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Date:   Wed, 22 Dec 2021 09:51:28 +0100
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To:     Jason Gunthorpe <jgg@nvidia.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
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On 21.12.21 20:07, Jason Gunthorpe wrote:
> On Tue, Dec 21, 2021 at 06:40:30PM +0100, David Hildenbrand wrote:
> 
>> 2) is certainly the cherry on top. But it just means that R/O pins don't
>> have to be the weird kid. And yes, achieving 2) would require
>> FAULT_FLAG_EXCLUSIVE / FAULT_FLAG_UNSHARED, but it would really 99% do
>> what existing COW logic does, just bypass the "map writable" and
>> "trigger write fault" semantics.
> 
> I still don't agree with this - when you come to patches can you have
> this work at the end and under a good cover letter? Maybe it will make
> more sense then.

Yes. But really, I think it's the logical consequence of what Linus said
[1]:

  "And then all GUP-fast would need to do is to refuse to look up a page
   that isn't exclusive to that VM. We already have the situation that
   GUP-fast can fail for non-writable pages etc, so it's just another
   test."

We must not FOLL_PIN a page that is not exclusive (not only on gup-fast,
but really, on any gup). If we special case R/O FOLL_PIN, we cannot
enable the sanity check on unpin as suggested by Linus [2]:

  "If we only set the exclusive VM bit on pages that get mapped into
   user space, and we guarantee that GUP only looks up such pages, then
   we can also add a debug test to the "unpin" case that the bit is
   still set."

There are really only two feasible options I see when we want to take a
R/O FOLL_PIN on a !PageAnonExclusive() anon page

(1) Fail the pinning completely. This implies that we'll have to fail
    O_DIRECT once converted to FOLL_PIN.
(2) Request to mark the page PageAnonExclusive() via a
    FAULT_FLAG_UNSHARE and let it succeed.


Anything else would require additional accounting that we already
discussed in the past is hard -- for example, to differentiate R/O from
R/W pins requiring two pin counters.

The only impact would be that FOLL_PIN after fork() has to go via a
FAULT_FLAG_UNSHARE once, to turn the page PageAnonExclusive. IMHO this
is the right thing to do for FOLL_LONGTERM. For !FOLL_LONGTERM it would
be nice to optimize this, to *not* do that, but again ... this would
require even more counters I think, for example, to differentiate
between "R/W short/long-term or R/O long-term pin" and "R/O short-term pin".

So unless we discover a way to do additional accounting for ordinary 4k
pages, I think we really can only do (1) or (2) to make sure we never
ever pin a !PageAnonExclusive() page.


[1]
https://lkml.kernel.org/r/CAHk-=wgQq3H6wfkW7+MmduVgBOqHeiXQN97yCMd+m1mM-1xCLQ@mail.gmail.com
[2]
https://lkml.kernel.org/r/CAHk-=wiyxQ==vnHFHW99S_OPwA=u1Qrfg2OGr_6zPcBAuhQY2w@mail.gmail.com


-- 
Thanks,

David / dhildenb