Re: [PATCH] mm: silently skip readahead for DAX inodes

[Andrew Morton]

On Wed, 24 Aug 2016 14:37:12 -0600 Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> wrote:

> For DAX inodes we need to be careful to never have page cache pages in the
> mapping->page_tree.  This radix tree should be composed only of DAX
> exceptional entries and zero pages.
> 
> ltp's readahead02 test was triggering a warning because we were trying to
> insert a DAX exceptional entry but found that a page cache page had already
> been inserted into the tree.  This page was being inserted into the radix
> tree in response to a readahead(2) call.
> 
> Readahead doesn't make sense for DAX inodes, but we don't want it to report
> a failure either.  Instead, we just return success and don't do any work.
> 
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -8,6 +8,7 @@
>   */
>  
>  #include <linux/kernel.h>
> +#include <linux/dax.h>
>  #include <linux/gfp.h>
>  #include <linux/export.h>
>  #include <linux/blkdev.h>
> @@ -544,6 +545,9 @@ do_readahead(struct address_space *mapping, struct file *filp,
>  	if (!mapping || !mapping->a_ops)
>  		return -EINVAL;
>  
> +	if (dax_mapping(mapping))
> +		return 0;
> +

Please don't force readers to go spend minutes putzing around in the
git tree trying to understand your code.
/* these things considered useful! */