Re: [PATCH v3 06/12] khugepaged: introduce khugepaged_scan_bitmap for mTHP support

[Nico Pache <npache@redhat.com>]

On Mon, Apr 14, 2025 at 4:07 PM Nico Pache <npache@redhat.com> 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 MIN_MTHP_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@redhat.com>
> ---
>  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..60d41215bc1a 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      3
Somehow managed to drop
#define KHUGEPAGED_MIN_MTHP_ORDER 2
When cleaning up my patches.

Sending a V4 of just this patch in reply to this email.

Sorry for the noise...



> +#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 dfecedc6a515..5a3be30096fc 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
> +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)) {
> +                       ret = collapse_huge_page(mm, address, referenced, unmapped, cc,
> +                                       mmap_locked, order, offset * KHUGEPAGED_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,
> @@ -1445,9 +1523,7 @@ 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);
> -               /* collapse_huge_page will return with the mmap_lock released */
> -               *mmap_locked = false;
> +                                           unmapped, cc, mmap_locked, HPAGE_PMD_ORDER, 0);
>         }
>  out:
>         trace_mm_khugepaged_scan_pmd(mm, &folio->page, writable, referenced,
> --
> 2.48.1
>