linux-kernel - Re: Extending page pinning into fs/direct-io.c

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Date:   Thu, 25 May 2023 10:04:53 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        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>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: Extending page pinning into fs/direct-io.c

On Thu, May 25, 2023 at 9:45 AM David Hildenbrand <david@...hat.com> wrote:
>
> I think the correct way to test for a zero page is
> is_zero_pfn(page_to_pfn(page).

Yeah. Except it's really ugly and strange, and we should probably add
a helper for that pattern.

The reason it has that odd "look at pfn" is just because I think the
first users were in the page table code, which had the pfn already,
and the test is basically based on the zero_page_mask thing that the
affected architectures have.

So I suspect we should add that

    is_zero_pfn(page_to_pfn(page))

as a helper inline function rather than write it out even more times
(that "is this 'struct page' a zero page" pattern already exists in
/proc and a few other places.

is_longterm_pinnable_page() already has it, so adding it as a helper
there in <linux/mm.h> is probably a good idea.

                Linus