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Subject[PATCH 2/6] NOMMU: struct vm_region's vm_usage count need not be atomic
Date
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>
---
include/linux/mm_types.h | 2 +-
mm/nommu.c | 14 +++++++-------
2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 84d020b..80cfa78 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -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 */
};
diff --git a/mm/nommu.c b/mm/nommu.c
index 1777386..5e39294 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -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);


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