linux-kernel - Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush

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Message-ID: <394e17bc-8bed-4d17-5dba-9ab8052c8bea@nvidia.com>
Date:   Thu, 7 Jan 2021 14:14:46 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yu Zhao <yuzhao@...gle.com>, Andy Lutomirski <luto@...nel.org>,
        Peter Xu <peterx@...hat.com>,
        Pavel Emelyanov <xemul@...nvz.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Minchan Kim" <minchan@...nel.org>, Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Leon Romanovsky <leonro@...dia.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce
 wrprotect_tlb_flush_pending

On 1/7/21 2:00 PM, Linus Torvalds wrote:
> On Thu, Jan 7, 2021 at 1:53 PM John Hubbard <jhubbard@...dia.com> wrote:
>>
>>>
>>> Now, I do agree that from a QoI standpoint, it would be really lovely
>>> if we actually enforced it. I'm not entirely sure we can, but maybe it
>>> would be reasonable to use that
>>>
>>>     mm->has_pinned && page_maybe_dma_pinned(page)
>>>
>>> at least as the beginning of a heuristic.
>>>
>>> In fact, I do think that "page_maybe_dma_pinned()" could possibly be
>>> made stronger than it is. Because at *THAT* point, we might say "we
>>
>> What exactly did you have in mind, to make it stronger? I think the
>> answer is in this email but I don't quite see it yet...
> 
> Literally just adding a " && page_mapcount(page) == 1" in there
> (probably best done inside page_maybe_dma_pinned() itself)

Well, that means that pages that are used for pinned DMA like this, can
not be shared with other processes. Is that an acceptable limitation
for the RDMA users? It seems a bit constraining, at first glance anyway.

> 
>> Direct IO pins, on the other hand, are more transient. We can probably live
>> without tagging Direct IO pages as FOLL_PIN. I think.
> 
> Yes. I think direct-IO writes should be able to just do a transient
> GUP, and if it causes a COW fault that isn't coherent, that's the
> correct semantics, I think (ie the direct-IO will see the original
> data, the COW faulter will get it's own private copy to make changes
> to).
> 
> I think pinning should be primarily limited to things that _require_
> coherency (ie you pin because you're going to do some active two-way
> communication using that page)
> 
> Does that match your thinking?
> 

Yes, perfectly. I'm going to update Documentation/core-api/pin_user_pages.rst
accordingly, once the dust settles on these discussions.

thanks,
-- 
John Hubbard
NVIDIA