[PATCH v5.1 19/19] mm/mmap: Move may_expand_vm() check in mmap_region()

[Bert Karwatzki]

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@xxxxxxxxxx>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
---
 mm/mmap.c | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index d2f47cd66650..e0c321b7c642 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1376,17 +1376,6 @@ 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 */
@@ -1414,6 +1403,10 @@ 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
 	 */
--
2.45.2