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

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Date:	Mon, 10 Dec 2007 15:38:20 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Nick Piggin <npiggin@...e.de>
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

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.

Christian
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