Re: [RFC 08/11] khugepaged: introduce khugepaged_scan_bitmap for mTHP support

Dev Jain
2025-01-10T09:06:06Z

On 09/01/25 5:01 am, Nico Pache wrote:
> 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 fully 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.
>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
> include/linux/khugepaged.h | 4 +-
> mm/khugepaged.c | 129 +++++++++++++++++++++++++++++++++++--
> 2 files changed, 126 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
> index 1f46046080f5..31cff8aeec4a 100644
> --- a/include/linux/khugepaged.h
> +++ b/include/linux/khugepaged.h
> @@ -1,7 +1,9 @@
> /* SPDX-License-Identifier: GPL-2.0 */
> #ifndef _LINUX_KHUGEPAGED_H
> #define _LINUX_KHUGEPAGED_H
> -

Nit: I don't think this line needs to be deleted.

> +#define MIN_MTHP_ORDER 3
> +#define MIN_MTHP_NR (1<<MIN_MTHP_ORDER)

Nit: Insert a space: (1 << 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
> extern struct attribute_group khugepaged_attr_group;
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 9eb161b04ee4..de1dc6ea3c71 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;
> + u8 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 */
> + unsigned long *mthp_bitmap;
> + unsigned long *mthp_bitmap_temp;
> + struct scan_bit_state *mthp_bitmap_stack;
> +};
> +
> +struct collapse_control khugepaged_collapse_control = {
> + .is_khugepaged = true,
> };
>
> /**
> @@ -389,6 +403,25 @@ int __init khugepaged_init(void)
> if (!mm_slot_cache)
> return -ENOMEM;
>
> + /*
> + * allocate the bitmaps dynamically since MTHP_BITMAP_SIZE is not known at
> + * compile time for some architectures.
> + */
> + khugepaged_collapse_control.mthp_bitmap = kmalloc_array(
> + BITS_TO_LONGS(MTHP_BITMAP_SIZE), sizeof(unsigned long), GFP_KERNEL);
> + if (!khugepaged_collapse_control.mthp_bitmap)
> + return -ENOMEM;
> +
> + khugepaged_collapse_control.mthp_bitmap_temp = kmalloc_array(
> + BITS_TO_LONGS(MTHP_BITMAP_SIZE), sizeof(unsigned long), GFP_KERNEL);
> + if (!khugepaged_collapse_control.mthp_bitmap_temp)
> + return -ENOMEM;
> +
> + khugepaged_collapse_control.mthp_bitmap_stack = kmalloc_array(
> + MTHP_BITMAP_SIZE, sizeof(struct scan_bit_state), GFP_KERNEL);
> + if (!khugepaged_collapse_control.mthp_bitmap_stack)
> + return -ENOMEM;
> +
> khugepaged_pages_to_scan = HPAGE_PMD_NR * 8;
> khugepaged_max_ptes_none = HPAGE_PMD_NR - 1;
> khugepaged_max_ptes_swap = HPAGE_PMD_NR / 8;
> @@ -400,6 +433,9 @@ int __init khugepaged_init(void)
> void __init khugepaged_destroy(void)
> {
> kmem_cache_destroy(mm_slot_cache);
> + kfree(khugepaged_collapse_control.mthp_bitmap);
> + kfree(khugepaged_collapse_control.mthp_bitmap_temp);
> + kfree(khugepaged_collapse_control.mthp_bitmap_stack);
> }
>
> static inline int khugepaged_test_exit(struct mm_struct *mm)
> @@ -850,10 +886,6 @@ 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;
> @@ -1102,7 +1134,8 @@ 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,
> + int order, int offset)
> {
> LIST_HEAD(compound_pagelist);
> pmd_t *pmd, _pmd;
> @@ -1115,6 +1148,11 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> struct mmu_notifier_range range;
> VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>
> + /* if collapsing mTHPs we may have already released the read_lock, and
> + * need to reaquire it to keep the proper locking order.
> + */
> + if (!*mmap_locked)
> + mmap_read_lock(mm);

There is no need to take the read lock again, because we drop it just
after this.

> /*
> * Before allocating the hugepage, release the mmap_lock read lock.
> * The allocation can take potentially a long time if it involves
> @@ -1122,6 +1160,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> * that. We will recheck the vma after taking it again in write mode.
> */
> mmap_read_unlock(mm);
> + *mmap_locked = false;
>
> result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER);
> if (result != SCAN_SUCCEED)
> @@ -1256,12 +1295,71 @@ 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, offset;
> + int num_chunks;
> + int bits_set, max_percent, threshold_bits;
> + int next_order, mid_offset;
> + 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;
> + offset = state.offset;
> + num_chunks = 1 << order;
> + // Skip mTHP orders that are not enabled
> + if (!(enabled_orders >> (order + MIN_MTHP_ORDER)) & 1)
> + 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);
> +
> + // Check if the region is "almost full" based on the threshold
> + max_percent = ((HPAGE_PMD_NR - khugepaged_max_ptes_none - 1) * 100)
> + / (HPAGE_PMD_NR - 1);
> + threshold_bits = (max_percent * num_chunks) / 100;
> +
> + if (bits_set >= threshold_bits) {
> + ret = collapse_huge_page(mm, address, referenced, unmapped, cc,
> + mmap_locked, order + MIN_MTHP_ORDER, offset * MIN_MTHP_NR);
> + if (ret == SCAN_SUCCEED)
> + collapsed += (1 << (order + MIN_MTHP_ORDER));
> + continue;
> + }
> +
> +next:
> + if (order > 0) {
> + next_order = 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,
> @@ -1430,7 +1528,7 @@ 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;
> }
> @@ -2767,6 +2865,21 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
> return -ENOMEM;
> cc->is_khugepaged = false;
>
> + cc->mthp_bitmap = kmalloc_array(
> + BITS_TO_LONGS(MTHP_BITMAP_SIZE), sizeof(unsigned long), GFP_KERNEL);
> + if (!cc->mthp_bitmap)
> + return -ENOMEM;
> +
> + cc->mthp_bitmap_temp = kmalloc_array(
> + BITS_TO_LONGS(MTHP_BITMAP_SIZE), sizeof(unsigned long), GFP_KERNEL);
> + if (!cc->mthp_bitmap_temp)
> + return -ENOMEM;
> +
> + cc->mthp_bitmap_stack = kmalloc_array(
> + MTHP_BITMAP_SIZE, sizeof(struct scan_bit_state), GFP_KERNEL);
> + if (!cc->mthp_bitmap_stack)
> + return -ENOMEM;
> +
> mmgrab(mm);
> lru_add_drain_all();
>
> @@ -2831,8 +2944,12 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
> out_nolock:
> mmap_assert_locked(mm);
> mmdrop(mm);
> + kfree(cc->mthp_bitmap);
> + kfree(cc->mthp_bitmap_temp);
> + kfree(cc->mthp_bitmap_stack);
> kfree(cc);
>
> +
> return thps == ((hend - hstart) >> HPAGE_PMD_SHIFT) ? 0
> : madvise_collapse_errno(last_fail);
> }