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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 25 May 2023 10:04:53 -0700
Message-ID: <CAHk-=wgPWUCyhiM+=S3nmh4JK8qtBQteYvtiXpoYpDjfKHnEhQ@mail.gmail.com>
Subject: Re: Extending page pinning into fs/direct-io.c
To:     David Hildenbrand <david@redhat.com>
Cc:     David Howells <dhowells@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        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>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Thu, May 25, 2023 at 9:45=E2=80=AFAM David Hildenbrand <david@redhat.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