Michal Hocko <mhocko@xxxxxxxxxx> writes: > On Mon 13-11-17 13:00:57, Michal Hocko wrote: > [...] >> 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. > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
