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Message-ID: <844af749-374e-49b3-91f0-a72e951981c8@redhat.com>
Date: Sat, 18 Oct 2025 00:23:34 +0200
From: David Hildenbrand <david@...hat.com>
To: Pedro Demarchi Gomes <pedrodemargomes@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Xu Xin <xu.xin16@....com.cn>, craftfever <craftfever@...mail.cc>,
Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] ksm: use range-walk function to jump over holes in
scan_get_next_rmap_item
On 16.10.25 03:22, Pedro Demarchi Gomes wrote:
> Currently, scan_get_next_rmap_item() walks every page address in a VMA
> to locate mergeable pages. This becomes highly inefficient when scanning
> large virtual memory areas that contain mostly unmapped regions.
>
> This patch replaces the per-address lookup with a range walk using
> walk_page_range(). The range walker allows KSM to skip over entire
> unmapped holes in a VMA, avoiding unnecessary lookups.
> This problem was previously discussed in [1].
>
> [1] https://lore.kernel.org/linux-mm/423de7a3-1c62-4e72-8e79-19a6413e420c@redhat.com/
>
> ---
This patch does to much in a single patch which makes it
rather hard to review.
As a first step, we should focus on leaving most of
scan_get_next_rmap_item() alone and only focus on replacing
folio_walk by walk_page_range_vma().
Follow-up cleanups could try cleaning up scan_get_next_rmap_item()
-- and boy oh boy, does that function scream for quite some cleanups.
This is something minimal based on your v3. I applied plenty of more
cleanups and I wish we could further shrink the pmd_entry function,
but I have to give up for today (well, it's already tomorrow :) ).
Briefly tested with ksm selftests and my machine did not burn down my building.
From d971b88056fe3fefe50e5d4fa5b359e8c8331b2c Mon Sep 17 00:00:00 2001
From: Pedro Demarchi Gomes <pedrodemargomes@...il.com>
Date: Wed, 15 Oct 2025 22:22:36 -0300
Subject: [PATCH] ksm: use range-walk function to jump over holes in
scan_get_next_rmap_item
Currently, scan_get_next_rmap_item() walks every page address in a VMA
to locate mergeable pages. This becomes highly inefficient when scanning
large virtual memory areas that contain mostly unmapped regions.
This patch replaces the per-address lookup with a range walk using
walk_page_range_vma(). The range walker allows KSM to skip over entire
unmapped holes in a VMA, avoiding unnecessary lookups.
This problem was previously discussed in [1].
[1] https://lore.kernel.org/linux-mm/423de7a3-1c62-4e72-8e79-19a6413e420c@redhat.com/
Reported-by: craftfever <craftfever@...mail.cc>
Closes: https://lkml.kernel.org/r/020cf8de6e773bb78ba7614ef250129f11a63781@murena.io
Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@...il.com>
Co-developed-by: David Hildenbrand <david@...hat.com>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
mm/ksm.c | 116 ++++++++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 103 insertions(+), 13 deletions(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index 3aed0478fdcef..8bd2b78c4f869 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2455,6 +2455,94 @@ static bool should_skip_rmap_item(struct folio *folio,
return true;
}
+struct ksm_next_page_arg {
+ struct folio *folio;
+ struct page *page;
+ unsigned long addr;
+};
+
+static int ksm_next_page_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned long end,
+ struct mm_walk *walk)
+{
+ struct ksm_next_page_arg *private = walk->private;
+ struct vm_area_struct *vma = walk->vma;
+ pte_t *start_ptep = NULL, *ptep, pte;
+ struct mm_struct *mm = walk->mm;
+ struct folio *folio;
+ struct page *page;
+ spinlock_t *ptl;
+ pmd_t pmd;
+
+ if (ksm_test_exit(mm))
+ return 0;
+ cond_resched();
+
+ pmd = pmdp_get_lockless(pmdp);
+ if (!pmd_present(pmd))
+ return 0;
+
+ if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && pmd_leaf(pmd)) {
+ ptl = pmd_lock(mm, pmdp);
+ pmd = pmdp_get(pmdp);
+
+ if (!pmd_present(pmd)) {
+ goto not_found_unlock;
+ } else if (pmd_leaf(pmd)) {
+ page = vm_normal_page_pmd(vma, addr, pmd);
+ if (!page)
+ goto not_found_unlock;
+ folio = page_folio(page);
+
+ if (folio_is_zone_device(folio) || !folio_test_anon(folio))
+ goto not_found_unlock;
+
+ page += ((addr & (PMD_SIZE - 1)) >> PAGE_SHIFT);
+ goto found_unlock;
+ }
+ spin_unlock(ptl);
+ }
+
+ 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 = ptep_get(ptep);
+
+ if (!pte_present(pte))
+ continue;
+
+ page = vm_normal_page(vma, addr, pte);
+ if (!page)
+ continue;
+ folio = page_folio(page);
+
+ if (folio_is_zone_device(folio) || !folio_test_anon(folio))
+ continue;
+ goto found_unlock;
+ }
+
+not_found_unlock:
+ spin_unlock(ptl);
+ if (start_ptep)
+ pte_unmap(start_ptep);
+ return 0;
+found_unlock:
+ folio_get(folio);
+ spin_unlock(ptl);
+ if (start_ptep)
+ pte_unmap(start_ptep);
+ private->page = page;
+ private->folio = folio;
+ private->addr = addr;
+ return 1;
+}
+
+static struct mm_walk_ops ksm_next_page_ops = {
+ .pmd_entry = ksm_next_page_pmd_entry,
+ .walk_lock = PGWALK_RDLOCK,
+};
+
static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
{
struct mm_struct *mm;
@@ -2542,21 +2630,23 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
ksm_scan.address = vma->vm_end;
while (ksm_scan.address < vma->vm_end) {
+ struct ksm_next_page_arg ksm_next_page_arg;
struct page *tmp_page = NULL;
- struct folio_walk fw;
struct folio *folio;
-
- if (ksm_test_exit(mm))
- break;
-
- folio = folio_walk_start(&fw, vma, ksm_scan.address, 0);
- if (folio) {
- if (!folio_is_zone_device(folio) &&
- folio_test_anon(folio)) {
- folio_get(folio);
- tmp_page = fw.page;
- }
- folio_walk_end(&fw, vma);
+ int found;
+
+ found = walk_page_range_vma(vma, ksm_scan.address,
+ vma->vm_end,
+ &ksm_next_page_ops,
+ &ksm_next_page_arg);
+
+ if (found > 0) {
+ folio = ksm_next_page_arg.folio;
+ tmp_page = ksm_next_page_arg.page;
+ ksm_scan.address = ksm_next_page_arg.addr;
+ } else {
+ VM_WARN_ON_ONCE(found < 0);
+ ksm_scan.address = vma->vm_end - PAGE_SIZE;
}
if (tmp_page) {
--
2.51.0
--
Cheers
David / dhildenb
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