Re: [RESEND][PATCH] 9p: add write-cache support to loose cache mode

[Andrew Morton]

> On Tue, 13 Feb 2007 17:55:31 -0600 Eric Van Hensbergen <ericvh@xxxxxxxxx> 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.
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