powerpc/mm/keys: Update documentation and remove unnecessary check

Adds more code comments. We also remove an unnecessary pkey check
after we check for pkey error in this patch.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/fault.c

@@ -297,7 +297,12 @@ static bool access_error(bool is_write, bool is_exec,
 
 	if (unlikely(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))))
 		return true;
-
+	/*
+	 * We should ideally do the vma pkey access check here. But in the
+	 * fault path, handle_mm_fault() also does the same check. To avoid
+	 * these multiple checks, we skip it here and handle access error due
+	 * to pkeys later.
+	 */
 	return false;
 }
 
@@ -518,26 +523,17 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 
 #ifdef CONFIG_PPC_MEM_KEYS
 	/*
-	 * if the HPTE is not hashed, hardware will not detect
-	 * a key fault. Lets check if we failed because of a
-	 * software detected key fault.
+	 * we skipped checking for access error due to key earlier.
+	 * Check that using handle_mm_fault error return.
 	 */
 	if (unlikely(fault & VM_FAULT_SIGSEGV) &&
-		!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
-			is_exec, 0)) {
-		/*
-		 * The PGD-PDT...PMD-PTE tree may not have been fully setup.
-		 * Hence we cannot walk the tree to locate the PTE, to locate
-		 * the key. Hence let's use vma_pkey() to get the key; instead
-		 * of get_mm_addr_key().
-		 */
+		!arch_vma_access_permitted(vma, is_write, is_exec, 0)) {
+
 		int pkey = vma_pkey(vma);
 
-		if (likely(pkey)) {
 		up_read(&mm->mmap_sem);
 		return bad_key_fault_exception(regs, address, pkey);
 	}
-	}
 #endif /* CONFIG_PPC_MEM_KEYS */
 
 	major |= fault & VM_FAULT_MAJOR;

arch/powerpc/mm/pkeys.c

@@ -119,18 +119,15 @@ int pkey_initialize(void)
 #else
 	os_reserved = 0;
 #endif
+	initial_allocation_mask = ~0x0;
+	pkey_amr_uamor_mask = ~0x0ul;
+	pkey_iamr_mask = ~0x0ul;
 	/*
-	 * Bits are in LE format. NOTE: 1, 0 are reserved.
+	 * key 0, 1 are reserved.
 	 * key 0 is the default key, which allows read/write/execute.
 	 * key 1 is recommended not to be used. PowerISA(3.0) page 1015,
 	 * programming note.
 	 */
-	initial_allocation_mask = ~0x0;
-
-	/* register mask is in BE format */
-	pkey_amr_uamor_mask = ~0x0ul;
-	pkey_iamr_mask = ~0x0ul;
-
 	for (i = 2; i < (pkeys_total - os_reserved); i++) {
 		initial_allocation_mask &= ~(0x1 << i);
 		pkey_amr_uamor_mask &= ~(0x3ul << pkeyshift(i));