Re: [PATCH v2 0/2] mm: introduce MAP_FIXED_SAFE

[David Goldblatt <davidtgoldblatt@gmail.com>]

(+cc the jemalloc jasone; -cc,+bcc the Google jasone).

The only time we would want MAP_FIXED (or rather, a non-broken
variant) is in the middle of trying to expand an allocation in place;
"atomic address range probing in the multithreaded programs" describes
our use case pretty well. That's in a pathway that usually fails; it's
pretty far down on our kernel mmap enhancements wish-list.

(Sorry if you get this twice, an html reply bounced).


On Wed, Dec 13, 2017 at 5:35 PM, David Goldblatt
<davidtgoldblatt@gmail.com> wrote:
> (+cc the jemalloc jasone; -cc,+bcc the Google jasone).
>
> The only time we would want MAP_FIXED (or rather, a non-broken variant) is
> in the middle of trying to expand an allocation in place; "atomic address
> range probing in the multithreaded programs" describes our use case pretty
> well. That's in a pathway that usually fails; it's pretty far down on our
> kernel mmap enhancements wish-list.
>
> On Wed, Dec 13, 2017 at 4:32 PM, Andrew Morton <akpm@linux-foundation.org>
> wrote:
>>
>> On Wed, 13 Dec 2017 10:25:48 +0100 Michal Hocko <mhocko@kernel.org> wrote:
>>
>> >
>> > Hi,
>> > I am resending with some minor updates based on Michael's review and
>> > ask for inclusion. There haven't been any fundamental objections for
>> > the RFC [1] nor the previous version [2].  The biggest discussion
>> > revolved around the naming. There were many suggestions flowing
>> > around MAP_REQUIRED, MAP_EXACT, MAP_FIXED_NOCLOBBER, MAP_AT_ADDR,
>> > MAP_FIXED_NOREPLACE etc...
>>
>> I like MAP_FIXED_CAREFUL :)
>>
>> > I am afraid we can bikeshed this to death and there will still be
>> > somebody finding yet another better name. Therefore I've decided to
>> > stick with my original MAP_FIXED_SAFE. Why? Well, because it keeps the
>> > MAP_FIXED prefix which should be recognized by developers and _SAFE
>> > suffix should also be clear that all dangerous side effects of the old
>> > MAP_FIXED are gone.
>> >
>> > If somebody _really_ hates this then feel free to nack and resubmit
>> > with a different name you can find a consensus for. I am sorry to be
>> > stubborn here but I would rather have this merged than go over few more
>> > iterations changing the name just because it seems like a good idea
>> > now. My experience tells me that chances are that the name will turn out
>> > to be "suboptimal" anyway over time.
>> >
>> > Some more background:
>> > This has started as a follow up discussion [3][4] resulting in the
>> > runtime failure caused by hardening patch [5] which removes MAP_FIXED
>> > from the elf loader because MAP_FIXED is inherently dangerous as it
>> > might silently clobber an existing underlying mapping (e.g. stack). The
>> > reason for the failure is that some architectures enforce an alignment
>> > for the given address hint without MAP_FIXED used (e.g. for shared or
>> > file backed mappings).
>> >
>> > One way around this would be excluding those archs which do alignment
>> > tricks from the hardening [6]. The patch is really trivial but it has
>> > been objected, rightfully so, that this screams for a more generic
>> > solution. We basically want a non-destructive MAP_FIXED.
>> >
>> > The first patch introduced MAP_FIXED_SAFE which enforces the given
>> > address but unlike MAP_FIXED it fails with EEXIST if the given range
>> > conflicts with an existing one. The flag is introduced as a completely
>> > new one rather than a MAP_FIXED extension because of the backward
>> > compatibility. We really want a never-clobber semantic even on older
>> > kernels which do not recognize the flag. Unfortunately mmap sucks wrt.
>> > flags evaluation because we do not EINVAL on unknown flags. On those
>> > kernels we would simply use the traditional hint based semantic so the
>> > caller can still get a different address (which sucks) but at least not
>> > silently corrupt an existing mapping. I do not see a good way around
>> > that. Except we won't export expose the new semantic to the userspace at
>> > all.
>> >
>> > It seems there are users who would like to have something like that.
>> > Jemalloc has been mentioned by Michael Ellerman [7]
>>
>> http://lkml.kernel.org/r/87efp1w7vy.fsf@concordia.ellerman.id.au.
>>
>> It would be useful to get feedback from jemalloc developers (please).
>> I'll add some cc's.
>>
>>
>> > Florian Weimer has mentioned the following:
>> > : glibc ld.so currently maps DSOs without hints.  This means that the
>> > kernel
>> > : will map right next to each other, and the offsets between them a
>> > completely
>> > : predictable.  We would like to change that and supply a random address
>> > in a
>> > : window of the address space.  If there is a conflict, we do not want
>> > the
>> > : kernel to pick a non-random address. Instead, we would try again with
>> > a
>> > : random address.
>> >
>> > John Hubbard has mentioned CUDA example
>> > : a) Searches /proc/<pid>/maps for a "suitable" region of available
>> > : VA space.  "Suitable" generally means it has to have a base address
>> > : within a certain limited range (a particular device model might
>> > : have odd limitations, for example), it has to be large enough, and
>> > : alignment has to be large enough (again, various devices may have
>> > : constraints that lead us to do this).
>> > :
>> > : This is of course subject to races with other threads in the process.
>> > :
>> > : Let's say it finds a region starting at va.
>> > :
>> > : b) Next it does:
>> > :     p = mmap(va, ...)
>> > :
>> > : *without* setting MAP_FIXED, of course (so va is just a hint), to
>> > : attempt to safely reserve that region. If p != va, then in most cases,
>> > : this is a failure (almost certainly due to another thread getting a
>> > : mapping from that region before we did), and so this layer now has to
>> > : call munmap(), before returning a "failure: retry" to upper layers.
>> > :
>> > :     IMPROVEMENT: --> if instead, we could call this:
>> > :
>> > :             p = mmap(va, ... MAP_FIXED_SAFE ...)
>> > :
>> > :         , then we could skip the munmap() call upon failure. This
>> > :         is a small thing, but it is useful here. (Thanks to Piotr
>> > :         Jaroszynski and Mark Hairgrove for helping me get that detail
>> > :         exactly right, btw.)
>> > :
>> > : c) After that, CUDA suballocates from p, via:
>> > :
>> > :      q = mmap(sub_region_start, ... MAP_FIXED ...)
>> > :
>> > : Interestingly enough, "freeing" is also done via MAP_FIXED, and
>> > : setting PROT_NONE to the subregion. Anyway, I just included (c) for
>> > : general interest.
>> >
>> > Atomic address range probing in the multithreaded programs in general
>> > sounds like an interesting thing to me.
>> >
>> > The second patch simply replaces MAP_FIXED use in elf loader by
>> > MAP_FIXED_SAFE. I believe other places which rely on MAP_FIXED should
>> > follow. Actually real MAP_FIXED usages should be docummented properly
>> > and they should be more of an exception.
>>
>

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