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Date:   Mon, 24 Jan 2022 13:18:19 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Christoph Hellwig <hch@lst.de>, Jan Kara <jack@suse.cz>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-block@vger.kernel.org
Subject: Re: RFA (Request for Advice): block/bio: get_user_pages() -->
 pin_user_pages()
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References: <e83cd4fe-8606-f4de-41ad-33a40f251648@nvidia.com>
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On Sun, Jan 23, 2022 at 11:52:07PM -0800, John Hubbard wrote:
> To ground this in reality, one of the partial call stacks is:
> 
> do_direct_IO()
>     dio_zero_block()
>         page = ZERO_PAGE(0); <-- This is a problem
> 
> I'm not sure what to use, instead of that zero page! The zero page
> doesn't need to be allocated nor tracked, and so any replacement
> approaches would need either other storage, or some horrid scheme that I
> won't go so far as to write on the screen. :)

I'm not really sure what the problem is.

include/linux/mm.h:             is_zero_pfn(page_to_pfn(page));

and release_pages() already contains:
                if (is_huge_zero_page(page))
                        continue;

Why can't the BIO release function contain an is_zero_pfn() check?