linux-kernel - Re: [PATCH] mm, fs: avoid page allocation beyond i

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Message-ID: <1377100012.2738.28.camel@menhir>
Date:	Wed, 21 Aug 2013 16:46:52 +0100
From:	Steven Whitehouse <swhiteho@...hat.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
	NeilBrown <neilb@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, fs: avoid page allocation beyond i_size on read

Hi,

On Wed, 2013-08-21 at 18:37 +0300, Kirill A. Shutemov wrote:
> I've noticed that we allocated unneeded page for cache on read beyond
> i_size. Simple test case (I checked it on ramfs):
> 
> $ touch testfile
> $ cat testfile
> 
> It triggers 'no_cached_page' code path in do_generic_file_read().
> 
> Looks like it's regression since commit a32ea1e. Let's fix it.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Acked-by: NeilBrown <neilb@...e.de>
> ---
>  mm/filemap.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 1905f0e..b1a4d35 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1163,6 +1163,10 @@ static void do_generic_file_read(struct file *filp, loff_t *ppos,
>  		loff_t isize;
>  		unsigned long nr, ret;
>  
> +		isize = i_size_read(inode);
> +		if (!isize || index > (isize - 1) >> PAGE_CACHE_SHIFT)
> +			goto out;
> +
>  		cond_resched();
>  find_page:
>  		page = find_get_page(mapping, index);

Please don't do that... there is no reason to think that i_size will be
correct at that moment. Why not just get readpage(s) to return the
correct return code in that case?

Steve.


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