[PATCH v4 4/8] mm: Move immutable fields of 'struct vm_fault' into anonymous struct

[Will Deacon]

'struct vm_fault' contains both information about the fault being
serviced alongside mutable fields contributing to the state of the
fault-handling logic. Unfortunately, the distinction between the two is
not clear-cut, and a number of callers end up manipulating the structure
temporarily before restoring it when returning.

Try to clean this up by moving the immutable fault information into an
anonymous struct, which will later be marked as 'const'. GCC will then
complain (with an error) about modification of these fields after they
have been initialised, although LLVM currently allows them without even
a warning:

https://bugs.llvm.org/show_bug.cgi?id=48755

Ideally, the 'flags' field would be part of the new structure too, but
it seems as though the ->page_mkwrite() path is not ready for this yet.

Cc: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
Suggested-by: Linus Torvalds <torvalds at linux-foundation.org>
Link: https://lore.kernel.org/r/CAHk-=whYs9XsO88iqJzN6NC=D-dp2m0oYXuOoZ=eWnvv=5OA+w@mail.gmail.com
Signed-off-by: Will Deacon <will at kernel.org>
---
 include/linux/mm.h | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 251a2339befb..b4a5cb9bff7d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -517,11 +517,14 @@ static inline bool fault_flag_allow_retry_first(unsigned int flags)
  * pgoff should be used in favour of virtual_address, if possible.
  */
 struct vm_fault {
-	struct vm_area_struct *vma;	/* Target VMA */
-	unsigned int flags;		/* FAULT_FLAG_xxx flags */
-	gfp_t gfp_mask;			/* gfp mask to be used for allocations */
-	pgoff_t pgoff;			/* Logical page offset based on vma */
-	unsigned long address;		/* Faulting virtual address */
+	struct {
+		struct vm_area_struct *vma;	/* Target VMA */
+		gfp_t gfp_mask;			/* gfp mask to be used for allocations */
+		pgoff_t pgoff;			/* Logical page offset based on vma */
+		unsigned long address;		/* Faulting virtual address */
+	};
+	unsigned int flags;		/* FAULT_FLAG_xxx flags
+					 * XXX: should really be 'const' */
 	pmd_t *pmd;			/* Pointer to pmd entry matching
 					 * the 'address' */
 	pud_t *pud;			/* Pointer to pud entry matching
-- 
2.30.0.284.gd98b1dd5eaa7-goog