linux-kernel - Re: [PATCH 0/2] mm: introduce MAP_FIXED

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Message-ID: <289f0cc8-9e24-fc38-cb83-4223f9923940@redhat.com>
Date:   Fri, 8 Dec 2017 21:47:21 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Pavel Machek <pavel@....cz>, Michael Ellerman <mpe@...erman.id.au>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Cyril Hrubis <chrubis@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Joel Stanley <joel@....id.au>
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

On 12/08/2017 03:27 PM, Pavel Machek wrote:
> On Fri 2017-12-08 22:08:07, Michael Ellerman wrote:
>> If we had a time machine, the right set of flags would be:
>>
>>    - MAP_FIXED:   don't treat addr as a hint, fail if addr is not free
>>    - MAP_REPLACE: replace an existing mapping (or force or clobber)

> Actually, if we had a time machine... would we even provide
> MAP_REPLACE functionality?

Probably yes.  ELF loading needs to construct a complex set of mappings 
from a single file.  munmap (to create a hole) followed by mmap would be 
racy because another thread could have reused the gap in the meantime. 
The only alternative to overriding existing mappings would be mremap 
with MREMAP_FIXED, and that doesn't look like an improvement API-wise.

(The glibc dynamic linker uses an mmap call with an increased length to 
reserve address space and then loads additional segments with MAP_FIXED 
at the offsets specified in the program header.)

Thanks,
Florian