linux-kernel - Re: [PATCH] mm: Re-allow pinning of zero pfns

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Message-ID: <6492e852-8acf-95a9-a3a0-8ce8bbaf46d6@redhat.com>
Date:   Thu, 28 Jul 2022 11:23:33 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        akpm@...ux-foundation.org, minchan@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, paulmck@...nel.org,
        jhubbard@...dia.com, joaodias@...gle.com
Subject: Re: [PATCH] mm: Re-allow pinning of zero pfns

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