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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 28 Sep 2020 09:17:09 -0700
Message-ID: <CAHk-=wj6aTsqq6BAUci-NYJ3b-EkDwVgz_NvW_kW8KBqGocouQ@mail.gmail.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Leon Romanovsky <leonro@nvidia.com>, Peter Xu <peterx@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, Michal Hocko <mhocko@suse.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Kirill Shutemov <kirill@shutemov.name>,
        Hugh Dickins <hughd@google.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>
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On Mon, Sep 28, 2020 at 5:49 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> Not seeing an obvious option besides adding a smp_mb() before
> page_maybe_dma_pinned() as Peter once suggested.

That is going to be prohibitively expensive - needing it for each pte
whether it's pinned or not.

I really think the better option is a "don't do that then". This has
_never_ worked before either except by pure luck.

I also doubt anybody does it. forking with threads is a bad idea to
begin with. Doing so while pinning pages even more so.

               Linus