Re: linux-next: Tree for Nov 7
From: Michael Ellerman <mpe@ellerman.id.au>
Date: 2017-11-14 09:02:21
Also in:
linux-arm-kernel, linux-mips, linux-next, linux-sh, lkml, sparclinux
Michal Hocko [off-list ref] writes:
On Mon 13-11-17 13:00:57, Michal Hocko wrote: [...]quoted
Yes, I have mentioned that in the previous email but the amount of code would be even larger. Basically every arch which reimplements arch_get_unmapped_area would have to special case new MAP_FIXED flag to do vma lookup.It turned out that this might be much more easier than I thought after all. It seems we can really handle that in the common code.
Ah nice. I should have read this before replying to your previous mail.
This would mean that we are exposing a new functionality to the userspace though. Myabe this would be useful on its own though.
Yes I think it would. At least jemalloc seems like it could use it: https://github.com/jemalloc/jemalloc/blob/9f455e2786685b443201c33119765c8093461174/src/pages.c#L65 And I have memories of some JIT code I read once which did a loop of mmap()s or something to try and get allocations below 4GB or some other limit - but I can't remember now what it was.
quoted hunk ↗ jump to hunk
Just a quick draft (not even compile tested) whether this makes sense in general. I would be worried about unexpected behavior when somebody set other bit without a good reason and we might fail with ENOMEM for such a call now. Elf loader would then use MAP_FIXED_SAFE rather than MAP_FIXED. ---diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h index 3b26cc62dadb..d021c21f9b01 100644 --- a/arch/alpha/include/uapi/asm/mman.h +++ b/arch/alpha/include/uapi/asm/mman.h@@ -31,6 +31,9 @@ #define MAP_STACK 0x80000 /* give out an address that is best suited for process/thread stacks */ #define MAP_HUGETLB 0x100000 /* create a huge page mapping */ +#define MAP_KEEP_MAPPING 0x2000000 +#define MAP_FIXED_SAFE MAP_FIXED|MAP_KEEP_MAPPING /* enforce MAP_FIXED without clobbering an existing mapping */
So bike-shedding a bit, but I think "SAFE" is too vague a name. Perhaps MAP_NO_CLOBBER - which has the single semantic of "do not clobber any existing mappings". It would be a flag on its own, so you could pass it with or without MAP_FIXED, but it would only change the behaviour when MAP_FIXED is specified also. cheers