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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 28 Sep 2020 12:50:03 -0700
Message-ID: <CAHk-=wiWr+gO0Ro4LvnJBMs90OiePNyrE3E+pJvc9PzdBShdmw@mail.gmail.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Peter Xu <peterx@redhat.com>, Leon Romanovsky <leonro@nvidia.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 12:36 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So I'll do the pte wrprotect/restore removal. Anybody willing to do
> and test the sequence count approach?

So the wrprotect removal is trivial, with most of it being about the comments.

However, when I look at this, I am - once again - tempted to just add a

        if (__page_mapcount(page) > 1)
                return 1;

there too. Because we know it's a private mapping (shared mappings we
checked for with the "is_cow_mapping()" earlier), and the only case we
really care about is the one where the page is only mapped in the
current mm (because that's what a write pinning will have done, and as
mentioned, a read pinning doesn't do anything wrt fork() right now
anyway).

So if it's mapped in another mm, the COW clearly hasn't been broken by
a pin, and a read pinned page had already gone through a fork.

But the more I look at this code, the more I go "ok, I want somebody
to actually test this with the rdma case".

So I'll attach my suggested patch, but I won't actually commit it. I'd
really like to have this tested, possibly _together_ with the sequence
count addition..

               Linus

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