From IEEE 1003.1: The mmap() function shall establish a mapping between a process’ address space and a file, shared memory object, or [TYM] typed memory object. It needs all the virtual address space because that’s exactly what mmap() does. The fact that it isn’t really running out of memory doesn’t matter – you can’t map … Read more
MAP_PRIVATE mappings require a memory reservation, as writing to these pages may result in copy-on-write allocations. This means that you can’t map something too much larger than your physical ram + swap. Try using a MAP_SHARED mapping instead. This means that writes to the mapping will be reflected on disk – as such, the kernel … Read more
What are the different ways I can write in kernel address space from user space? I’m not sure if there’re other methods, but you can access physical memory using /dev/mem & system call mmap(). /dev/mem is a character device file that is an image of the main memory of the computer. It may be used, … Read more
If you are using MAMP be sure to add the unix_socket key with a value of the path that the mysql.sock resides in MAMP. ‘mysql’ => array( ‘driver’ => ‘mysql’, ‘host’ => ‘localhost’, ‘unix_socket’ => ‘/Applications/MAMP/tmp/mysql/mysql.sock’, ‘database’ => ‘database’, ‘username’ => ‘root’, ‘password’ => ‘root’, ‘charset’ => ‘utf8’, ‘collation’ => ‘utf8_unicode_ci’, ‘prefix’ => ”, ),
I think I’ve found the issue — it’s to do with /dev/mem memory mapping protection on the x86. Pl refer to this LWN article: “x86: introduce /dev/mem restrictions with a config option” http://lwn.net/Articles/267427/ CONFIG_NONPROMISC_DEVMEM Now (i tested this on a recent 3.2.21 kernel), the config option seems to be called CONFIG_STRICT_DEVMEM. I changed my kernel … Read more
mmap works differently. It is anticipatory and adapts to the program’s access pattern. Also, specific policies can be set via madvise to further fine tune usage. For a much more thorough discussion of how mmap works in a demand paging environment, see my answer here: Which segments are affected by a copy-on-write? as it also … Read more
You’re being too conservative: A memory-mapped file can be larger than the address space. The view of the memory-mapped file is limited by OS memory constraints, but that’s only the part of the file you’re looking at at one time. (And I guess technically you could map multiple views of discontinuous parts of the file … Read more
Linux has an execution domain called READ_IMPLIES_EXEC, which causes all pages allocated with PROT_READ to also be given PROT_EXEC. Older Linux kernels used to use this for executables that used the equivalent of gcc -z execstack. This program will show you whether that’s enabled for itself: #include <stdio.h> #include <sys/personality.h> int main(void) { printf(“Read-implies-exec is … Read more
mmap is great if you have multiple processes accessing data in a read only fashion from the same file, which is common in the kind of server systems I write. mmap allows all those processes to share the same physical memory pages, saving a lot of memory. mmap also allows the operating system to optimize … Read more
The following analysis applies only to glibc (based on the ptmalloc2 algorithm). There are certain options that seem helpful to return the freed memory back to the system: mallopt() (defined in malloc.h) does provide an option to set the trim threshold value using one of the parameter option M_TRIM_THRESHOLD, this indicates the minimum amount of … Read more