Re: [PATCH] mmap.2: ENOMEM possible when exceeding VA space

["Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>]

Hi Topi,

On 11/11/21 19:04, Topi Miettinen wrote:
> A bit surprisingly, mmap(2) returns ENOMEM when the virtual address
> space of the CPU is exceeded.
> 
> The expectation could be EINVAL instead ("We don't like _addr_,
> length, or offset (e.g., they are _too large_, or not aligned on a
> page boundary)").
> 
> This is demonstrated with the following program:
> 
>   #include <stdio.h>
>   #include <sys/mman.h>
> 
>   int main(void) {
>   	for (int i = 12; i < 64; i++) {
> 		void *addr = mmap((void *)(1UL << i), 4096, PROT_NONE,
> 				  MAP_ANON | MAP_FIXED_NOREPLACE | MAP_PRIVATE,
> 				  -1, 0);
> 		if (addr == MAP_FAILED)
> 			fprintf(stderr, "mmap %lx: %m\n", (1UL << i));
> 		continue;
> 		munmap(addr, 4096);
> 	}
>   }
> 
> It gives the following output when running on CPU with 48 bit VA
> space:
> 
> mmap 800000000000: Cannot allocate memory
> mmap 1000000000000: Cannot allocate memory
> mmap 2000000000000: Cannot allocate memory
> mmap 4000000000000: Cannot allocate memory
> mmap 8000000000000: Cannot allocate memory
> mmap 10000000000000: Cannot allocate memory
> mmap 20000000000000: Cannot allocate memory
> mmap 40000000000000: Cannot allocate memory
> mmap 80000000000000: Cannot allocate memory
> mmap 100000000000000: Cannot allocate memory
> mmap 200000000000000: Cannot allocate memory
> mmap 400000000000000: Cannot allocate memory
> mmap 800000000000000: Cannot allocate memory
> mmap 1000000000000000: Cannot allocate memory
> mmap 2000000000000000: Cannot allocate memory
> mmap 4000000000000000: Cannot allocate memory
> mmap 8000000000000000: Cannot allocate memory
> 
> Signed-off-by: Topi Miettinen <toiwoton@gmail.com>

Patch applied!

Thanks,
Alex

> ---
>   man2/mmap.2 | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/man2/mmap.2 b/man2/mmap.2
> index 96b7444b0..59fd5c904 100644
> --- a/man2/mmap.2
> +++ b/man2/mmap.2
> @@ -603,6 +603,11 @@ limit, described in
>   .BR getrlimit (2),
>   would have been exceeded.
>   .TP
> +.B ENOMEM
> +We don't like
> +.IR addr ,
> +because it exceeds the virtual address space of the CPU.
> +.TP
>   .B EOVERFLOW
>   On 32-bit architecture together with the large file extension
>   (i.e., using 64-bit
> 

-- 
Alejandro Colomar
Linux man-pages comaintainer; http://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/