The patch titled
Subject: mm/ksm: convert break_ksm() to use walk_page_range_vma()
has been added to the -mm mm-unstable branch. Its filename is
mm-ksm-convert-break_ksm-to-use-walk_page_range_vma.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-ksm-convert-break_ksm-to-use-walk_page_range_vma.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm/ksm: convert break_ksm() to use walk_page_range_vma()
Date: Fri, 30 Sep 2022 16:19:30 +0200
FOLL_MIGRATION exists only for the purpose of break_ksm(), and actually,
there is not even the need to wait for the migration to finish, we only
want to know if we're dealing with a KSM page.
Using follow_page() just to identify a KSM page overcomplicates GUP code.
Let's use walk_page_range_vma() instead, because we don't actually care
about the page itself, we only need to know a single property -- no need
to even grab a reference on the page.
In my setup (AMD Ryzen 9 3900X), running the KSM selftest to test unmerge
performance on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this results in
a performance degradation of ~4% (old: ~5010 MiB/s, new: ~4800 MiB/s). I
don't think we particularly care for now.
Link: https://lkml.kernel.org/r/20220930141931.174362-7-david@xxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/ksm.c | 70 ++++++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 62 insertions(+), 8 deletions(-)
--- a/mm/ksm.c~mm-ksm-convert-break_ksm-to-use-walk_page_range_vma
+++ a/mm/ksm.c
@@ -39,6 +39,7 @@
#include <linux/freezer.h>
#include <linux/oom.h>
#include <linux/numa.h>
+#include <linux/pagewalk.h>
#include <asm/tlbflush.h>
#include "internal.h"
@@ -419,6 +420,60 @@ static inline bool ksm_test_exit(struct
return atomic_read(&mm->mm_users) == 0;
}
+int break_ksm_pud_entry(pud_t *pud, unsigned long addr, unsigned long next,
+ struct mm_walk *walk)
+{
+ /* We only care about page tables to walk to a single base page. */
+ if (pud_leaf(*pud) || !pud_present(*pud))
+ return 1;
+ return 0;
+}
+
+int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
+ struct mm_walk *walk)
+{
+ bool *ksm_page = walk->private;
+ struct page *page = NULL;
+ pte_t *pte, ptent;
+ spinlock_t *ptl;
+
+ /* We only care about page tables to walk to a single base page. */
+ if (pmd_leaf(*pmd) || !pmd_present(*pmd))
+ return 1;
+
+ /*
+ * We only lookup a single page (a) no need to iterate; and (b)
+ * always return 1 to exit immediately and not iterate in the caller.
+ */
+ pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
+ ptent = *pte;
+
+ if (pte_none(ptent))
+ return 1;
+ if (!pte_present(ptent)) {
+ swp_entry_t entry = pte_to_swp_entry(ptent);
+
+ /*
+ * We only care about migration of KSM pages. As KSM pages
+ * remain KSM pages until freed, no need to wait here for
+ * migration to end to identify such.
+ */
+ if (is_migration_entry(entry))
+ page = pfn_swap_entry_to_page(entry);
+ } else {
+ page = vm_normal_page(walk->vma, addr, ptent);
+ }
+ if (page && PageKsm(page))
+ *ksm_page = true;
+ pte_unmap_unlock(pte, ptl);
+ return 1;
+}
+
+static const struct mm_walk_ops break_ksm_ops = {
+ .pud_entry = break_ksm_pud_entry,
+ .pmd_entry = break_ksm_pmd_entry,
+};
+
/*
* We use break_ksm to break COW on a ksm page by triggering unsharing,
* such that the ksm page will get replaced by an exclusive anonymous page.
@@ -434,20 +489,19 @@ static inline bool ksm_test_exit(struct
*/
static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
{
- struct page *page;
vm_fault_t ret = 0;
+ if (WARN_ON_ONCE(!IS_ALIGNED(addr, PAGE_SIZE)))
+ return -EINVAL;
+
do {
bool ksm_page = false;
cond_resched();
- page = follow_page(vma, addr,
- FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);
- if (IS_ERR_OR_NULL(page))
- break;
- if (PageKsm(page))
- ksm_page = true;
- put_page(page);
+ ret = walk_page_range_vma(vma, addr, addr + PAGE_SIZE,
+ &break_ksm_ops, &ksm_page);
+ if (WARN_ON_ONCE(ret < 0))
+ return ret;
if (!ksm_page)
return 0;
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
selftests-vm-add-test-to-measure-madv_unmergeable-performance.patch
mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch
mm-remove-vm_fault_write.patch
mm-ksm-fix-ksm-cow-breaking-with-userfaultfd-wp-via-fault_flag_unshare.patch
mm-pagewalk-add-walk_page_range_vma.patch
mm-ksm-convert-break_ksm-to-use-walk_page_range_vma.patch
mm-gup-remove-foll_migration.patch
