[PATCH v6 1/3] mm/mmap: separate writenotify and dirty tracking logic

[Lorenzo Stoakes <lstoakes@gmail.com>]

vma_wants_writenotify() is specifically intended for setting PTE page table
flags, accounting for existing PTE flag state and whether that might
already be read-only while mixing this check with a check whether the
filesystem performs dirty tracking.

Separate out the notions of dirty tracking and a PTE write notify checking
in order that we can invoke the dirty tracking check from elsewhere.

Note that this change introduces a very small duplicate check of the
separated out vm_ops_needs_writenotify(). This is necessary to avoid making
vma_needs_dirty_tracking() needlessly complicated (e.g. passing a
check_writenotify flag or having it assume this check was already
performed). This is such a small check that it doesn't seem too egregious
to do this.

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Mika Penttilä <mpenttil@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
---
 include/linux/mm.h |  1 +
 mm/mmap.c          | 36 +++++++++++++++++++++++++++---------
 2 files changed, 28 insertions(+), 9 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 27ce77080c79..7b1d4e7393ef 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2422,6 +2422,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
 #define  MM_CP_UFFD_WP_ALL                 (MM_CP_UFFD_WP | \
 					    MM_CP_UFFD_WP_RESOLVE)
 
+bool vma_needs_dirty_tracking(struct vm_area_struct *vma);
 int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot);
 static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma)
 {
diff --git a/mm/mmap.c b/mm/mmap.c
index 5522130ae606..295c5f2e9bd9 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1475,6 +1475,31 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
 }
 #endif /* __ARCH_WANT_SYS_OLD_MMAP */
 
+/* Do VMA operations imply write notify is required? */
+static bool vm_ops_needs_writenotify(const struct vm_operations_struct *vm_ops)
+{
+	return vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite);
+}
+
+/*
+ * Does this VMA require the underlying folios to have their dirty state
+ * tracked?
+ */
+bool vma_needs_dirty_tracking(struct vm_area_struct *vma)
+{
+	/* Does the filesystem need to be notified? */
+	if (vm_ops_needs_writenotify(vma->vm_ops))
+		return true;
+
+	/* Specialty mapping? */
+	if (vma->vm_flags & VM_PFNMAP)
+		return false;
+
+	/* Can the mapping track the dirty pages? */
+	return vma->vm_file && vma->vm_file->f_mapping &&
+		mapping_can_writeback(vma->vm_file->f_mapping);
+}
+
 /*
  * Some shared mappings will want the pages marked read-only
  * to track write events. If so, we'll downgrade vm_page_prot
@@ -1484,14 +1509,13 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
 int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
 {
 	vm_flags_t vm_flags = vma->vm_flags;
-	const struct vm_operations_struct *vm_ops = vma->vm_ops;
 
 	/* If it was private or non-writable, the write bit is already clear */
 	if ((vm_flags & (VM_WRITE|VM_SHARED)) != ((VM_WRITE|VM_SHARED)))
 		return 0;
 
 	/* The backer wishes to know when pages are first written to? */
-	if (vm_ops && (vm_ops->page_mkwrite || vm_ops->pfn_mkwrite))
+	if (vm_ops_needs_writenotify(vma->vm_ops))
 		return 1;
 
 	/* The open routine did something to the protections that pgprot_modify
@@ -1511,13 +1535,7 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
 	if (userfaultfd_wp(vma))
 		return 1;
 
-	/* Specialty mapping? */
-	if (vm_flags & VM_PFNMAP)
-		return 0;
-
-	/* Can the mapping track the dirty pages? */
-	return vma->vm_file && vma->vm_file->f_mapping &&
-		mapping_can_writeback(vma->vm_file->f_mapping);
+	return vma_needs_dirty_tracking(vma);
 }
 
 /*
-- 
2.40.1