
> --- linux-2.6.orig/mm/filemap.c	2008-10-03 11:21:31.000000000 +1000
> +++ linux-2.6/mm/filemap.c	2008-10-03 12:00:17.000000000 +1000
> @@ -1304,11 +1304,8 @@ generic_file_aio_read(struct kiocb *iocb
>  			goto out; /* skip atime */
>  		size = i_size_read(inode);
>  		if (pos < size) {
> -			retval = filemap_write_and_wait(mapping);
> -			if (!retval) {
> -				retval = mapping->a_ops->direct_IO(READ, iocb,
> +			retval = mapping->a_ops->direct_IO(READ, iocb,
>  							iov, pos, nr_segs);
> -			}

So why is it safe to get rid of this?  Can't this result in reading
stale data from disk?

The rest looks good to me.  I ran the aio-dio-regress tests against this
kernel on a UP machine, and they all passed.  The kernel didn't boot on
my SMP box, though.  Nick, any chance you could grab that test suite and
run it on an smp system?
  http://git.kernel.org/?p=linux/kernel/git/zab/aio-dio-regress.git;a=summary

Thanks,

Jeff