linux-kernel - Re: [RFC PATCH v2 3/3] block: Use iov_iter_extract

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Message-ID: <520477.1685090027@warthog.procyon.org.uk>
Date:   Fri, 26 May 2023 09:33:47 +0100
From:   David Howells <dhowells@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     dhowells@...hat.com, David Hildenbrand <david@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH v2 3/3] block: Use iov_iter_extract_pages() and page pinning in direct-io.c

Christoph Hellwig <hch@...radead.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