Date: Mon, 31 Jul 2006 20:15:11 +0200 From: "Attilio Rao" <attilio@freebsd.org> To: "Divacky Roman" <xdivac02@stud.fit.vutbr.cz> Cc: freebsd-hackers@freebsd.org Subject: Re: VM question related to faults Message-ID: <3bbf2fe10607311115ua9b7b9axd9d3e6dcff2c1daf@mail.gmail.com> In-Reply-To: <20060730200354.GA82547@stud.fit.vutbr.cz> References: <20060730105731.GA64955@stud.fit.vutbr.cz> <20060730200354.GA82547@stud.fit.vutbr.cz>
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2006/7/30, Divacky Roman <xdivac02@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
--
Peace can only be achieved by understanding - A. Einstein
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