nommu: struct vm_region's vm_usage count need not be atomic
David Howells [Sat, 16 Jan 2010 01:01:33 +0000 (17:01 -0800)]
The vm_usage count field in struct vm_region does not need to be atomic as
it's only even modified whilst nommu_region_sem is write locked.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Greg Ungerer <gerg@snapgear.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm_types.h
mm/nommu.c

index 84d020b..80cfa78 100644 (file)
@@ -122,7 +122,7 @@ struct vm_region {
        unsigned long   vm_pgoff;       /* the offset in vm_file corresponding to vm_start */
        struct file     *vm_file;       /* the backing file or NULL */
 
-       atomic_t        vm_usage;       /* region usage count */
+       int             vm_usage;       /* region usage count (access under nommu_region_sem) */
        bool            vm_icache_flushed : 1; /* true if the icache has been flushed for
                                                * this region */
 };
index 1777386..5e39294 100644 (file)
@@ -552,11 +552,11 @@ static void free_page_series(unsigned long from, unsigned long to)
 static void __put_nommu_region(struct vm_region *region)
        __releases(nommu_region_sem)
 {
-       kenter("%p{%d}", region, atomic_read(&region->vm_usage));
+       kenter("%p{%d}", region, region->vm_usage);
 
        BUG_ON(!nommu_region_tree.rb_node);
 
-       if (atomic_dec_and_test(&region->vm_usage)) {
+       if (--region->vm_usage == 0) {
                if (region->vm_top > region->vm_start)
                        delete_nommu_region(region);
                up_write(&nommu_region_sem);
@@ -1205,7 +1205,7 @@ unsigned long do_mmap_pgoff(struct file *file,
        if (!vma)
                goto error_getting_vma;
 
-       atomic_set(&region->vm_usage, 1);
+       region->vm_usage = 1;
        region->vm_flags = vm_flags;
        region->vm_pgoff = pgoff;
 
@@ -1272,7 +1272,7 @@ unsigned long do_mmap_pgoff(struct file *file,
                        }
 
                        /* we've found a region we can share */
-                       atomic_inc(&pregion->vm_usage);
+                       pregion->vm_usage++;
                        vma->vm_region = pregion;
                        start = pregion->vm_start;
                        start += (pgoff - pregion->vm_pgoff) << PAGE_SHIFT;
@@ -1289,7 +1289,7 @@ unsigned long do_mmap_pgoff(struct file *file,
                                        vma->vm_region = NULL;
                                        vma->vm_start = 0;
                                        vma->vm_end = 0;
-                                       atomic_dec(&pregion->vm_usage);
+                                       pregion->vm_usage--;
                                        pregion = NULL;
                                        goto error_just_free;
                                }
@@ -1444,7 +1444,7 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma,
        /* we're only permitted to split anonymous regions that have a single
         * owner */
        if (vma->vm_file ||
-           atomic_read(&vma->vm_region->vm_usage) != 1)
+           vma->vm_region->vm_usage != 1)
                return -ENOMEM;
 
        if (mm->map_count >= sysctl_max_map_count)
@@ -1518,7 +1518,7 @@ static int shrink_vma(struct mm_struct *mm,
 
        /* cut the backing region down to size */
        region = vma->vm_region;
-       BUG_ON(atomic_read(&region->vm_usage) != 1);
+       BUG_ON(region->vm_usage != 1);
 
        down_write(&nommu_region_sem);
        delete_nommu_region(region);