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

* [PATCH] mmap.2: ENOMEM possible when exceeding VA space
@ 2021-11-11 18:04 Topi Miettinen
  2021-11-22 16:15 ` Jakub Wilk
  2021-11-22 17:00 ` Alejandro Colomar (man-pages)
  0 siblings, 2 replies; 4+ messages in thread
From: Topi Miettinen @ 2021-11-11 18:04 UTC (permalink / raw)
  To: linux-man, alx.manpages, mtk.manpages; +Cc: linux-mm, 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


^ permalink raw reply related	[flat|nested] 4+ messages in thread