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

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Date:   Fri, 29 Jul 2022 12:49:00 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        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


David Hildenbrand <david@...hat.com> writes:

> 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.

Ok. Well I've looked at this now so may as well stick

Reviewed-by: Alistair Popple <apopple@...dia.com>

on it. However I think it needs rebasing, should I send an updated
version?

>>
>> 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.