From: Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
To: John Hubbard <jhubbard-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
Cc: Mike Rapoport
<rppt-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>,
Michael Kerrisk
<mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
linux-man <linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
Michael Ellerman <mpe-Gsx/Oe8HsFggBc27wqDAHg@public.gmane.org>,
linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org,
LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
linux-arch-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
Jann Horn <jannh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>,
Matthew Wilcox <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Subject: Re: [PATCH v2] mmap.2: MAP_FIXED updated documentation
Date: Tue, 5 Dec 2017 08:08:38 +0100 [thread overview]
Message-ID: <20171205070838.u3br5lvshywkwxby@dhcp22.suse.cz> (raw)
In-Reply-To: <6777116d-ad9e-48c9-0009-01d10274135e-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
On Mon 04-12-17 18:52:27, John Hubbard wrote:
> On 12/04/2017 03:31 AM, Mike Rapoport wrote:
> > On Sun, Dec 03, 2017 at 06:14:11PM -0800, john.hubbard-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org wrote:
> >> From: John Hubbard <jhubbard-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> >>
> [...]
> >> +.IP
> >> +Given the above limitations, one of the very few ways to use this option
> >> +safely is: mmap() a region, without specifying MAP_FIXED. Then, within that
> >> +region, call mmap(MAP_FIXED) to suballocate regions. This avoids both the
> >> +portability problem (because the first mmap call lets the kernel pick the
> >> +address), and the address space corruption problem (because the region being
> >> +overwritten is already owned by the calling thread).
> >
> > Maybe "address space corruption problem caused by implicit calls to mmap"?
> > The region allocated with the first mmap is not exactly owned by the
> > thread and a multi-thread application can still corrupt its memory if
> > different threads use mmap(MAP_FIXED) for overlapping regions.
> >
> > My 2 cents.
> >
>
> Hi Mike,
>
> Yes, thanks for picking through this, and I agree that the above is misleading.
> It should definitely not use the word "owned" at all. Re-doing the whole
> paragraph in order to make it all fit together nicely, I get this:
>
> "Given the above limitations, one of the very few ways to use this option
> safely is: mmap() an enclosing region, without specifying MAP_FIXED.
> Then, within that region, call mmap(MAP_FIXED) to suballocate regions
> within the enclosing region. This avoids both the portability problem
> (because the first mmap call lets the kernel pick the address), and the
> address space corruption problem (because implicit calls to mmap will
> not affect the already-mapped enclosing region)."
>
> ...how's that sound to you? I'll post a v3 soon with this.
It sounds to me you are trying to tell way to much while actually being
a bit misleading. Even sub-range MAP_FIXED is not multi-thread safe.
Really the more corner cases you will try to cover the worse the end
result will end up. I would just try to be simple here and mention the
address space corruption issues you've had earlier and be done with it.
Maybe add a note that some architectures might need a special alignement
and fail if it is not the case but nothing really specific.
--
Michal Hocko
SUSE Labs
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next prev parent reply other threads:[~2017-12-05 7:08 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-04 2:14 [PATCH v2] mmap.2: MAP_FIXED updated documentation john.hubbard
[not found] ` <20171204021411.4786-1-jhubbard-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
2017-12-04 10:55 ` Cyril Hrubis
2017-12-05 2:14 ` John Hubbard
[not found] ` <efb6eae4-7f30-42c3-0efe-0ab5fbf0fdb4-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
2017-12-05 7:05 ` Michal Hocko
2017-12-05 7:42 ` John Hubbard
2017-12-05 8:52 ` Michal Hocko
[not found] ` <2cff594a-b481-269d-dd91-ff2cc2f4100a-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
2017-12-06 10:01 ` Cyril Hrubis
2017-12-06 21:21 ` John Hubbard
2017-12-07 12:58 ` Cyril Hrubis
2017-12-07 14:02 ` Michal Hocko
2017-12-09 17:19 ` Pavel Machek
2017-12-10 7:44 ` John Hubbard
2017-12-04 11:31 ` Mike Rapoport
2017-12-05 2:52 ` John Hubbard
[not found] ` <6777116d-ad9e-48c9-0009-01d10274135e-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
2017-12-05 7:08 ` Michal Hocko [this message]
2017-12-05 7:43 ` John Hubbard
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