Re: [PATCH] x86/sgx: Reduce delay and interference of enclave release

[Jarkko Sakkinen <jarkko@kernel.org>]

On Mon, Oct 24, 2022 at 11:56:39AM -0700, Reinette Chatre wrote:
> Hi Jarkko,
> 
> On 10/23/2022 1:06 PM, Jarkko Sakkinen wrote:
> > On Tue, Oct 18, 2022 at 03:42:47PM -0700, Reinette Chatre wrote:
> 
> ...
> 
> >> diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
> >> index 1ec20807de1e..f7365c278525 100644
> >> --- a/arch/x86/kernel/cpu/sgx/encl.c
> >> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> >> @@ -682,9 +682,12 @@ void sgx_encl_release(struct kref *ref)
> >>  	struct sgx_encl *encl = container_of(ref, struct sgx_encl, refcount);
> >>  	struct sgx_va_page *va_page;
> >>  	struct sgx_encl_page *entry;
> >> -	unsigned long index;
> >> +	unsigned long count = 0;
> >> +
> >> +	XA_STATE(xas, &encl->page_array, PFN_DOWN(encl->base));
> >>  
> >> -	xa_for_each(&encl->page_array, index, entry) {
> >> +	xas_lock(&xas);
> >> +	xas_for_each(&xas, entry, PFN_DOWN(encl->base + encl->size  - 1)) {
> > 
> > I would add to declarations:
> > 
> > unsigned long nr_pages = PFN_DOWN(encl->base + encl->size  - 1);
> > 
> > Makes this more readable.
> 
> Will do, but I prefer to name it "max_page_index" or something related instead.
> "nr_pages" implies "number of pages" to me, which is not what
> PFN_DOWN(encl->base + encl->size - 1) represents. What is represented is the
> highest possible index of a page in page_array, where an index is the
> pfn of a page.

Yeah, makes sense.

> 
> > 
> >>  		if (entry->epc_page) {
> >>  			/*
> >>  			 * The page and its radix tree entry cannot be freed
> >> @@ -699,9 +702,20 @@ void sgx_encl_release(struct kref *ref)
> >>  		}
> >>  
> >>  		kfree(entry);
> >> -		/* Invoke scheduler to prevent soft lockups. */
> >> -		cond_resched();
> >> +		/*
> >> +		 * Invoke scheduler on every XA_CHECK_SCHED iteration
> >> +		 * to prevent soft lockups.
> >> +		 */
> >> +		if (!(++count % XA_CHECK_SCHED)) {
> >> +			xas_pause(&xas);
> >> +			xas_unlock(&xas);
> >> +
> >> +			cond_resched();
> >> +
> >> +			xas_lock(&xas);
> >> +		}
> >>  	}
> > 
> >         WARN_ON(count != nr_pages);
> > 
> 
> nr_pages as assigned in your example does not represent a count of the
> enclave pages but instead a pfn index into the page_array. Comparing it
> to count, the number of removed enclave pages that are not being held
> by reclaimer, is not appropriate.
> 
> This check would be problematic even if we create a "nr_pages" from
> the range of possible indices. This is because of how enclave sizes are
> required to be power-of-two that makes it likely for there to be indices
> without pages associated with it.

Ok.

> 
> >> +	xas_unlock(&xas);
> >>  
> >>  	xa_destroy(&encl->page_array);
> >>  
> >> -- 
> >> 2.34.1
> >>
> 
> Reinette

BR, Jarkko