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

From: Nico Pache
Date: Mon Apr 14 2025 - 19:40:40 EST

On Mon, Apr 14, 2025 at 5:06 PM Nico Pache <npache@xxxxxxxxxx> wrote:
>
> On Mon, Apr 14, 2025 at 4:07 PM Nico Pache <npache@xxxxxxxxxx> 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@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..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...

Sorry more noise...

The #define KHUGEPAGED_MIN_MTHP_ORDER 2 fixup got merged into the
wrong commit, and is actually in 07/12. If we take this V4, the merge
will clean up the 07/12 commit with no additional changes.

If sending out a V4 of 07/12 is needed please let me know.
>
>
>
> > +#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
> >