(+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@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx> > wrote: >> >> On Wed, 13 Dec 2017 10:25:48 +0100 Michal Hocko <mhocko@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxx. >> >> 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. >> > -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
