linux-kernel - [PATCH v5 4/7] mm: Introduce FPB_RESPECT_WRITE for PTE batching infrastructure

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Message-Id: <20250718090244.21092-5-dev.jain@arm.com>
Date: Fri, 18 Jul 2025 14:32:41 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org
Cc: ryan.roberts@....com,
	david@...hat.com,
	willy@...radead.org,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
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	Dev Jain <dev.jain@....com>
Subject: [PATCH v5 4/7] mm: Introduce FPB_RESPECT_WRITE for PTE batching infrastructure

Patch 6 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.

Signed-off-by: Dev Jain <dev.jain@....com>
---
 mm/internal.h | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 5b0f71e5434b..28d2d5b051df 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -208,17 +208,20 @@ 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)
 {
@@ -226,7 +229,9 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
 		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);
 }
 
 /**
-- 
2.30.2