linux-kernel - [RFC PATCH 04/14] mm/mprotect: preserve write with MM_CP_TRY_CHANGE

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Date:   Mon, 18 Jul 2022 05:02:02 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>
Subject: [RFC PATCH 04/14] mm/mprotect: preserve write with MM_CP_TRY_CHANGE_WRITABLE

From: Nadav Amit <namit@...are.com>

When MM_CP_TRY_CHANGE_WRITABLE is used, change_pte_range() tries to set
PTEs as writable.

Yet, writable PTEs might still become read-only, due to various
limitations of the logic that determines whether a PTE can become
writable (see can_change_pte_writable()). Anyhow, it is much easier to
keep the writable bit set when MM_CP_TRY_CHANGE_WRITABLE is used than to
first clear it and then figure out whether it can be set again.

Preserve the write-bit when MM_CP_TRY_CHANGE_WRITABLE is used, similarly
to the way it is done with NUMA.

Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: David Hildenbrand <david@...hat.com>
Cc: Peter Xu <peterx@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Will Deacon <will@...nel.org>
Cc: Yu Zhao <yuzhao@...gle.com>
Cc: Nick Piggin <npiggin@...il.com>
Signed-off-by: Nadav Amit <namit@...are.com>
---
 mm/mprotect.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index da5b9bf8204f..92bfb17dcb8a 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -84,6 +84,7 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
 	bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
 	bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
 	bool will_need = cp_flags & MM_CP_WILL_NEED;
+	bool try_change_writable = cp_flags & MM_CP_TRY_CHANGE_WRITABLE;
 
 	tlb_change_page_size(tlb, PAGE_SIZE);
 
@@ -114,7 +115,8 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
 		oldpte = *pte;
 		if (pte_present(oldpte)) {
 			pte_t ptent;
-			bool preserve_write = prot_numa && pte_write(oldpte);
+			bool preserve_write = (prot_numa || try_change_writable) &&
+					       pte_write(oldpte);
 
 			/*
 			 * Avoid trapping faults against the zero or KSM
@@ -190,8 +192,7 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
 			 * example, if a PTE is already dirty and no other
 			 * COW or special handling is required.
 			 */
-			if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
-			    !pte_write(ptent) &&
+			if (try_change_writable && !pte_write(ptent) &&
 			    can_change_pte_writable(vma, addr, ptent)) {
 				ptent = pte_mkwrite(ptent);
 				if (will_need)
-- 
2.25.1