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Date: Tue, 28 Jan 2014 13:09:10 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Mark Rutland <mark.rutland@arm.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: readahead: fix do_readahead for no readpage(s)
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On Tue, 28 Jan 2014 23:01:26 +0200 "Kirill A. Shutemov" <kirill@shutemov.name> wrote:

> On Tue, Jan 28, 2014 at 12:03:01PM -0800, Andrew Morton wrote:
> > On Tue, 28 Jan 2014 11:14:19 +0000 Mark Rutland <mark.rutland@arm.com> wrote:
> > 
> > > Commit 63d0f0a3c7e1 (mm/readahead.c:do_readhead(): don't check for
> > > ->readpage) unintentionally made do_readahead return 0 for all valid
> > > files regardless of whether readahead was supported, rather than the
> > > expected -EINVAL. This gets forwarded on to userspace, and results in
> > > sys_readahead appearing to succeed in cases that don't make sense (e.g.
> > > when called on pipes or sockets). This issue is detected by the LTP
> > > readahead01 testcase.
> > 
> > How can this be?
> > 
> > : static ssize_t
> > : do_readahead(struct address_space *mapping, struct file *filp,
> > : 	     pgoff_t index, unsigned long nr)
> > : {
> > : 	if (!mapping || !mapping->a_ops)
> > : 		return -EINVAL;
> > : 
> > : 	return force_page_cache_readahead(mapping, filp, index, nr);
> 
> It's not what we have in Linus' tree. force_page_cache_readahead() return
> code is unused:
> 
> 	force_page_cache_readahead(mapping, filp, index, nr);
> 	return 0;
> 

ah, oops, I was looking at the code with Mark's patch applied.

Yes, the force_page_cache_readahead() return value should be propagated.

And I guess the lobotomising of the force_page_cache_readahead() return
value is OK.  force_page_cache_readahead() presently returns an errno
even if it has performed some reads.  A read()-style function shouldn't
do that - it should return a short read in that case.