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

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

[Topi Miettinen]

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>
---
 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
-- 
2.33.0

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

[Jakub Wilk]

* Topi Miettinen <toiwoton@gmail.com>, 2021-11-11, 20:04:
>+.B ENOMEM
>+We don't like
>+.IR addr ,
>+because it exceeds the virtual address space of the CPU.
>+.TP

Who is "we"?

FWIW, failing with ENOMEM in this case may seem weird, but this is what 
POSIX prescribes.

-- 
Jakub Wilk

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

[Alejandro Colomar (man-pages)]

Hi Jakub,

On 11/22/21 17:15, Jakub Wilk wrote:
> * Topi Miettinen <toiwoton@gmail.com>, 2021-11-11, 20:04:
>> +.B ENOMEM
>> +We don't like
>> +.IR addr ,
>> +because it exceeds the virtual address space of the CPU.
>> +.TP
> 
> Who is "we"?

I was going to ask the same, but that page already describes other 
errors like that.  If only for consistency, and avoiding a complete 
rewrite of that ERRORS section, I applied the patch as is.

But yes, "we" doesn't sound like very technical.  Maybe we should fix this.

> 
> FWIW, failing with ENOMEM in this case may seem weird, but this is what 
> POSIX prescribes.
> 

Thanks,
Alex


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

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

[Alejandro Colomar (man-pages)]

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/