Re: [PATCH 2/2] powerpc/64s: __find_linux_pte synchronization vs pmdp_invalidate

Christophe Leroy's on June 7, 2019 3:35 pm:

Le 07/06/2019 à 05:56, Nicholas Piggin a écrit :

quoted

The change to pmdp_invalidate to mark the pmd with _PAGE_INVALID broke
the synchronisation against lock free lookups, __find_linux_pte's
pmd_none check no longer returns true for such cases.

Fix this by adding a check for this condition as well.

Fixes: da7ad366b497 ("powerpc/mm/book3s: Update pmd_present to look at _PAGE_PRESENT bit")
Cc: Christophe Leroy <redacted>
Suggested-by: Aneesh Kumar K.V <redacted>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
  arch/powerpc/mm/pgtable.c | 16 ++++++++++++++--
  1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index db4a6253df92..533fc6fa6726 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c

@@ -372,13 +372,25 @@ pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
  	pdshift = PMD_SHIFT;
  	pmdp = pmd_offset(&pud, ea);
  	pmd  = READ_ONCE(*pmdp);
+
  	/*
-	 * A hugepage collapse is captured by pmd_none, because
-	 * it mark the pmd none and do a hpte invalidate.
+	 * A hugepage collapse is captured by this condition, see
+	 * pmdp_collapse_flush.
  	 */
  	if (pmd_none(pmd))
  		return NULL;
  
+#ifdef CONFIG_PPC_BOOK3S_64
+	/*
+	 * A hugepage split is captured by this condition, see
+	 * pmdp_invalidate.
+	 *
+	 * Huge page modification can be caught here too.
+	 */
+	if (pmd_is_serializing(pmd))
+		return NULL;
+#endif
+

Could get rid of that #ifdef by adding the following in book3s32 and 
nohash pgtable.h:

static inline bool pmd_is_serializing()  { return false; }

I don't mind either way. If it's an isolated case like this, sometimes 
I'm against polluting the sub arch code with it.

It's up to you I can change that if you prefer.

Thanks,
Nick