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Date: Tue, 13 Feb 2007 16:16:07 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Eric Van Hensbergen <ericvh@gmail.com>
Cc: linux-kernel@vger.kernel.org, v9fs-developer@lists.sourceforge.net,
       ericvh@gmail.com
Subject: Re: [RESEND][PATCH] 9p: add write-cache support to loose cache mode
Message-Id: <20070213161607.0ca21ea4.akpm@linux-foundation.org>
In-Reply-To: <11714109311262-git-send-email-ericvh@gmail.com>
References: <11714109311262-git-send-email-ericvh@gmail.com>
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> On Tue, 13 Feb 2007 17:55:31 -0600 Eric Van Hensbergen <ericvh@gmail.com> wrote:
> +int v9fs_prepare_write(struct file *file, struct page *page,
> +		       unsigned from, unsigned to)
> +{
> +	if (!PageUptodate(page)) {
> +		if (to - from != PAGE_CACHE_SIZE) {
> +			void *kaddr = kmap_atomic(page, KM_USER0);
> +			memset(kaddr, 0, from);
> +			memset(kaddr + to, 0, PAGE_CACHE_SIZE - to);
> +			flush_dcache_page(page);
> +			kunmap_atomic(kaddr, KM_USER0);
> +		}
> +		SetPageUptodate(page);
> +	}

This will mark the page uptodate while the piece between `to' and `from' is
uninitialised.  A concurrent pagefault can come in and permit a read of
that uninitialised data.  Because filemap_nopage() doesn't lock the page if
it is uptodate.