Reclaim space from unused ramdisk?

Reclaim space from unused ramdisk?

[Mike Mohr]

I wonder if it would be possible to somehow reclaim space that has
been previously reserved for a ramdisk without rebooting.  I read the
ramdisk docs in the latest kernel source and it seems that it is not
currently possible.  However, the kernel keeps track of the memory
allocated for said ramdisks; would it not be possible with root (or
even kernel) permissions to remove the flag that prevents the VM
subsystem from reclaiming that space?  I realize that rot permissions
may not be high enough.  In that case, could a module be written that
takes a device name as a parameter then uses it to look up the
reserved memory that device uses, then resets the necessary flag and
finally unloads itself?  It would have to check that the filesystem
was unmounted, of course.

How difficult would this be to write?

Re: Reclaim space from unused ramdisk?

[Marc Ballarin]

On Tue, 26 Jul 2005 15:16:58 -0700
Mike Mohr <akihana@gmail.com> wrote:

> I wonder if it would be possible to somehow reclaim space that has
> been previously reserved for a ramdisk without rebooting.  I read the
> ramdisk docs in the latest kernel source and it seems that it is not
> currently possible.  However, the kernel keeps track of the memory
> allocated for said ramdisks; would it not be possible with root (or
> even kernel) permissions to remove the flag that prevents the VM
> subsystem from reclaiming that space?  I realize that rot permissions
> may not be high enough.  In that case, could a module be written that
> takes a device name as a parameter then uses it to look up the
> reserved memory that device uses, then resets the necessary flag and
> finally unloads itself?  It would have to check that the filesystem
> was unmounted, of course.
> 
> How difficult would this be to write?

Hi,

ramfs (always there) and tmpfs (optional) already do this.
tmpfs can be swapped out, ramfs always uses physical memory.

mount -t ramfs none /mnt/blah
mount -t tmpfs none /mnt/blah

Regards

Re: Reclaim space from unused ramdisk?

[Jan Engelhardt]

>I wonder if it would be possible to somehow reclaim space that has
>been previously reserved for a ramdisk without rebooting.

free_ramdisk.c:

#include <sys/ioctl.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int main(int argc, const char **argv) {
    int eax = 0;
    while(*argv != NULL) {
        int fd = open(*argv, O_RDWR);
        if(fd < 0) {
            fprintf(stderr, "Warning: Cannot open %s: %s\n",
             *argv, strerror(errno));
            if(eax == 0) { eax = errno; }
            continue;
        }
        ioctl(fd, BLKFLSBUF, 0);
        close(fd);
        ++argv;
    }
    return eax == 0;
}

//eof


>How difficult would this be to write?


Jan Engelhardt
-- 
| Alphagate Systems, http://alphagate.hopto.org/

Re: Reclaim space from unused ramdisk?

[Andrew Morton]

Jan Engelhardt <jengelh@linux01.gwdg.de> wrote:
>
> >I wonder if it would be possible to somehow reclaim space that has
> >been previously reserved for a ramdisk without rebooting.
> 
> free_ramdisk.c:
> 
> #include <sys/ioctl.h>
> #include <sys/mount.h>
> #include <sys/stat.h>
> #include <sys/types.h>
> #include <errno.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <string.h>
> #include <unistd.h>
> 
> int main(int argc, const char **argv) {
>     int eax = 0;
>     while(*argv != NULL) {
>         int fd = open(*argv, O_RDWR);
>         if(fd < 0) {
>             fprintf(stderr, "Warning: Cannot open %s: %s\n",
>              *argv, strerror(errno));
>             if(eax == 0) { eax = errno; }
>             continue;
>         }
>         ioctl(fd, BLKFLSBUF, 0);
>         close(fd);
>         ++argv;
>     }
>     return eax == 0;
> }
> 

hmm, yes.  That's a special-case in the ramdisk driver.

The command `blockdev --flushbufs /dev/ram0' should have the same effect.

Re: Reclaim space from unused ramdisk?

[Jan Engelhardt]

>> }
>> 
>
>hmm, yes.  That's a special-case in the ramdisk driver.
>
>The command `blockdev --flushbufs /dev/ram0' should have the same effect.

Interesting. Command not found, here. URL?



Jan Engelhardt
-- 

Re: Reclaim space from unused ramdisk?

[Andrew Morton]

Jan Engelhardt <jengelh@linux01.gwdg.de> wrote:
>
> >> }
> >> 
> >
> >hmm, yes.  That's a special-case in the ramdisk driver.
> >
> >The command `blockdev --flushbufs /dev/ram0' should have the same effect.
> 
> Interesting. Command not found, here. URL?
> 

It ships with util-linux.  Weird that a distro would omit it.

Re: Reclaim space from unused ramdisk?

[Jan Engelhardt]

>> >> }
>> >
>> >hmm, yes.  That's a special-case in the ramdisk driver.
>> >The command `blockdev --flushbufs /dev/ram0' should have the same effect.
>> 
>> Interesting. Command not found, here. URL?
>
>It ships with util-linux.  Weird that a distro would omit it.

Don't look at me, look at SUSE.

Euh, is it already included in util-linux-2.12q?


Jan Engelhardt
-- 

Re: Reclaim space from unused ramdisk?

[Jan Engelhardt]

>>It ships with util-linux.  Weird that a distro would omit it.
>
>Euh, is it already included in util-linux-2.12q?

I hate me. It was not in $PATH (/sbin).