linux-kernel - Re: [patch] mm: fix XIP file writes

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Date:	Wed, 12 Dec 2007 05:03:28 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>, rob@...dley.net,
	Jens Axboe <axboe@...nel.dk>, cotte@...ibm.com
Subject: Re: [patch] mm: fix XIP file writes

On Mon, Dec 10, 2007 at 03:38:20PM +0100, Christian Borntraeger wrote:
> Hi Nick,
> 
> > Here we go. See, brd already found a bug ;)
> > Can you apply the ext2 XIP patch too? And I'll resend the brd XIP patch.
> [...]
> > Writing to XIP files at a non-page-aligned offset results in data corruption
> > because the writes were always sent to the start of the page.
> [...]
> > @@ -314,7 +314,7 @@ __xip_file_write(struct file *filp, cons
> >  		fault_in_pages_readable(buf, bytes);
> >  		kaddr = kmap_atomic(page, KM_USER0);
> >  		copied = bytes -
> > -			__copy_from_user_inatomic_nocache(kaddr, buf, bytes);
> > +			__copy_from_user_inatomic_nocache(kaddr + offset, buf, bytes);
> >  		kunmap_atomic(kaddr, KM_USER0);
> >  		flush_dcache_page(page);
> 
> I asked myself why this problem never happened before. So I asked our testers
> to reproduce this problem on 2.6.23 and service levels. As the testcase did
> not trigger, I looked into the 2.6.23 code. This problem was introduced by
> commit 4a9e5ef1f4f15205e477817a5cefc34bd3f65f55 (mm: write iovec cleanup from
> Nick Piggin) during 2.6.24-rc:
> --------snip-------
> -		copied = filemap_copy_from_user(page, offset, buf, bytes);
> [...]
> +		copied = bytes -
> +			__copy_from_user_inatomic_nocache(kaddr, buf, bytes);
> -------------------
> 
> So yes, its good to have xip on brd. It even tests your changes ;-)
> Good news is, that we dont need anything for stable.

Heh ;) that explains a lot. Thanks!
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