Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20200915160553.GJ1221970@ziepe.ca> <20200915182933.GM1221970@ziepe.ca>
 <20200915191346.GD2949@xz-x1> <20200915193838.GN1221970@ziepe.ca>
 <20200915213330.GE2949@xz-x1> <20200915232238.GO1221970@ziepe.ca>
 <e6c352f8-7ee9-0702-10a4-122d2c4422fc@nvidia.com> <20200916174804.GC8409@ziepe.ca>
 <20200916184619.GB40154@xz-x1> <20200917112538.GD8409@ziepe.ca> <20200917181411.GA133226@xz-x1>
In-Reply-To: <20200917181411.GA133226@xz-x1>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 17 Sep 2020 11:26:01 -0700
Message-ID: <CAHk-=wgMVPAhD7C24ipe03+MScgp6F=zMS-roOznvxJ+hOGfSA@mail.gmail.com>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
To:     Peter Xu <peterx@redhat.com>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>, John Hubbard <jhubbard@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Maya B . Gokhale" <gokhale2@llnl.gov>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Marty Mcfadden <mcfadden8@llnl.gov>,
        Kirill Shutemov <kirill@shutemov.name>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Jan Kara <jack@suse.cz>, Kirill Tkhai <ktkhai@virtuozzo.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, Sep 17, 2020 at 11:14 AM Peter Xu <peterx@redhat.com> wrote:
>
> In my humble opinion, the real solution is still to use MADV_DONTFORK properly
> so we should never share the DMA pages with others when we know the fact.

Is this all just because somebody does a fork() after doing page pinning?

If so, I feel this should be trivially fixed in  copy_one_pte().
That's where we currently do

        /*
         * If it's a COW mapping, write protect it both
         * in the parent and the child
         */
        if (is_cow_mapping(vm_flags) && pte_write(pte)) {
                ptep_set_wrprotect(src_mm, addr, src_pte);
                pte = pte_wrprotect(pte);
        }

and I feel that that is where we could just change the code to do a
COW event for pinned pages (and *not* mark the parent write protected,
since the parent page now isn't a COW page).

Because if that's the case that Jason is hitting, then I feel that
really is the correct fix: make sure that the pinning action is
meaningful.

As mentioned, I really think the whole (and only) point of page
pinning is that it should keep the page locked in the page tables. And
by "locked" I mean exactly that: not just present, but writable.

And then the "we never COW a pinned page" comes not from the COW code
doing magic, but by it simply never becoming non-writable - because
the page table entry is locked!

               Linus