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Date:   Thu, 28 Jul 2022 11:23:33 +0200
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Subject: Re: [PATCH] mm: Re-allow pinning of zero pfns
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To:     Alistair Popple <apopple@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>
Cc:     Alex Williamson <alex.williamson@redhat.com>,
        akpm@linux-foundation.org, minchan@kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, paulmck@kernel.org,
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 28.07.22 10:45, Alistair Popple wrote:
> 
> Looks like the original patch might need rebasing. I am about to post a
> clean-up for the tortured logic in check_and_migrate_movable_pages() so
> can incorporate it there, but I'm wondering what the consensus was for
> pinning of zero pfn?

We have to keep it working right now, but in most cases (inside
MAP_PRIVATE regions) it's shaky and undesired.

> 
> Currently my clean-up will result in PUP returning an error for the zero
> pfn rather than looping indefinitely in the kernel. However it wasn't
> clear from this thread if returning an error is ok, or if R/O pinning
> of the zero pfn should succeed?

I'm working on proper COW breaking in MAP_PRIVATE mappings, which will,
for example, unshare the shared zeropage and properly replace it by
exclusive anon pages first in the FOLL_LONGTERM case.

-- 
Thanks,

David / dhildenb