Re: [PATCH v5 06/12] khugepaged: introduce khugepaged_scan_bitmap for mTHP support

From: Baolin Wang
Date: Thu May 01 2025 - 21:24:15 EST



On 2025/5/2 07:03, Nico Pache wrote:
On Wed, Apr 30, 2025 at 12:56 PM Nico Pache <npache@xxxxxxxxxx> wrote:

On Wed, Apr 30, 2025 at 4:08 AM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:



On 2025/4/29 02:12, Nico Pache wrote:
khugepaged scans anons PMD ranges for potential collapse to a hugepage.
To add mTHP support we use this scan to instead record chunks of utilized
sections of the PMD.

khugepaged_scan_bitmap uses a stack struct to recursively scan a bitmap
that represents chunks of utilized regions. We can then determine what
mTHP size fits best and in the following patch, we set this bitmap while
scanning the anon PMD.

max_ptes_none is used as a scale to determine how "full" an order must
be before being considered for collapse.

When attempting to collapse an order that has its order set to "always"
lets always collapse to that order in a greedy manner without
considering the number of bits set.

Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
---
include/linux/khugepaged.h | 4 ++
mm/khugepaged.c | 94 ++++++++++++++++++++++++++++++++++----
2 files changed, 89 insertions(+), 9 deletions(-)

diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
index 1f46046080f5..18fe6eb5051d 100644
--- a/include/linux/khugepaged.h
+++ b/include/linux/khugepaged.h
@@ -1,6 +1,10 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_KHUGEPAGED_H
#define _LINUX_KHUGEPAGED_H
+#define KHUGEPAGED_MIN_MTHP_ORDER 2

Still better to add some comments to explain explicitly why choose 2 as
the MIN_MTHP_ORDER.
Ok i'll add a note that explicitly states that the min order of anon mTHPs is 2

+#define KHUGEPAGED_MIN_MTHP_NR (1<<KHUGEPAGED_MIN_MTHP_ORDER)
+#define MAX_MTHP_BITMAP_SIZE (1 << (ilog2(MAX_PTRS_PER_PTE) - KHUGEPAGED_MIN_MTHP_ORDER))
+#define MTHP_BITMAP_SIZE (1 << (HPAGE_PMD_ORDER - KHUGEPAGED_MIN_MTHP_ORDER))

extern unsigned int khugepaged_max_ptes_none __read_mostly;
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e21998a06253..6e67db86409a 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -94,6 +94,11 @@ static DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);

static struct kmem_cache *mm_slot_cache __ro_after_init;

+struct scan_bit_state {
+ u8 order;
+ u16 offset;
+};
+
struct collapse_control {
bool is_khugepaged;

@@ -102,6 +107,18 @@ struct collapse_control {

/* nodemask for allocation fallback */
nodemask_t alloc_nmask;
+
+ /*
+ * bitmap used to collapse mTHP sizes.
+ * 1bit = order KHUGEPAGED_MIN_MTHP_ORDER mTHP
+ */
+ DECLARE_BITMAP(mthp_bitmap, MAX_MTHP_BITMAP_SIZE);
+ DECLARE_BITMAP(mthp_bitmap_temp, MAX_MTHP_BITMAP_SIZE);
+ struct scan_bit_state mthp_bitmap_stack[MAX_MTHP_BITMAP_SIZE];
+};
+
+struct collapse_control khugepaged_collapse_control = {
+ .is_khugepaged = true,
};

/**
@@ -851,10 +868,6 @@ static void khugepaged_alloc_sleep(void)
remove_wait_queue(&khugepaged_wait, &wait);
}

