[PATCH v2 2/3] ksm: perform a range-walk in break_ksm

Pedro Demarchi Gomes
2025-10-31T17:48:48Z

Make break_ksm() receive an address range and change
break_ksm_pmd_entry() to perform a range-walk and return the address of
the first ksm page found.

This change allows break_ksm() to skip unmapped regions instead of
iterating every page address. When unmerging large sparse VMAs, this
significantly reduces runtime.

In a benchmark unmerging a 32 TiB sparse virtual address space where
only one page was populated, the runtime dropped from 9 minutes to less
then 5 seconds.

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
---
mm/ksm.c | 88 ++++++++++++++++++++++++++++++--------------------------
1 file changed, 48 insertions(+), 40 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 922d2936e206..64d66699133d 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -607,35 +607,55 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
return atomic_read(&mm->mm_users) == 0;
}

-static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
+struct break_ksm_arg {
+ unsigned long addr;
+};
+
+static int break_ksm_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned long end,
struct mm_walk *walk)
{
- struct folio *folio = NULL;
+ unsigned long *found_addr = (unsigned long *) walk->private;
+ struct mm_struct *mm = walk->mm;
+ pte_t *start_ptep, *ptep;
spinlock_t *ptl;
- pte_t *pte;
- pte_t ptent;
- int ret;
+ int found = 0;

- pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
- if (!pte)
+ if (ksm_test_exit(walk->mm))
return 0;
- ptent = ptep_get(pte);
- if (pte_present(ptent)) {
- folio = vm_normal_folio(walk->vma, addr, ptent);
- } else if (!pte_none(ptent)) {
- swp_entry_t entry = pte_to_swp_entry(ptent);

- /*
- * As KSM pages remain KSM pages until freed, no need to wait
- * here for migration to end.
- */
- if (is_migration_entry(entry))
- folio = pfn_swap_entry_folio(entry);
+ if (signal_pending(current))
+ return -ERESTARTSYS;
+
+ start_ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
+ if (!start_ptep)
+ return 0;
+
+ for (ptep = start_ptep; addr < end; ptep++, addr += PAGE_SIZE) {
+ pte_t pte = ptep_get(ptep);
+ struct folio *folio = NULL;
+
+ if (pte_present(pte)) {
+ folio = vm_normal_folio(walk->vma, addr, pte);
+ } else if (!pte_none(pte)) {
+ swp_entry_t entry = pte_to_swp_entry(pte);
+
+ /*
+ * As KSM pages remain KSM pages until freed, no need to wait
+ * here for migration to end.
+ */
+ if (is_migration_entry(entry))
+ folio = pfn_swap_entry_folio(entry);
+ }
+ /* return 1 if the page is an normal ksm page or KSM-placed zero page */
+ found = (folio && folio_test_ksm(folio)) || is_ksm_zero_pte(pte);
+ if (found) {
+ *found_addr = addr;
+ goto out_unlock;
+ }
}
- /* return 1 if the page is an normal ksm page or KSM-placed zero page */
- ret = (folio && folio_test_ksm(folio)) || is_ksm_zero_pte(ptent);
- pte_unmap_unlock(pte, ptl);
- return ret;
+out_unlock:
+ pte_unmap_unlock(ptep, ptl);
+ return found;
}

static const struct mm_walk_ops break_ksm_ops = {
@@ -661,7 +681,8 @@ static const struct mm_walk_ops break_ksm_lock_vma_ops = {
* of the process that owns 'vma'. We also do not want to enforce
* protection keys here anyway.
*/
-static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_vma)
+static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
+ unsigned long end, bool lock_vma)
{
vm_fault_t ret = 0;
const struct mm_walk_ops *ops = lock_vma ?
@@ -671,11 +692,9 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_v
int ksm_page;

cond_resched();
- ksm_page = walk_page_range_vma(vma, addr, addr + 1, ops, NULL);
- if (WARN_ON_ONCE(ksm_page < 0))
+ ksm_page = walk_page_range_vma(vma, addr, end, ops, &addr);
+ if (ksm_page <= 0)
return ksm_page;
- if (!ksm_page)
- return 0;
ret = handle_mm_fault(vma, addr,
FAULT_FLAG_UNSHARE | FAULT_FLAG_REMOTE,
NULL);
@@ -761,7 +780,7 @@ static void break_cow(struct ksm_rmap_item *rmap_item)
mmap_read_lock(mm);
vma = find_mergeable_vma(mm, addr);
if (vma)
- break_ksm(vma, addr, false);
+ break_ksm(vma, addr, addr + PAGE_SIZE, false);
mmap_read_unlock(mm);
}

@@ -1072,18 +1091,7 @@ static void remove_trailing_rmap_items(struct ksm_rmap_item **rmap_list)
static int unmerge_ksm_pages(struct vm_area_struct *vma,
unsigned long start, unsigned long end, bool lock_vma)
{
- unsigned long addr;
- int err = 0;
-
- for (addr = start; addr < end && !err; addr += PAGE_SIZE) {
- if (ksm_test_exit(vma->vm_mm))
- break;
- if (signal_pending(current))
- err = -ERESTARTSYS;
- else
- err = break_ksm(vma, addr, lock_vma);
- }
- return err;
+ return break_ksm(vma, start, end, lock_vma);
}

static inline
--
2.43.0