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From:   Nadav Amit <nadav.amit@gmail.com>
To:     linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Nadav Amit <namit@vmware.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Nick Piggin <npiggin@gmail.com>
Subject: [RFC PATCH 02/14] userfaultfd: try to map write-unprotected pages
Date:   Mon, 18 Jul 2022 05:02:00 -0700
Message-Id: <20220718120212.3180-3-namit@vmware.com>
In-Reply-To: <20220718120212.3180-1-namit@vmware.com>
References: <20220718120212.3180-1-namit@vmware.com>
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From: Nadav Amit <namit@vmware.com>

When using userfaultfd write-(un)protect ioctl, try to change the PTE to
be writable. This would save a page-fault afterwards.

Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Nick Piggin <npiggin@gmail.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 mm/userfaultfd.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e0492f5f06a0..6013b217e9f3 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -799,6 +799,8 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
 	cp_flags = enable_wp ? MM_CP_UFFD_WP : MM_CP_UFFD_WP_RESOLVE;
 	if (uffd_flags & (UFFD_FLAGS_ACCESS_LIKELY|UFFD_FLAGS_WRITE_LIKELY))
 		cp_flags |= MM_CP_WILL_NEED;
+	if (!enable_wp && (uffd_flags & UFFD_FLAGS_WRITE_LIKELY))
+		cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
 
 	tlb_gather_mmu(&tlb, dst_mm);
 	change_protection(&tlb, dst_vma, start, start + len, newprot, cp_flags);
-- 
2.25.1