mm/shmem: persist uffd-wp bit across zapping for file-backed

File-backed memory is prone to being unmapped at any time.  It means all
information in the pte will be dropped, including the uffd-wp flag.

To persist the uffd-wp flag, we'll use the pte markers.  This patch
teaches the zap code to understand uffd-wp and know when to keep or drop
the uffd-wp bit.

Add a new flag ZAP_FLAG_DROP_MARKER and set it in zap_details when we
don't want to persist such an information, for example, when destroying
the whole vma, or punching a hole in a shmem file.  For the rest cases we
should never drop the uffd-wp bit, or the wr-protect information will get
lost.

The new ZAP_FLAG_DROP_MARKER needs to be put into mm.h rather than
memory.c because it'll be further referenced in hugetlb files later.

Link: https://lkml.kernel.org/r/20220405014847.14295-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 53 insertions, 0 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index d63ba0e0e068..61786259e52a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3428,4 +3428,14 @@ madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
 }
 #endif
 
+typedef unsigned int __bitwise zap_flags_t;
+
+/*
+ * Whether to drop the pte markers, for example, the uffd-wp information for
+ * file-backed memory.  This should only be specified when we will completely
+ * drop the page in the mm, either by truncation or unmapping of the vma.  By
+ * default, the flag is not set.
+ */
+#define  ZAP_FLAG_DROP_MARKER        ((__force zap_flags_t) BIT(0))
+
 #endif /* _LINUX_MM_H */
diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index ac32125745ab..7b25b53c474a 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -6,6 +6,8 @@
 #include <linux/huge_mm.h>
 #include <linux/swap.h>
 #include <linux/string.h>
+#include <linux/userfaultfd_k.h>
+#include <linux/swapops.h>
 
 /**
  * folio_is_file_lru - Should the folio be on a file LRU or anon LRU?
@@ -316,5 +318,46 @@ static inline bool mm_tlb_flush_nested(struct mm_struct *mm)
 	return atomic_read(&mm->tlb_flush_pending) > 1;
 }
 
+/*
+ * If this pte is wr-protected by uffd-wp in any form, arm the special pte to
+ * replace a none pte.  NOTE!  This should only be called when *pte is already
+ * cleared so we will never accidentally replace something valuable.  Meanwhile
+ * none pte also means we are not demoting the pte so tlb flushed is not needed.
+ * E.g., when pte cleared the caller should have taken care of the tlb flush.
+ *
+ * Must be called with pgtable lock held so that no thread will see the none
+ * pte, and if they see it, they'll fault and serialize at the pgtable lock.
+ *
+ * This function is a no-op if PTE_MARKER_UFFD_WP is not enabled.
+ */
+static inline void
+pte_install_uffd_wp_if_needed(struct vm_area_struct *vma, unsigned long addr,
+			      pte_t *pte, pte_t pteval)
+{
+#ifdef CONFIG_PTE_MARKER_UFFD_WP
+	bool arm_uffd_pte = false;
+
+	/* The current status of the pte should be "cleared" before calling */
+	WARN_ON_ONCE(!pte_none(*pte));
+
+	if (vma_is_anonymous(vma) || !userfaultfd_wp(vma))
+		return;
+
+	/* A uffd-wp wr-protected normal pte */
+	if (unlikely(pte_present(pteval) && pte_uffd_wp(pteval)))
+		arm_uffd_pte = true;
+
+	/*
+	 * A uffd-wp wr-protected swap pte.  Note: this should even cover an
+	 * existing pte marker with uffd-wp bit set.
+	 */
+	if (unlikely(pte_swp_uffd_wp_any(pteval)))
+		arm_uffd_pte = true;
+
+	if (unlikely(arm_uffd_pte))
+		set_pte_at(vma->vm_mm, addr, pte,
+			   make_pte_marker(PTE_MARKER_UFFD_WP));
+#endif
+}
 
 #endif