Re: [PATCH v39 11/24] x86/sgx: Add SGX enclave driver

[Matthew Wilcox <willy@infradead.org>]

On Mon, Oct 05, 2020 at 02:41:53AM +0300, Jarkko Sakkinen wrote:
> On Sun, Oct 04, 2020 at 11:27:50PM +0100, Matthew Wilcox wrote:
> > 	int ret = 0;
> > 
> > 	mutex_lock(&encl->lock);
> > 	rcu_read_lock();
> 
> Right, so xa_*() take RCU lock implicitly and xas_* do not.

Not necessarily the RCU lock ... I did document all this in xarray.rst:

https://www.kernel.org/doc/html/latest/core-api/xarray.html

> > 	while (xas.index < idx_end) {
> > 		page = xas_next(&xas);
> 
> It should iterate through every possible page index within the range,
> even the ones that do not have an entry, i.e. this loop also checks
> that there are no empty slots.
> 
> Does xas_next() go through every possible index, or skip the non-empty
> ones?

xas_next(), as its documentation says, will move to the next array
index:

https://www.kernel.org/doc/html/latest/core-api/xarray.html#c.xas_next

> > 		if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
> > 			ret = -EACCESS;
> > 			break;
> > 		}
> > 	}
> > 	rcu_read_unlock();
> > 	mutex_unlock(&encl->lock);
> 
> In my Geminilake NUC the maximum size of the address space is 64GB for
> an enclave, and it is not fixed but can grow in microarchitectures
> beyond that.
> 
> That means that in (*artificial*) worst case the locks would be kept for
> 64*1024*1024*1024/4096 = 16777216 iterations.

Oh, there's support for that on the XArray API too.

        xas_lock_irq(&xas);
        xas_for_each_marked(&xas, page, end, PAGECACHE_TAG_DIRTY) {
                xas_set_mark(&xas, PAGECACHE_TAG_TOWRITE);
                if (++tagged % XA_CHECK_SCHED)
                        continue;

                xas_pause(&xas);
                xas_unlock_irq(&xas);
                cond_resched();
                xas_lock_irq(&xas);
        }
        xas_unlock_irq(&xas);