[PATCH v5 20/21] mm/mmap: Move may_expand_vm() check in mmap_region()

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>

The may_expand_vm() check requires the count of the pages within the
munmap range.  Since this is needed for accounting and obtained later,
the reodering of may_expand_vm() to later in the call stack, after the
vma munmap struct (vms) is initialised and the gather stage is
potentially run, will allow for a single loop over the vmas.  The gather
sage does not commit any work and so everything can be undone in the
case of a failure.

The MAP_FIXED page count is available after the vms_gather_munmap_vmas()
call, so use it instead of looping over the vmas twice.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/mmap.c | 36 ++++--------------------------------
 1 file changed, 4 insertions(+), 32 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index ca752317adef..5e74f5cb7be4 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -402,27 +402,6 @@ anon_vma_interval_tree_post_update_vma(struct vm_area_struct *vma)
 		anon_vma_interval_tree_insert(avc, &avc->anon_vma->rb_root);
 }
 
-static unsigned long count_vma_pages_range(struct mm_struct *mm,
-		unsigned long addr, unsigned long end,
-		unsigned long *nr_accounted)
-{
-	VMA_ITERATOR(vmi, mm, addr);
-	struct vm_area_struct *vma;
-	unsigned long nr_pages = 0;
-
-	*nr_accounted = 0;
-	for_each_vma_range(vmi, vma, end) {
-		unsigned long vm_start = max(addr, vma->vm_start);
-		unsigned long vm_end = min(end, vma->vm_end);
-
-		nr_pages += PHYS_PFN(vm_end - vm_start);
-		if (vma->vm_flags & VM_ACCOUNT)
-			*nr_accounted += PHYS_PFN(vm_end - vm_start);
-	}
-
-	return nr_pages;
-}
-
 static void __vma_link_file(struct vm_area_struct *vma,
 			    struct address_space *mapping)
 {
@@ -2977,17 +2956,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	pgoff_t vm_pgoff;
 	int error = -ENOMEM;
 	VMA_ITERATOR(vmi, mm, addr);
-	unsigned long nr_pages, nr_accounted;
-
-	nr_pages = count_vma_pages_range(mm, addr, end, &nr_accounted);
-
-	/*
-	 * Check against address space limit.
-	 * MAP_FIXED may remove pages of mappings that intersects with requested
-	 * mapping. Account for the pages it would unmap.
-	 */
-	if (!may_expand_vm(mm, vm_flags, pglen - nr_pages))
-		return -ENOMEM;
 
 
 	/* Find the first overlapping VMA */
@@ -3015,6 +2983,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 			vma_iter_next_range(&vmi);
 	}
 
+	/* Check against address space limit. */
+	if (!may_expand_vm(mm, vm_flags, pglen - vms.nr_pages))
+		goto abort_munmap;
+
 	/*
 	 * Private writable mapping: check memory availability
 	 */
-- 
2.43.0