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

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Message-ID: <CAHk-=wjaqHgd4u63XdZoTPs1YCJnDZ7-GQHKKdFrT32y2-__tw@mail.gmail.com>
Date:   Thu, 25 May 2023 09:31:44 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Howells <dhowells@...hat.com>
Cc:     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>,
        David Hildenbrand <david@...hat.com>,
        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 Wed, May 24, 2023 at 1:47 AM David Howells <dhowells@...hat.com> wrote:
>
> True - but I was thinking of just treating the zero_page specially and never
> 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 *page)
>     {
>             if (page == 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 = 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