Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <ZGxfrOLZ4aN9/MvE@infradead.org> <20230522205744.2825689-1-dhowells@redhat.com>
 <3068545.1684872971@warthog.procyon.org.uk> <ZG2m0PGztI2BZEn9@infradead.org> <3215177.1684918030@warthog.procyon.org.uk>
In-Reply-To: <3215177.1684918030@warthog.procyon.org.uk>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 25 May 2023 09:31:44 -0700
Message-ID: <CAHk-=wjaqHgd4u63XdZoTPs1YCJnDZ7-GQHKKdFrT32y2-__tw@mail.gmail.com>
Subject: Re: Extending page pinning into fs/direct-io.c
To:     David Howells <dhowells@redhat.com>
Cc:     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>,
        David Hildenbrand <david@redhat.com>,
        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
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Wed, May 24, 2023 at 1:47=E2=80=AFAM David Howells <dhowells@redhat.com>=
 wrote:
>
> True - but I was thinking of just treating the zero_page specially and ne=
ver
> hold a pin or a ref on it.  It can be checked by address, e.g.:
>
>     static inline void bio_release_page(struct bio *bio, struct page *pag=
e)
>     {
>             if (page =3D=3D ZERO_PAGE(0))
>                     return;

That won't actually work.

We do have cases that try to use the page coloring that we support.

Admittedly it seems to be only rmda that does it directly with
something like this:

        vmf->page =3D ZERO_PAGE(vmf->address);

but you can get arbitrary zero pages by pinning or GUPing them from
user space mappings.

Now, the only architectures that *use* multiple zero pages are - I
think - MIPS (including Loongarch) and s390.

So it's rare, but it does happen.

             Linus