mm: introduce FPB_RESPECT_WRITE for PTE batching infrastructure

Patch 6 ("mm: Optimize mprotect() by PTE batching") optimizes mprotect()
by batch clearing the ptes, masking in the new protections, and batch
setting the ptes.  Suppose that the first pte of the batch is writable -
with the current implementation of folio_pte_batch(), it is not guaranteed
that the other ptes in the batch are already writable too, so we may
incorrectly end up setting the writable bit on all ptes via
modify_prot_commit_ptes().

Therefore, introduce FPB_RESPECT_WRITE so that all ptes in the batch are
writable or not.

Link: https://lkml.kernel.org/r/20250718090244.21092-5-dev.jain@arm.com
Signed-off-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Cc: Zhenhua Huang <quic_zhenhuah@quicinc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Dev Jain

mm/internal.h

@@ -208,25 +208,30 @@ typedef int __bitwise fpb_t;
 /* Compare PTEs respecting the soft-dirty bit. */
 #define FPB_RESPECT_SOFT_DIRTY		((__force fpb_t)BIT(1))
 
+/* Compare PTEs respecting the writable bit. */
+#define FPB_RESPECT_WRITE		((__force fpb_t)BIT(2))
+
 /*
  * Merge PTE write bits: if any PTE in the batch is writable, modify the
  * PTE at @ptentp to be writable.
  */
-#define FPB_MERGE_WRITE			((__force fpb_t)BIT(2))
+#define FPB_MERGE_WRITE			((__force fpb_t)BIT(3))
 
 /*
  * Merge PTE young and dirty bits: if any PTE in the batch is young or dirty,
  * modify the PTE at @ptentp to be young or dirty, respectively.
  */
-#define FPB_MERGE_YOUNG_DIRTY		((__force fpb_t)BIT(3))
+#define FPB_MERGE_YOUNG_DIRTY		((__force fpb_t)BIT(4))
 
 static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
 {
 	if (!(flags & FPB_RESPECT_DIRTY))
 		pte = pte_mkclean(pte);
 	if (likely(!(flags & FPB_RESPECT_SOFT_DIRTY)))
 		pte = pte_clear_soft_dirty(pte);
-	return pte_wrprotect(pte_mkold(pte));
+	if (likely(!(flags & FPB_RESPECT_WRITE)))
+		pte = pte_wrprotect(pte);
+	return pte_mkold(pte);
 }
 
 /**