Merge tag 'powerpc-5.2-4' of git://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux

Pull powerpc fixes from Michael Ellerman:
 "One fix for a regression introduced by our 32-bit KASAN support, which
  broke booting on machines with "bootx" early debugging enabled.

  A fix for a bug which broke kexec on 32-bit, introduced by changes to
  the 32-bit STRICT_KERNEL_RWX support in v5.1.

  Finally two fixes going to stable for our THP split/collapse handling,
  discovered by Nick. The first fixes random crashes and/or corruption
  in guests under sufficient load.

  Thanks to: Nicholas Piggin, Christophe Leroy, Aaro Koskinen, Mathieu
  Malaterre"

* tag 'powerpc-5.2-4' of git://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux:
  powerpc/32s: fix booting with CONFIG_PPC_EARLY_DEBUG_BOOTX
  powerpc/64s: __find_linux_pte() synchronization vs pmdp_invalidate()
  powerpc/64s: Fix THP PMD collapse serialisation
  powerpc: Fix kexec failure on book3s/32

arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -876,6 +876,23 @@ static inline int pmd_present(pmd_t pmd)
 	return false;
 }
 
+static inline int pmd_is_serializing(pmd_t pmd)
+{
+	/*
+	 * If the pmd is undergoing a split, the _PAGE_PRESENT bit is clear
+	 * and _PAGE_INVALID is set (see pmd_present, pmdp_invalidate).
+	 *
+	 * This condition may also occur when flushing a pmd while flushing
+	 * it (see ptep_modify_prot_start), so callers must ensure this
+	 * case is fine as well.
+	 */
+	if ((pmd_raw(pmd) & cpu_to_be64(_PAGE_PRESENT | _PAGE_INVALID)) ==
+						cpu_to_be64(_PAGE_INVALID))
+		return true;
+
+	return false;
+}
+
 static inline int pmd_bad(pmd_t pmd)
 {
 	if (radix_enabled())
@@ -1092,6 +1109,19 @@ static inline int pmd_protnone(pmd_t pmd)
 #define pmd_access_permitted pmd_access_permitted
 static inline bool pmd_access_permitted(pmd_t pmd, bool write)
 {
+	/*
+	 * pmdp_invalidate sets this combination (which is not caught by
+	 * !pte_present() check in pte_access_permitted), to prevent
+	 * lock-free lookups, as part of the serialize_against_pte_lookup()
+	 * synchronisation.
+	 *
+	 * This also catches the case where the PTE's hardware PRESENT bit is
+	 * cleared while TLB is flushed, which is suboptimal but should not
+	 * be frequent.
+	 */
+	if (pmd_is_serializing(pmd))
+		return false;
+
 	return pte_access_permitted(pmd_pte(pmd), write);
 }