[PATCH] fix madvise(MADV_DONTNEED) for nonlinear vmas

From: Hugh Dickins <hugh@veritas.com>

Jamie points out that madvise(MADV_DONTNEED) should unmap pages from a
nonlinear area in such a way that the nonlinear offsets are preserved if the
pages do turn out to be needed later after all, instead of reverting them to
linearity: needs to pass down a zap_details block.

(But this still leaves mincore unaware of nonlinear vmas: bigger job.)

include/linux/mm.h

@@ -439,7 +439,16 @@ struct file *shmem_file_setup(char * name, loff_t size, unsigned long flags);
 void shmem_lock(struct file * file, int lock);
 int shmem_zero_setup(struct vm_area_struct *);
 
-struct zap_details;
+/*
+ * Parameter block passed down to zap_pte_range in exceptional cases.
+ */
+struct zap_details {
+	struct vm_area_struct *nonlinear_vma;	/* Check page->index if set */
+	struct address_space *check_mapping;	/* Check page->mapping if set */
+	pgoff_t	first_index;			/* Lowest page->index to unmap */
+	pgoff_t last_index;			/* Highest page->index to unmap */
+};
+
 void zap_page_range(struct vm_area_struct *vma, unsigned long address,
 		unsigned long size, struct zap_details *);
 int unmap_vmas(struct mmu_gather **tlbp, struct mm_struct *mm,

mm/madvise.c

@@ -92,9 +92,18 @@ static long madvise_willneed(struct vm_area_struct * vma,
 static long madvise_dontneed(struct vm_area_struct * vma,
 			     unsigned long start, unsigned long end)
 {
+	struct zap_details details;
+
 	if (vma->vm_flags & VM_LOCKED)
 		return -EINVAL;
 
+	if (unlikely(vma->vm_flags & VM_NONLINEAR)) {
+		details.check_mapping = NULL;
+		details.nonlinear_vma = vma;
+		details.first_index = 0;
+		details.last_index = ULONG_MAX;
+		zap_page_range(vma, start, end - start, &details);
+	} else
 		zap_page_range(vma, start, end - start, NULL);
 	return 0;
 }

mm/memory.c

@@ -384,16 +384,6 @@ skip_copy_pmd_range:	address = (address + PGDIR_SIZE) & PGDIR_MASK;
 	return -ENOMEM;
 }
 
-/*
- * Parameter block passed down to zap_pte_range in exceptional cases.
- */
-struct zap_details {
-	struct vm_area_struct *nonlinear_vma;	/* Check page->index if set */
-	struct address_space *check_mapping;	/* Check page->mapping if set */
-	pgoff_t	first_index;			/* Lowest page->index to unmap */
-	pgoff_t last_index;			/* Highest page->index to unmap */
-};
-
 static void zap_pte_range(struct mmu_gather *tlb,
 		pmd_t *pmd, unsigned long address,
 		unsigned long size, struct zap_details *details)