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Message-ID: <20250211003028.213461-7-npache@redhat.com>
Date: Mon, 10 Feb 2025 17:30:25 -0700
From: Nico Pache <npache@...hat.com>
To: linux-kernel@...r.kernel.org,
linux-trace-kernel@...r.kernel.org,
linux-mm@...ck.org
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Subject: [RFC v2 6/9] khugepaged: introduce khugepaged_scan_bitmap for mTHP support
khugepaged scans PMD ranges for potential collapse to a hugepage. To add
mTHP support we use this scan to instead record chunks of fully utilized
sections of the PMD.
create a bitmap to represent a PMD in order MTHP_MIN_ORDER chunks.
by default we will set this to order 3. The reasoning is that for 4K 512
PMD size this results in a 64 bit bitmap which has some optimizations.
For other arches like ARM64 64K, we can set a larger order if needed.
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 PMD.
max_ptes_none is used as a scale to determine how "full" an order must
be before being considered for collapse.
If a order is set to "always" lets always collapse to that order in a
greedy manner.
Signed-off-by: Nico Pache <npache@...hat.com>
---
include/linux/khugepaged.h | 4 ++
mm/khugepaged.c | 89 +++++++++++++++++++++++++++++++++++---
2 files changed, 86 insertions(+), 7 deletions(-)
diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
index 1f46046080f5..1fe0c4fc9d37 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 MIN_MTHP_ORDER 3
+#define MIN_MTHP_NR (1<<MIN_MTHP_ORDER)
+#define MAX_MTHP_BITMAP_SIZE (1 << (ilog2(MAX_PTRS_PER_PTE * PAGE_SIZE) - MIN_MTHP_ORDER))
+#define MTHP_BITMAP_SIZE (1 << (HPAGE_PMD_ORDER - 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 3776055bd477..c8048d9ec7fb 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 +107,15 @@ struct collapse_control {
/* nodemask for allocation fallback */
nodemask_t alloc_nmask;
+
+ /* bitmap used to collapse mTHP sizes. 1bit = order 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 +865,6 @@ 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;
@@ -1112,7 +1122,8 @@ 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;
@@ -1130,8 +1141,12 @@ 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)
@@ -1266,12 +1281,71 @@ 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
+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;
+
+ cc->mthp_bitmap_stack[++top] = (struct scan_bit_state)
+ { HPAGE_PMD_ORDER - MIN_MTHP_ORDER, 0 };
+
+ while (top >= 0) {
+ state = cc->mthp_bitmap_stack[top--];
+ order = state.order + 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
+ || test_bit(order, &huge_anon_orders_always)) {
+ ret = collapse_huge_page(mm, address, referenced, unmapped, cc,
+ mmap_locked, order, offset * MIN_MTHP_NR);
+ if (ret == SCAN_SUCCEED) {
+ collapsed += (1 << order);
+ continue;
+ }
+ }
+
+next:
+ if (state.order > 0) {
+ next_order = state.order - 1;
+ mid_offset = offset + (num_chunks / 2);
+ cc->mthp_bitmap_stack[++top] = (struct scan_bit_state)
+ { next_order, mid_offset };
+ cc->mthp_bitmap_stack[++top] = (struct scan_bit_state)
+ { next_order, offset };
+ }
+ }
+ return collapsed;
+}
+
static int khugepaged_scan_pmd(struct mm_struct *mm,
struct vm_area_struct *vma,
unsigned long address, bool *mmap_locked,
@@ -1440,7 +1514,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
pte_unmap_unlock(pte, ptl);
if (result == SCAN_SUCCEED) {
result = collapse_huge_page(mm, address, referenced,
- unmapped, cc);
+ unmapped, cc, mmap_locked, HPAGE_PMD_ORDER, 0);
/* collapse_huge_page will return with the mmap_lock released */
*mmap_locked = false;
}
@@ -2856,6 +2930,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
mmdrop(mm);
kfree(cc);
+
return thps == ((hend - hstart) >> HPAGE_PMD_SHIFT) ? 0
: madvise_collapse_errno(last_fail);
}
--
2.48.1
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