Re: [uml-devel] Re: madvise(DONTNEED) on tmpfs pages instead of /dev/anon

[Jeff Dike <jdike@addtoit.com>]

On Fri, Aug 12, 2005 at 07:00:26PM +0200, Blaisorblade wrote:
> Yes, but for the VM, there is nothing else than pagecache and swapcache and 
> swap entries. Since that call will drop them, on normal files backing store 
> will remain, on tmpfs everything will vanish!
> 
> Hope you'll implement this soon, if it works.

There was a small omission in my quick analysis, which you alluded to above.

It's removed from the address space, but preserved in the page cache.

Run the test below to see this.

With a tmpfs /tmp, MADV_DONTNEED preserves the data.

				Jeff

#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/param.h>
#include <sys/mman.h>
#include <asm/page.h>

int main(int argc, char **argv)
{
	int fd, i;
	unsigned char *addr, c = 0xff;

	fd = open("/tmp/test", O_RDWR | O_CREAT);
	if(fd < 0){
		perror("Opening /tmp/test");
		exit(1);
	}

	for(i = 0; i < PAGE_SIZE; i++)
		write(fd, &c, 1);
	close(fd);

	fd = open("/tmp/test", O_RDWR);
	if(fd < 0){
		perror("Opening /tmp/test for mmap");
		exit(1);
	}

	addr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE,
		    fd, 0);
	if(addr == MAP_FAILED){
		perror("mmapping test file 1");
		exit(1);
	}
		
	for(i = 0; i < PAGE_SIZE; i++){
		if(addr[i] != 0xff){
			printf("First mmap - bytes not == 0xff\n");
			exit(1);
		}
	}

	if(madvise(addr, PAGE_SIZE, MADV_DONTNEED) < 0){
		perror("MADV_DONTNEED");
		exit(1);
	}


	munmap(addr, PAGE_SIZE);

	addr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE,
		    fd, 0);
	if(addr == MAP_FAILED){
		perror("mmapping test file 2");
		exit(1);
	}

	for(i = 0; i < PAGE_SIZE; i++){
		if(addr[i] != 0xff){
			printf("Second mmap - bytes not == 0xff\n");
			exit(1);
		}
	}

	printf("Data was preserved across MADV_DONTNEED\n");
}


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