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

From: Linus Torvalds <torvalds@linux-foundation.org>
To: David Howells <dhowells@redhat.com>
Cc: David Hildenbrand <david@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
Subject: Re: Extending page pinning into fs/direct-io.c
Date: Thu, 25 May 2023 10:25:42 -0700	[thread overview]
Message-ID: <CAHk-=wirrJNoVSOSVK2ae9pa7Q5kJKFYMk2ad-_6YrS1cEScqA@mail.gmail.com> (raw)
In-Reply-To: <144598.1685034915@warthog.procyon.org.uk>

On Thu, May 25, 2023 at 10:15 AM David Howells <dhowells@redhat.com> wrote:
>
>  It doesn't seem I can add it to mm.h as an inline function.

What? We already have that pattern inside is_longterm_pinnable_page(),
so that's really strange.

But regardless, please don't duplicate that odd conditional for no
reason, and don't scream.

So regardless of where it is, make that "is_zero_folio()" just do
"is_zero_page(&folio->page)" rather than repeat the question.

I also wonder whether we shouldn't just use the "transparent union"
argument thing more aggressively. Something like

   typedef union {
        struct page *page;
        struct folio *folio;
   } page_or_folio_t __attribute__ ((__transparent_union__));

and then you should be able to do something like this:

    static inline bool is_zero_page(const page_or_folio_t arg)
    {
        return is_zero_pfn(page_to_pfn(arg.page));
    }

and we don't have to keep generating the two versions over and over
and over again.

                Linus