Re: mmap + mprotect + malloc strange behaviour
From: Kaloian Manassiev
Date: Mon Jul 26 2004 - 08:05:45 EST
Nevermind, I found it :)
I just found out (by reading
/usr/src/linux-2.4.20-8/Documentation/sysctl/vm.txt)
that there is a limit on the number of mappings that a
process may have and that for some reason malloc
consumes mappings.
I just increased the limit by editing the file
/proc/sys/vm/max_map_count. This works okay for my
application...
Does someone know what repercussions this could have
on the "normal" operation of the system?
Cheers,
Kaloian.
--- Kaloian Manassiev <kmanassieff@xxxxxxxxx> wrote:
> Hi all,
>
> I am writing a tool which keeps track of the
> accessed
> memory pages and needs to persist the modified pages
> back to disk on exit. For the purpose, it mmaps a
> large file (512MB or 131072 pages), then protects
> all
> its pages for both reading and writing using
> mprotect(
> ptr, PAGE_SIZE, 0). I do this in order to be able to
> keep track of the accessed pages.
>
> Throughout the code I also use malloc in order to
> allocate heap memory for some of the internal
> structures. The problem I have is that all mallocs
> that follow the mmapping and mprotect described
> above
> return NULL, no matter what the passed size is.
>
> I noticed that this behaviour is caused by the
> mprotection of the pages one-by-one rather than as a
> whole chunk. E.g., if I mprotect my whole mmapped
> space with a single call everything works fine at
> the
> beginning. However, this is not very helpful,
> because
> in the process of execution, I eventually mprotect
> many separate pages, and malloc starts to fail
> again.
>
> I checked my limits using the "ulimit" BASH command
> and it shows that I have no limits concerning
> memory-related issues.
>
> ------------------
> core file size (blocks, -c) 0
> data seg size (kbytes, -d) unlimited
> file size (blocks, -f) unlimited
> max locked memory (kbytes, -l) unlimited
> max memory size (kbytes, -m) unlimited
> open files (-n) 1024
> pipe size (512 bytes, -p) 8
> stack size (kbytes, -s) 10240
> cpu time (seconds, -t) unlimited
> max user processes (-u) 4092
> virtual memory (kbytes, -v) unlimited
> -------------------
>
> With my lack of knowledge about Linux VM management
> I
> can only suspect that is some way I am exceeding the
> process' virtual address space slots and it cannot
> bookkeep the additional malloc entries.
>
> I am using Fedora Linux on a dual AMD Athlon MP
> 2600+
> processor machine with 512MB of physical memory and
> 1GB swap disk space. However, I observe the same
> effects on many other machines and distributions
> (for
> example, 4 Intel Xeon processor machine with 4GB RAM
> +
> 6GB swap running Red Hat Linux 9).
>
> Has anyone had a similar problem, and does anyone
> know
> a remedy for it? At the bottom of this post I have
> pasted a sample program that exhibits this
> behaviour.
>
> Any help will be much appreciated. Please, if
> possible, reply to (or CC) my personal email address
> in answers to this post.
>
> Many thanks in advance.
>
> Cheers,
> Kaloian.
>
>
> ----- Sample code (buildable)
> --------------------------
>
> #include <stdio.h>
> #include <errno.h>
> #include <sys/mman.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <assert.h>
>
> #define PAGE_SIZE 4096
> #define NUM_PAGES 131072
> #define SIZE PAGE_SIZE * NUM_PAGES
> #define MMAP_ADDR ((void *)0x10000000L)
>
> int array[ NUM_PAGES * 64];
> int * mem_load;
>
> int main( int argc, char ** argv) {
> char c;
> int i;
> int fd_mmap = open( "/tmp/swap.tmp", O_CREAT |
> O_RDWR, S_IRWXU);
>
> if ( fd_mmap) {
> lseek( fd_mmap, SIZE - 1, SEEK_SET);
> write( fd_mmap, &c, 1);
> }
>
> printf( "Allocating %d bytes of memory...\n",
> SIZE);
> mem_load = malloc( SIZE);
> printf( "mem_load = %p\n", mem_load);
> array[ NUM_PAGES * 64 - 1] = 10000;
>
> if ( NULL == mem_load)
> printf( "Failed...\n");
>
> void * p = mmap( MMAP_ADDR, SIZE, PROT_READ |
> PROT_WRITE,
> MAP_FIXED | MAP_SHARED, fd_mmap, PAGE_SIZE);
> printf( "File %d mapped at %p.\n", fd_mmap, p);
> if ( !p) {
> exit( 1);
> }
> void * alias = mmap( MMAP_ADDR + 2 * SIZE, SIZE,
> PROT_READ |
> PROT_WRITE, MAP_SHARED, fd_mmap, PAGE_SIZE);
> if ( !alias) {
> exit( 2);
> }
> void * pg = p;
> for ( i = 0; i < NUM_PAGES; i++) {
> if ( 0 > mprotect( pg, PAGE_SIZE, 0)) {
> printf( "Failed to mprotect.\n");
> exit( 4);
> }
> pg += PAGE_SIZE;
> }
>
> /* If you substitute the above with the call
> below,
> malloc suddenly starts to work. */
> // mprotect( pg, SIZE, 0);
>
> printf( "Alias mapped at %p.\n", alias);
>
> assert( mem_load > p + SIZE || ( (mem_load < p +
> SIZE) && ((
> mem_load + SIZE) < alias)));
>
> free( mem_load);
> mem_load = malloc( SIZE);
> printf( "mem_load = %p\n", mem_load);
>
> return 0;
> }
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