linux-kernel - agp: two-stage page destruction issue

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Message-Id: <4858E330.76E4.0078.0@novell.com>
Date:	Wed, 18 Jun 2008 09:28:00 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Dave Airlie" <airlied@...ux.ie>
Cc:	<linux-kernel@...r.kernel.org>
Subject: agp: two-stage page destruction issue

Dave,

besides it apparently being useful only in 2.6.24 (the changes in 2.6.25
really mean that it could be converted back to a single-stage mechanism),
I'm seeing an issue in Xen Dom0 kernels, which is caused by the calling
of gart_to_virt() in the second stage invocations of the destroy function.
I think that besides this being a real issue with Xen (where
unmap_page_from_agp() is not just a page table attribute change), this
also is invalid from a theoretical perspective: One should not assume that
gart_to_virt() is still valid after unmapping a page. So minimally (keeping
the 2-stage mechanism) a patch like the one below would be needed.

Jan

--- a/drivers/char/agp/backend.c
+++ b/drivers/char/agp/backend.c
@@ -188,10 +188,10 @@ static int agp_backend_initialize(struct
 
 err_out:
 	if (bridge->driver->needs_scratch_page) {
-		bridge->driver->agp_destroy_page(gart_to_virt(bridge->scratch_page_real),
-						 AGP_PAGE_DESTROY_UNMAP);
-		bridge->driver->agp_destroy_page(gart_to_virt(bridge->scratch_page_real),
-						 AGP_PAGE_DESTROY_FREE);
+		void *va = gart_to_virt(bridge->scratch_page_real);
+
+		bridge->driver->agp_destroy_page(va, AGP_PAGE_DESTROY_UNMAP);
+		bridge->driver->agp_destroy_page(va, AGP_PAGE_DESTROY_FREE);
 	}
 	if (got_gatt)
 		bridge->driver->free_gatt_table(bridge);
@@ -215,10 +215,10 @@ static void agp_backend_cleanup(struct a
 
 	if (bridge->driver->agp_destroy_page &&
 	    bridge->driver->needs_scratch_page) {
-		bridge->driver->agp_destroy_page(gart_to_virt(bridge->scratch_page_real),
-						 AGP_PAGE_DESTROY_UNMAP);
-		bridge->driver->agp_destroy_page(gart_to_virt(bridge->scratch_page_real),
-						 AGP_PAGE_DESTROY_FREE);
+		void *va = gart_to_virt(bridge->scratch_page_real);
+
+		bridge->driver->agp_destroy_page(va, AGP_PAGE_DESTROY_UNMAP);
+		bridge->driver->agp_destroy_page(va, AGP_PAGE_DESTROY_FREE);
 	}
 }
 
--- a/drivers/char/agp/generic.c
+++ b/drivers/char/agp/generic.c
@@ -202,10 +202,13 @@ void agp_free_memory(struct agp_memory *
 	}
 	if (curr->page_count != 0) {
 		for (i = 0; i < curr->page_count; i++) {
-			curr->bridge->driver->agp_destroy_page(gart_to_virt(curr->memory[i]), AGP_PAGE_DESTROY_UNMAP);
+			curr->memory[i] = (unsigned long)gart_to_virt(curr->memory[i]);
+			curr->bridge->driver->agp_destroy_page((void *)curr->memory[i],
+							       AGP_PAGE_DESTROY_UNMAP);
 		}
 		for (i = 0; i < curr->page_count; i++) {
-			curr->bridge->driver->agp_destroy_page(gart_to_virt(curr->memory[i]), AGP_PAGE_DESTROY_FREE);
+			curr->bridge->driver->agp_destroy_page((void *)curr->memory[i],
+							       AGP_PAGE_DESTROY_FREE);
 		}
 	}
 	agp_free_key(curr->key);
--- a/drivers/char/agp/intel-agp.c
+++ b/drivers/char/agp/intel-agp.c
@@ -428,9 +428,11 @@ static void intel_i810_free_by_type(stru
 		if (curr->page_count == 4)
 			i8xx_destroy_pages(gart_to_virt(curr->memory[0]));
 		else {
-			agp_bridge->driver->agp_destroy_page(gart_to_virt(curr->memory[0]),
+			void *va = gart_to_virt(curr->memory[0]);
+
+			agp_bridge->driver->agp_destroy_page(va,
 							     AGP_PAGE_DESTROY_UNMAP);
-			agp_bridge->driver->agp_destroy_page(gart_to_virt(curr->memory[0]),
+			agp_bridge->driver->agp_destroy_page(va,
 							     AGP_PAGE_DESTROY_FREE);
 		}
 		agp_free_page_array(curr);


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