[folded-merged] mm-improve-mprotectrw-efficiency-on-pages-referenced-once-v5.patch removed from -mm tree

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The patch titled
     Subject: mm: improve mprotect(R|W) efficiency on pages referenced once
has been removed from the -mm tree.  Its filename was
     mm-improve-mprotectrw-efficiency-on-pages-referenced-once-v5.patch

This patch was dropped because it was folded into mm-improve-mprotectrw-efficiency-on-pages-referenced-once.patch

------------------------------------------------------
From: Peter Collingbourne <pcc@xxxxxxxxxx>
Subject: mm: improve mprotect(R|W) efficiency on pages referenced once

add comments, prohibit optimization for NUMA pages

Link: https://lkml.kernel.org/r/20210601185926.2623183-1-pcc@xxxxxxxxxx
Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx>
Link: https://linux-review.googlesource.com/id/I98d75ef90e20330c578871c87494d64b1df3f1b8
Link: [1] https://source.android.com/devices/tech/debug/scudo
Link: [2] https://cs.android.com/android/platform/superproject/+/master:bionic/benchmarks/stdlib_benchmark.cpp;l=53;drc=e8693e78711e8f45ccd2b610e4dbe0b94d551cc9
Link: [3] https://github.com/pcc/llvm-project/commit/scudo-mprotect-secondary2
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Evgenii Stepanov <eugenis@xxxxxxxxxx>
Cc: Kostya Kortchinsky <kostyak@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/mprotect.c |   23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

--- a/mm/mprotect.c~mm-improve-mprotectrw-efficiency-on-pages-referenced-once-v5
+++ a/mm/mprotect.c
@@ -35,10 +35,16 @@
 
 #include "internal.h"
 
+/* Determine whether we can avoid taking write faults for known dirty pages. */
 static bool may_avoid_write_fault(pte_t pte, struct vm_area_struct *vma,
 				  unsigned long cp_flags)
 {
+	/*
+	 * The dirty accountable bit indicates that we can always make the page
+	 * writable regardless of the number of references.
+	 */
 	if (!(cp_flags & MM_CP_DIRTY_ACCT)) {
+		/* Otherwise, we must have exclusive access to the page. */
 		if (!(vma_is_anonymous(vma) && (vma->vm_flags & VM_WRITE)))
 			return false;
 
@@ -46,15 +52,31 @@ static bool may_avoid_write_fault(pte_t
 			return false;
 	}
 
+	/*
+	 * Don't do this optimization for clean pages as we need to be notified
+	 * of the transition from clean to dirty.
+	 */
 	if (!pte_dirty(pte))
 		return false;
 
+	/* Same for softdirty. */
 	if (!pte_soft_dirty(pte) && (vma->vm_flags & VM_SOFTDIRTY))
 		return false;
 
+	/*
+	 * For userfaultfd the user program needs to monitor write faults so we
+	 * can't do this optimization.
+	 */
 	if (pte_uffd_wp(pte))
 		return false;
 
+	/*
+	 * It is unclear whether this optimization can be done safely for NUMA
+	 * pages.
+	 */
+	if (cp_flags & MM_CP_PROT_NUMA)
+		return false;
+
 	return true;
 }
 
@@ -153,7 +175,6 @@ static unsigned long change_pte_range(st
 				ptent = pte_clear_uffd_wp(ptent);
 			}
 
-			/* Avoid taking write faults for known dirty pages */
 			if (may_avoid_write_fault(ptent, vma, cp_flags))
 				ptent = pte_mkwrite(ptent);
 			ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
_

Patches currently in -mm which might be from pcc@xxxxxxxxxx are

mm-improve-mprotectrw-efficiency-on-pages-referenced-once.patch




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