VM question related to faults

[Attilio Rao]

2006/7/30, Divacky Roman <xdivac02 at stud.fit.vutbr.cz>:
> On Sun, Jul 30, 2006 at 12:57:32PM +0200, Divacky Roman wrote:
> > hi,
> >
> > while working on SoC linuxolator project I am in a need of this:
> >
> > I need to do some operation on memory like mem1 = mem1 + mem2 etc.
> > where the mem1/mem2 access can trigger fault. (memory not mapped or something)
>
> to make it clear.. I am trying to access user-space memory from kernel.
> This needs to be atomic (its an implementation of linux futexes)
>
> I need to check from kernel if some memory is accessible and then perform an
> operation on this memory. All atomically.
>
> hence I need two things - function which checks wheter the memory is accessible
> and something which makes it atomic (some mutex/something which prevents other
> process to enter VM to unmap/etc. the memory in question)
>
> hope its a bit more clear now

You would use something like:

#include <sys/cdefs.h>
#include <sys/param.h>
#include <sys/time.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/systm.h>
#include <sys/resource.h>

#include <vm/vm_page.h>

...
int
lock_and_fetch(const void* mem1, const void *mem2)
{

        mtx_lock(&vm_page_queue_mtx);
        if (fubyte(mem1) == -1 || fubyte(mem2) == -1) {
                mtx_unlock(&vm_page_queue_mtx);
                return(EINVAL);
        }
        /* Operations... */
        mtx_unlock(&vm_page_queue_mtx);

        return(0);
}

It prevents to virtual pages to be passed through queues.

Attilio


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