Re: [PATCHV2 3/3] x86, ras: Add mcsafe_memcpy() function to recover from machine checks

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Fri, Dec 11, 2015 at 09:19:17PM +0000, Luck, Tony wrote:
> > I still don't get the BIT(63) thing.  Can you explain it?
> 
> It will be more obvious when I get around to writing copy_from_user().
> 
> Then we will have a function that can take page faults if there are pages
> that are not present.  If the page faults can't be fixed we have a -EFAULT
> condition. We can also take machine checks if we reads from a location with an
> uncorrected error.
> 
> We need to distinguish these two cases because the action we take is
> different. For the unresolved page fault we already have the ABI that the
> copy_to/from_user() functions return zero for success, and a non-zero
> return is the number of not-copied bytes.
> 
> So for my new case I'm setting bit63 ... this is never going to be set for
> a failed page fault.

Isn't 63 NX?

> 
> copy_from_user() conceptually will look like this:
> 
> int copy_from_user(void *to, void *from, unsigned long n)
> {
> 	u64 ret = mcsafe_memcpy(to, from, n);
> 
> 	if (COPY_HAD_MCHECK(r)) {
> 		if (memory_failure(COPY_MCHECK_PADDR(ret) >> PAGE_SIZE, ...))
> 			force_sig(SIGBUS, current);
> 		return something;
> 	} else
> 		return ret;
> }
> 
> -Tony
> _______________________________________________
> Linux-nvdimm mailing list
> Linux-nvdimm@lists.01.org
> https://lists.01.org/mailman/listinfo/linux-nvdimm

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>