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From:   David Howells <dhowells@redhat.com>
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To:     Christoph Hellwig <hch@infradead.org>
Cc:     dhowells@redhat.com, David Hildenbrand <david@redhat.com>,
        Jens Axboe <axboe@kernel.dk>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Hillf Danton <hdanton@sina.com>,
        Christian Brauner <brauner@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH v2 3/3] block: Use iov_iter_extract_pages() and page pinning in direct-io.c
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Date:   Fri, 26 May 2023 09:33:47 +0100
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Christoph Hellwig <hch@infradead.org> wrote:

> I'm not seeing where we skip the unpin of the zero page, as commented
> in patch 1 (but maybe I'm not reviewing carefully enough as I'm at a
> conference right now).

It's done by unpin_user_page{,s}(), hidden away in gup.c.  See the commit
message for patch 1:

	Make pin_user_pages*() leave a ZERO_PAGE unpinned if it extracts a
	pointer to it from the page tables and make unpin_user_page*()
	correspondingly ignore a ZERO_PAGE when unpinning.

David