linux-kernel - Re: [patch] mremap.2: Add description of old

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Date:   Mon, 25 Sep 2017 14:52:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, mtk.manpages@...il.com,
        linux-man@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-mm@...ck.org
Subject: Re: [patch] mremap.2: Add description of old_size == 0 functionality

On Mon 25-09-17 14:40:42, Florian Weimer wrote:
> On 09/25/2017 02:35 PM, Michal Hocko wrote:
> > What would be the usecase. I mean why don't you simply create a new
> > mapping by a plain mmap when you have no guarantee about the same
> > content?
> 
> I plan to use it for creating an unbounded number of callback thunks at run
> time, from a single set of pages in libc.so, in case we need this
> functionality.
> 
> The idea is to duplicate existing position-independent machine code in
> libc.so, prefixed by a data mapping which controls its behavior.  Each
> data/code combination would only give us a fixed number of thunks, so we'd
> need to create a new mapping to increase the total number.
> 
> Instead, we could re-map the code from the executable in disk, but not if
> chroot has been called or glibc has been updated on disk.  Creating an alias
> mapping does not have these problems.
> 
> Another application (but that's for anonymous memory) would be to duplicate
> class metadata in a Java-style VM, so that you can use bits in the class
> pointer in each Java object (which is similar to the vtable pointer in C++)
> for the garbage collector, without having to mask it when accessing the
> class metadata in regular (mutator) code.

So, how are you going to deal with the CoW and the implementation which
basically means that the newm mmap content is not the same as the
original one?
-- 
Michal Hocko
SUSE Labs