[PATCH v9 14/69] kernel/fork: use maple tree for dup_mmap() during forking

Liam Howlett
2022-05-04T01:09:13Z

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

The maple tree was already tracking VMAs in this function by an earlier
commit, but the rbtree iterator was being used to iterate the list.
Change the iterator to use a maple tree native iterator and switch to the
maple tree advanced API to avoid multiple walks of the tree during insert
operations. Unexport the now-unused vma_store() function.

For performance reasons we bulk allocate the maple tree nodes. The node
calculations are done internally to the tree and use the VMA count and
assume the worst-case node requirements. The VM_DONT_COPY flag does not
allow for the most efficient copy method of the tree and so a bulk loading
algorithm is used.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
---
include/linux/mm.h | 2 --
kernel/fork.c | 12 +++++++++---
2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9d9b0d96433a..c259f15c58ac 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2605,8 +2605,6 @@ extern bool arch_has_descending_max_zone_pfns(void);
/* nommu.c */
extern atomic_long_t mmap_pages_allocated;
extern int nommu_shrink_inode_mappings(struct inode *, size_t, size_t);
-/* mmap.c */
-void vma_mas_store(struct vm_area_struct *vma, struct ma_state *mas);

/* interval_tree.c */
void vma_interval_tree_insert(struct vm_area_struct *node,
diff --git a/kernel/fork.c b/kernel/fork.c
index bb985cc624fd..79af6e908539 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -583,9 +583,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
struct vm_area_struct *mpnt, *tmp, *prev, **pprev;
struct rb_node **rb_link, *rb_parent;
int retval;
- unsigned long charge;
- LIST_HEAD(uf);
+ unsigned long charge = 0;
+ MA_STATE(old_mas, &oldmm->mm_mt, 0, 0);
MA_STATE(mas, &mm->mm_mt, 0, 0);
+ LIST_HEAD(uf);

uprobe_start_dup_mmap();
if (mmap_write_lock_killable(oldmm)) {
@@ -622,7 +623,12 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
goto out;

prev = NULL;
- for (mpnt = oldmm->mmap; mpnt; mpnt = mpnt->vm_next) {
+
+ retval = mas_expected_entries(&mas, oldmm->map_count);
+ if (retval)
+ goto out;
+
+ mas_for_each(&old_mas, mpnt, ULONG_MAX) {
struct file *file;

if (mpnt->vm_flags & VM_DONTCOPY) {
--
2.35.1