Re: [PATCH v1 6/7] mm/ksm: convert break_ksm() to use walk_page_range_vma()
From: David Hildenbrand
Date: Fri Oct 21 2022 - 05:13:16 EST
On 06.10.22 21:28, Peter Xu wrote:
On Thu, Oct 06, 2022 at 11:20:42AM +0200, David Hildenbrand wrote:
+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;
+}
Is this needed? I thought the pgtable walker handlers this already.
[...]
Most probably yes. I was trying to avoid about PUD splits, but I guess we
simply should not care in VMAs that are considered by KSM (MERGABLE). Most
probably never ever happens.
I was surprised the split is the default approach; didn't really notice
that before. Yeah maybe better to keep it.
Interestingly, one callback reduces the benchmark result by 100-200 MiB.
With only break_ksm_pmd_entry(), I get ~4900 MiB/s instead of ~5010 MiB/s (~2%).
I came to the conclusion that we really shouldn't have to worry about pud
THPs here: it could only be a file PUD and splitting that only zaps the
entry, but doesn't PMD- or PTE-map it.
Also, I think we will barely see large pud THP in a mergable mapping ... :)
[...]
My main motivation is to remove most of that GUP hackery here, which is
1) Getting a reference on a page and waiting for migration to finish
even though both is unnecessary.
2) As we don't have sufficient control, we added FOLL_MIGRATION hacks to
MM core to work around limitations in the GUP-based approacj.
I saw one thing of adding FOLL_MIGRATION from Hugh was to have a hint for
follow page users:
I'd have preferred to avoid another flag, and do it every time, in case
someone else makes the same easy mistake..
Though..
The important thing I think is that FOLL_MIGRATION really only applies to
follow_page(). In case of "modern" GUP we will just wait for migration
entries, handle swap entries ... when triggering a page fault.
3) We rely on legacy follow_page() interface that we should really get
rid of in the long term.
..this is part of effort to remove follow_page()? More context will be
helpful in that case.
The comment from Hugh is another example why follow_page() adds complexity.
One might wonder, how pages in the swapcache, device coherent pages, ...
would have to be handled.
Short-term, I want to cleanup GUP. Long-term we might want to consider
removing follow_page() completely.
[...]
Yes, we have to extend page walking code, but it's just the natural,
non-hacky way of doing it.
Regarding the 4% performance degradation (if I wouldn't have added the
benchmarks, nobody would know and probably care ;) ), I am not quite sure
why that is the case. We're just walking page tables after all in both
cases. Maybe the callback-based implementation of pagewalk code is less
efficient, but we might be able to improve that implementation if we really
care about performance here. Maybe removing break_ksm_pud_entry() already
improves the numbers slightly.
Yeah it could be the walker is just slower. And for !ksm walking your code
should be faster when hit migration entries, but that should really be rare
anyway.
I have the following right now:
From 7f767f9e9e673a29793cd35f1c3d66ed593b67cb Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@xxxxxxxxxx>
Date: Mon, 25 Jul 2022 10:36:20 +0200
Subject: [PATCH] mm/ksm: convert break_ksm() to use walk_page_range_vma()
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.
So, get rid of follow_page() usage such that we can get rid of
FOLL_MIGRATION now and eventually be able to get rid of follow_page() in
the future.
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 ~2% (old: ~5010 MiB/s, new: ~4900 MiB/s).
I don't think we particularly care for now.
Interestingly, the benchmark reduction is due to the single callback.
Adding a second callback (e.g., pud_entry()) reduces the benchmark by
another 100-200 MiB/s.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
mm/ksm.c | 49 +++++++++++++++++++++++++++++++++++++++----------
1 file changed, 39 insertions(+), 10 deletions(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index c6f58aa6e731..5cdb852ff132 100644
--- a/mm/ksm.c
+++ b/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 +420,39 @@ 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 mm_walk *walk)
+{
+ struct page *page = NULL;
+ spinlock_t *ptl;
+ pte_t *pte;
+ int ret;
+
+ if (pmd_leaf(*pmd) || !pmd_present(*pmd))
+ return 0;
+
+ pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
+ if (pte_present(*pte)) {
+ page = vm_normal_page(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))
+ page = pfn_swap_entry_to_page(entry);
+ }
+ ret = page && PageKsm(page);
+ pte_unmap_unlock(pte, ptl);
+ return ret;
+}
+
+static const struct mm_walk_ops break_ksm_ops = {
+ .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.
@@ -434,21 +468,16 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
*/
static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
{
- struct page *page;
vm_fault_t ret = 0;
do {
- bool ksm_page = false;
+ int ksm_page;
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);
-
+ ksm_page = walk_page_range_vma(vma, addr, addr + 1,
+ &break_ksm_ops, NULL);
+ if (WARN_ON_ONCE(ksm_page < 0))
+ return ksm_page;
if (!ksm_page)
return 0;
ret = handle_mm_fault(vma, addr,
--
2.37.3
--
Thanks,
David / dhildenb