Re: [PATCH 3/6] dax: Avoid page invalidation races and unnecessary radix tree traversals

[Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Thu, Nov 24, 2016 at 10:46:33AM +0100, Jan Kara wrote:
> Currently each filesystem (possibly through generic_file_direct_write()
> or iomap_dax_rw()) takes care of invalidating page tables and evicting

Just some nits about the commit message: the DAX I/O path function is now
called dax_iomap_rw(), and no filesystems still use
generic_file_direct_write() for DAX so you can probably remove it from the
changelog - up to you.

Aside from that:
Reviewed-by: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>

> hole pages from the radix tree when write(2) to the file happens. This
> invalidation is only necessary when there is some block allocation
> resulting from write(2). Furthermore in current place the invalidation
> is racy wrt page fault instantiating a hole page just after we have
> invalidated it.
> 
> So perform the page invalidation inside dax_do_io() where we can do it
> only when really necessary and after blocks have been allocated so
> nobody will be instantiating new hole pages anymore.
> 
> Reviewed-by: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
> Signed-off-by: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
> ---
>  fs/dax.c | 28 +++++++++++-----------------
>  1 file changed, 11 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 4534f0e232e9..ddf77ef2ca18 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -984,6 +984,17 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>  	if (WARN_ON_ONCE(iomap->type != IOMAP_MAPPED))
>  		return -EIO;
>  
> +	/*
> +	 * Write can allocate block for an area which has a hole page mapped
> +	 * into page tables. We have to tear down these mappings so that data
> +	 * written by write(2) is visible in mmap.
> +	 */
> +	if ((iomap->flags & IOMAP_F_NEW) && inode->i_mapping->nrpages) {
> +		invalidate_inode_pages2_range(inode->i_mapping,
> +					      pos >> PAGE_SHIFT,
> +					      (end - 1) >> PAGE_SHIFT);
> +	}
> +
>  	while (pos < end) {
>  		unsigned offset = pos & (PAGE_SIZE - 1);
>  		struct blk_dax_ctl dax = { 0 };
> @@ -1042,23 +1053,6 @@ dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
>  	if (iov_iter_rw(iter) == WRITE)
>  		flags |= IOMAP_WRITE;
>  
> -	/*
> -	 * Yes, even DAX files can have page cache attached to them:  A zeroed
> -	 * page is inserted into the pagecache when we have to serve a write
> -	 * fault on a hole.  It should never be dirtied and can simply be
> -	 * dropped from the pagecache once we get real data for the page.
> -	 *
> -	 * XXX: This is racy against mmap, and there's nothing we can do about
> -	 * it. We'll eventually need to shift this down even further so that
> -	 * we can check if we allocated blocks over a hole first.
> -	 */
> -	if (mapping->nrpages) {
> -		ret = invalidate_inode_pages2_range(mapping,
> -				pos >> PAGE_SHIFT,
> -				(pos + iov_iter_count(iter) - 1) >> PAGE_SHIFT);
> -		WARN_ON_ONCE(ret);
> -	}
> -
>  	while (iov_iter_count(iter)) {
>  		ret = iomap_apply(inode, pos, iov_iter_count(iter), flags, ops,
>  				iter, dax_iomap_actor);
> -- 
> 2.6.6
>