[RFC PATCH 04/14] mm/mprotect: preserve write with MM_CP_TRY_CHANGE_WRITABLE

Nadav Amit
2022-07-18T19:37:20Z

From: Nadav Amit <namit@vmware.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@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/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