[RFC PATCH 2/6] arm64: mm: allow huge vmap permission adjustments with bbml2_no_abort

["Adrian Barnaś" <abarnas@google.com>]

Remove the protection against huge vmap permission adjustments on
systems that support the bbml2_no_abort CPU feature.

Splitting live kernel VA section mappings into page mappings was
restricted because it could cause TLB Conflict Aborts. This forced
permission adjustments on memory allocated with VM_ALLOW_HUGE_VMAP to be
rejected, resulting in performance drops (e.g., when enforcing rodata=on
disables huge mappings).

The bbml2_no_abort feature (which mirrors the architectural guarantees of
FEAT_BBML3) ensures that changing between table and block sizes without
following a break-before-make sequence will not generate a TLB Conflict
Abort. This hardware guarantee makes it safe to allow dynamic permission
adjustments on huge vmap regions.

Signed-off-by: Adrian Barnaś <abarnas@google.com>
---
 arch/arm64/mm/pageattr.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 358d1dc9a576..88720bbba892 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -157,23 +157,29 @@ static int change_memory_common(unsigned long addr, int numpages,
 	}
 
 	/*
-	 * Kernel VA mappings are always live, and splitting live section
-	 * mappings into page mappings may cause TLB conflicts. This means
-	 * we have to ensure that changing the permission bits of the range
-	 * we are operating on does not result in such splitting.
-	 *
 	 * Let's restrict ourselves to mappings created by vmalloc (or vmap).
-	 * Disallow VM_ALLOW_HUGE_VMAP mappings to guarantee that only page
-	 * mappings are updated and splitting is never needed.
 	 *
 	 * So check whether the [addr, addr + size) interval is entirely
 	 * covered by precisely one VM area that has the VM_ALLOC flag set.
 	 */
 	area = find_vm_area((void *)addr);
+
 	if (!area ||
 	    ((unsigned long)kasan_reset_tag((void *)end) >
 	     (unsigned long)kasan_reset_tag(area->addr) + area->size) ||
-	    ((area->flags & (VM_ALLOC | VM_ALLOW_HUGE_VMAP)) != VM_ALLOC))
+	    !(area->flags & VM_ALLOC))
+		return -EINVAL;
+
+	/*
+	 * Kernel VA mappings are always live, and splitting live section
+	 * mappings into page mappings may cause TLB conflicts if bbml2_noabort
+	 * is not present.
+	 *
+	 * While bbml2_noabort is not present disallow VM_ALLOW_HUGE_VMAP mappings
+	 * to guarantee that only page mappings are updated and splitting is not
+	 * needed.
+	 */
+	if (!system_supports_bbml2_noabort() && (area->flags & (VM_ALLOW_HUGE_VMAP)))
 		return -EINVAL;
 
 	if (!numpages)
-- 
2.54.0.1136.gdb2ca164c4-goog