arm64/mm: improve comment in contpte_ptep_get_lockless()

Make clear the atmicity/consistency requirements of the API and how we
achieve them.

Link: https://lore.kernel.org/linux-mm/Zc-Tqqfksho3BHmU@arm.com/
Link: https://lkml.kernel.org/r/20240226120321.1055731-3-ryan.roberts@arm.com
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
index be0a226c4ff9bbf4e5bce09903ae50b8bb7ac400..1b64b4c3f8bf8af49d53d1d835ad9eaa3dda5407 100644 (file)
--- a/arch/arm64/mm/contpte.c
+++ b/arch/arm64/mm/contpte.c
@@ -183,16 +183,20 @@ EXPORT_SYMBOL_GPL(contpte_ptep_get);
 pte_t contpte_ptep_get_lockless(pte_t *orig_ptep)
 {
        /*
-        * Gather access/dirty bits, which may be populated in any of the ptes
-        * of the contig range. We may not be holding the PTL, so any contiguous
-        * range may be unfolded/modified/refolded under our feet. Therefore we
-        * ensure we read a _consistent_ contpte range by checking that all ptes
-        * in the range are valid and have CONT_PTE set, that all pfns are
-        * contiguous and that all pgprots are the same (ignoring access/dirty).
-        * If we find a pte that is not consistent, then we must be racing with
-        * an update so start again. If the target pte does not have CONT_PTE
-        * set then that is considered consistent on its own because it is not
-        * part of a contpte range.
+        * The ptep_get_lockless() API requires us to read and return *orig_ptep
+        * so that it is self-consistent, without the PTL held, so we may be
+        * racing with other threads modifying the pte. Usually a READ_ONCE()
+        * would suffice, but for the contpte case, we also need to gather the
+        * access and dirty bits from across all ptes in the contiguous block,
+        * and we can't read all of those neighbouring ptes atomically, so any
+        * contiguous range may be unfolded/modified/refolded under our feet.
+        * Therefore we ensure we read a _consistent_ contpte range by checking
+        * that all ptes in the range are valid and have CONT_PTE set, that all
+        * pfns are contiguous and that all pgprots are the same (ignoring
+        * access/dirty). If we find a pte that is not consistent, then we must
+        * be racing with an update so start again. If the target pte does not
+        * have CONT_PTE set then that is considered consistent on its own
+        * because it is not part of a contpte range.
         */
 
        pgprot_t orig_prot;