linux-kernel - Re: [PATCH v2 1/2] x86/sgx: Do not update sgx_nr_free_pages in sgx_setup_epc

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Date:   Thu, 8 Apr 2021 11:48:46 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-sgx@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] x86/sgx: Do not update sgx_nr_free_pages in
 sgx_setup_epc_section()

On Wed, Apr 07, 2021 at 06:18:11PM +0200, Borislav Petkov wrote:
> On Wed, Apr 07, 2021 at 07:03:47PM +0300, Jarkko Sakkinen wrote:
> > > Which leads to my question: what is sgx_nr_free_pages supposed to denote?
> > > 
> > > Because I understand the callpath
> > > 
> > > sgx_page_cache_init
> > > ...
> > > for (i = 0; i < ARRAY_SIZE(sgx_epc_sections); i++) {
> > > 	...
> > > 	sgx_setup_epc_section
> > > 	...
> > > 		sgx_nr_free_pages += nr_pages;
> > > 
> > > as adding the number of pages of each new EPC section to the total
> > > number of the free pages. Unless that variable accounts something else.
> > > 
> > > So what does this variable actually mean?
> > 
> > It's used for only to trigger watermark for reclaiming. I.e. causes
> > ksgxd to trigger. And it gives the number of total free EPC pages in
> > all NUMA nodes.
> 
> So the callpath I laid out above is adding the number of pages of each
> section to the total free EPC pages number.
> 
> Why is that wrong and why is your patch needed?
 
As part of "x86/sgx: Replace section->init_laundry_list with sgx_dirty_page_list"
pages are processed from a global list by ksgxd.

This in turn introduces change to sanitization:

-		if (!ret)
-			list_move(&page->list, &section->page_list);
-		else
+		if (!ret) {
+			/*
+			 * page is now sanitized.  Make it available via the SGX
+			 * page allocator:
+			 */
+			list_del(&page->list);
+			sgx_free_epc_page(page);
+		} else {
+			/* The page is not yet clean - move to the dirty list. */
 			list_move_tail(&page->list, &dirty);
-
-		spin_unlock(&section->lock);
+		}

This is done for the reason that it is best to keep the logic to assign
available-for-use EPC pages to correct NUMA lists in a single location.

The regression is that the sgx_nr_free_pages is also incremented by
sgx_free_epc_pages(), and thus it ends up having double the number of
pages available.

/Jarkko