On Mon, Jan 02, 2017 at 10:08:28PM -0800, Andy Lutomirski wrote: > On Mon, Jan 2, 2017 at 12:44 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > > On Tuesday, December 27, 2016 4:54:13 AM CET Kirill A. Shutemov wrote: > >> As with other resources you can set the limit lower than current usage. > >> It would affect only future virtual address space allocations. > > I still don't buy all these use cases: > > >> > >> Use-cases for new rlimit: > >> > >> - Bumping the soft limit to RLIM_INFINITY, allows current process all > >> its children to use addresses above 47-bits. > > OK, I get this, but only as a workaround for programs that make > assumptions about the address space and don't use some mechanism (to > be designed?) to work correctly in spite of a larger address space. I guess you've misread the case. It's opt-in for large adrress space, not other way around. I believe 47-bit VA by default is right way to go to make the transition without breaking userspace. > >> - Bumping the soft limit to RLIM_INFINITY after fork(2), but before > >> exec(2) allows the child to use addresses above 47-bits. > > Ditto. > > >> > >> - Lowering the hard limit to 47-bits would prevent current process all > >> its children to use addresses above 47-bits, unless a process has > >> CAP_SYS_RESOURCES. > > I've tried and I can't imagine any reason to do this. That's just if something went wrong and we want to stop an application from use addresses above 47-bit. > >> - It’s also can be handy to lower hard or soft limit to arbitrary > >> address. User-mode emulation in QEMU may lower the limit to 32-bit > >> to emulate 32-bit machine on 64-bit host. > > I don't understand. QEMU user-mode emulation intercepts all syscalls. > What QEMU would *actually* want is a way to say "allocate me some > memory with the high N bits clear". mmap-via-int80 on x86 should be > fixed to do this, but a new syscall with an explicit parameter would > work, as would a prctl changing the current limit. Look at mess in mmap_find_vma(). QEmu has to guess where is free virtual memory. That's unnessesary complex. prctl would work for this too. new-mmap would *not*: there are more ways to allocate vitual address space: shmat(), mremap(). Changing all of them just for this is stupid. > >> > >> TODO: > >> - port to non-x86; > >> > >> Not-yet-signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > >> Cc: linux-api@xxxxxxxxxxxxxxx > > > > This seems to nicely address the same problem on arm64, which has > > run into the same issue due to the various page table formats > > that can currently be chosen at compile time. > > On further reflection, I think this has very little to do with paging > formats except insofar as paging formats make us notice the problem. > The issue is that user code wants to be able to assume an upper limit > on an address, and it gets an upper limit right now that depends on > architecture due to paging formats. But someone really might want to > write a *portable* 64-bit program that allocates memory with the high > 16 bits clear. So let's add such a mechanism directly. > > As a thought experiment, what if x86_64 simply never allocated "high" > (above 2^47-1) addresses unless a new mmap-with-explicit-limit syscall > were used? Old glibc would continue working. Old VMs would work. > New programs that want to use ginormous mappings would have to use the > new syscall. This would be totally stateless and would have no issues > with CRIU. Except, we need more than mmap as I mentioned. And what about stack? I'm not sure that everybody would be happy with stack in the middle of address space. > If necessary, we could also have a prctl that changes a > "personality-like" limit that is in effect when the old mmap was used. > I say "personality-like" because it would reset under exactly the same > conditions that personality resets itself. > > Thoughts? > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
