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

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Message-Id: <20160824134212.e9b50aa36523fbfcbcfe2f55@linux-foundation.org>
Date:   Wed, 24 Aug 2016 13:42:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jan Kara <jack@...e.com>, linux-mm@...ck.org,
        linux-nvdimm@...1.01.org, Jeff Moyer <jmoyer@...hat.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] mm: silently skip readahead for DAX inodes

On Wed, 24 Aug 2016 14:37:12 -0600 Ross Zwisler <ross.zwisler@...ux.intel.com> 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! */