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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 14 Sep 2020 15:59:31 -0700
Message-ID: <CAHk-=wgdn5sJ0UEVZRQvj6r5kqOkU24jA_V6cPkqb9tqoAKBJg@mail.gmail.com>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Peter Xu <peterx@redhat.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>
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On Mon, Sep 14, 2020 at 3:55 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> Just as an aside, the RDMA stuff is also supposed to set MADV_DONTFORK
> on these regions, so I'm a bit puzzled what is happening here.

Did the fork perhaps happen _before_ , so the pages are shared when
you do the pin?

MADV_DONTFORK doesn't mean COW doesn't happen. It just means that the
next fork() won't be copying that memory area.

That said, it's possible that the test cases do something invalid - or
maybe we've broken MADV_DONTFORK - and it all just happened to work
before.

So it's possible the breakage is exposing some other bug..

             Linus