pressuring dirty pages (2.3.99-pre6)

[Rik van Riel <riel@conectiva.com.br>]

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Hi,

I've been trying to fix the VM balance for a week or so now,
and things are mostly fixed except for one situation.

If there is a *heavy* write going on and the data is in the
page cache only .. ie. no buffer heads available, then the
page cache will grow almost without bounds and kswapd and
the rest of the system will basically spin in shrink_mmap()...

What mechanism do we use to flush back dirty pages from eg.
mmap()s?  How could I push those pages to disk the way we
do with buffers (by waking up bdflush)?

(yes, this is a big bug, please try the attached program by
Juan Quintela and set the #defines as wanted .. it'll make
painfully clear that this bug exists and should be fixed)

regards,

Rik
--
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/*
 * Memory tester by Quintela.
 */
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

#define FILENAME "/tmp/testing_file"
/* Put here 2times your memory or less */
#define SIZE     (128 * 1024 * 1024) 


void error_string(char *msg)
{
        perror(msg);
        exit(EXIT_FAILURE);
}

int main(int argc, char * argv[])
{
        char *array;
        int i;
        int fd = open(FILENAME, O_RDWR | O_CREAT, 0666);
        if (fd == -1)
                error_string("Problems opening the file");

        if (lseek(fd, SIZE, SEEK_SET) != SIZE)
                error_string("Problems doing the lseek");

        if (write(fd,"\0",1) !=1)
                error_string("Problems writing");
 
        array = mmap(0, SIZE, PROT_WRITE, MAP_SHARED,fd,0);
        if (array == MAP_FAILED)
                error_string("The mmap has failed");
        
        for(i = 0; i < SIZE; i++) {
                array[i] = i;
        } 
        msync(array, SIZE, MS_SYNC);
        close(fd);
        exit(EXIT_SUCCESS);
}