linux-kernel - Re: [PATCH 1/5] mm: Introduce mm_struct.has

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Date:   Tue, 22 Sep 2020 09:25:04 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Peter Xu <peterx@...hat.com>, Jann Horn <jannh@...gle.com>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

On Tue, Sep 22, 2020 at 8:56 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> I thought MAP_PRIVATE without PROT_WRITE was nonsensical,

MAP_PRIVATE without PROT_WRITE is very common.

It's what happens for every executable mapping, for example.

And no, it's not the same as MAP_SHARED, for a couple of simple
reasons. It does end up being similar for all the *normal* cases, but
there are various cases where it isn't.

 - mprotect() and friends. MAP_PRIVATE is fixed, but it might have
been writable in the past, and it might become writable in the future.

 - breakpoints and ptrace. This will punch through even a non-writable
mapping and force a COW (since that's the whole point: executables are
not writable, but to do a SW breakpoint you have to write to the page)

So no, MAP_PRIVATE is not nonsensical without PROT_WRITE, and it's not
even remotely unusual.

              Linus