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Date: Fri, 9 Feb 2007 11:14:55 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Nick Piggin <npiggin@suse.de>
Cc: Linux Filesystems <linux-fsdevel@vger.kernel.org>,
       Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [patch 3/3] ext2: use perform_write aop
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On Thu,  8 Feb 2007 14:07:46 +0100 (CET) Nick Piggin <npiggin@suse.de> wrote:

> +void page_zero_new_buffers(struct page *page, unsigned from, unsigned to)
> +{
> +	unsigned int block_start, block_end;
> +	struct buffer_head *head, *bh;
> +
> +	BUG_ON(!PageLocked(page));
> +	if (!page_has_buffers(page))
> +		return;
> +
> +	bh = head = page_buffers(page);
> +	block_start = 0;
> +	do {
> +		block_end = block_start + bh->b_size;
> +
> +		if (buffer_new(bh)) {
> +			if (block_end > from && block_start < to) {
> +				if (!PageUptodate(page)) {
> +					unsigned start, end;
> +					void *kaddr;
> +
> +					start = max(from, block_start);
> +					end = min(to, block_end);
> +
> +					kaddr = kmap_atomic(page, KM_USER0);
> +					memset(kaddr+start, 0, block_end-end);
> +					flush_dcache_page(page);
> +					kunmap_atomic(kaddr, KM_USER0);
> +					set_buffer_uptodate(bh);
> +				}

I don't see how this differs from the previous attempts to solve the
deadlock via atomic copt_from_user().  Here we temporarily zero out the
pagecache page then block_perform_write() unlocks the page.  So another
thread can come in, read the page and see the temporary zeroes?  

If so, that might be preventable by leaving the buffer nonuptodate.