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

[Nico Pache <npache@redhat.com>]

On Wed, Apr 30, 2025 at 4:08 AM Baolin Wang
<baolin.wang@linux.alibaba.com> 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@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..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.
    +             if (!test_bit(order, &enabled_orders))
    +                     goto next;
>
> > +                     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,
>