-struct collapse_control khugepaged_collapse_control = {
- .is_khugepaged = true,
-};
-
static bool khugepaged_scan_abort(int nid, struct collapse_control *cc)
{
int i;
@@ -1118,7 +1131,8 @@ static int alloc_charge_folio(struct folio **foliop, struct mm_struct *mm,

static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
int referenced, int unmapped,
- struct collapse_control *cc)
+ struct collapse_control *cc, bool *mmap_locked,
+ u8 order, u16 offset)
{
LIST_HEAD(compound_pagelist);
pmd_t *pmd, _pmd;
@@ -1137,8 +1151,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
* The allocation can take potentially a long time if it involves
* sync compaction, and we do not need to hold the mmap_lock during
* that. We will recheck the vma after taking it again in write mode.
+ * If collapsing mTHPs we may have already released the read_lock.
*/
- mmap_read_unlock(mm);
+ if (*mmap_locked) {
+ mmap_read_unlock(mm);
+ *mmap_locked = false;
+ }

result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
if (result != SCAN_SUCCEED)
@@ -1273,12 +1291,72 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
out_up_write:
mmap_write_unlock(mm);
out_nolock:
+ *mmap_locked = false;
if (folio)
folio_put(folio);
trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result);
return result;
}

+// Recursive function to consume the bitmap

Nit: please use '/* Xxxx */' for comments in this patch.

+static int khugepaged_scan_bitmap(struct mm_struct *mm, unsigned long address,
+ int referenced, int unmapped, struct collapse_control *cc,
+ bool *mmap_locked, unsigned long enabled_orders)
+{
+ u8 order, next_order;
+ u16 offset, mid_offset;
+ int num_chunks;
+ int bits_set, threshold_bits;
+ int top = -1;
+ int collapsed = 0;
+ int ret;
+ struct scan_bit_state state;
+ bool is_pmd_only = (enabled_orders == (1 << HPAGE_PMD_ORDER));
+
+ cc->mthp_bitmap_stack[++top] = (struct scan_bit_state)
+ { HPAGE_PMD_ORDER - KHUGEPAGED_MIN_MTHP_ORDER, 0 };
+
+ while (top >= 0) {
+ state = cc->mthp_bitmap_stack[top--];
+ order = state.order + KHUGEPAGED_MIN_MTHP_ORDER;
+ offset = state.offset;
+ num_chunks = 1 << (state.order);
+ // Skip mTHP orders that are not enabled
+ if (!test_bit(order, &enabled_orders))
+ goto next;
+
+ // copy the relavant section to a new bitmap
+ bitmap_shift_right(cc->mthp_bitmap_temp, cc->mthp_bitmap, offset,
+ MTHP_BITMAP_SIZE);
+
+ bits_set = bitmap_weight(cc->mthp_bitmap_temp, num_chunks);
+ threshold_bits = (HPAGE_PMD_NR - khugepaged_max_ptes_none - 1)
+ >> (HPAGE_PMD_ORDER - state.order);
+
+ //Check if the region is "almost full" based on the threshold
+ if (bits_set > threshold_bits || is_pmd_only
+ || test_bit(order, &huge_anon_orders_always)) {

When testing this patch, I disabled the PMD-sized THP and enabled
64K-sized mTHP, but it still attempts to collapse into a PMD-sized THP
(since bits_set > threshold_bits is ture). This doesn't seem reasonable?
We are still required to have PMD enabled for mTHP collapse to work.
It's a limitation of the current khugepaged code (it currently only
adds mm_slots when PMD is enabled).
We've discussed this in the past and are looking for a proper way
forward, but the solution becomes tricky.

However I'm surprised that it still collapses due to the code below.
I'll test this out later today.

Following up, you are correct, if I disable the PMD size within the
mTHP enabled settings (echo never >
/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled) it still
collapses to PMDs. I believe the global variable takes precedent. I'm
not sure what the correct behavior is... I will look into it further

+ if (!test_bit(order, &enabled_orders))
+ goto next;

IMO, we should respect the mTHP sysfs control interfaces and use the 'TVA_ENFORCE_SYSFS' flag when determining allowable orders through thp_vma_allowable_orders().