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Date:   Thu, 25 May 2023 18:45:07 +0200
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Subject: Re: Extending page pinning into fs/direct-io.c
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        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>,
        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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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 25.05.23 18:31, Linus Torvalds wrote:
> On Wed, May 24, 2023 at 1:47 AM David Howells <dhowells@redhat.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.

I think the correct way to test for a zero page is 
is_zero_pfn(page_to_pfn(page).

Using my_zero_pfn(vmf->address) in do_anonymous_page() these can easily 
end up in any process.

-- 
Thanks,

David / dhildenb