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Message-ID: <CAA1CXcAFyQDZ_n2TLSLVCY0dqt0tudwaOceb+i9tDOLNo9rzZw@mail.gmail.com>
Date: Thu, 17 Jul 2025 01:23:42 -0600
From: Nico Pache <npache@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, linux-doc@...r.kernel.org,
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Subject: Re: [PATCH v9 06/14] khugepaged: introduce collapse_scan_bitmap for
mTHP support
On Wed, Jul 16, 2025 at 8:03 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 14.07.25 02:31, 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.
> >
> > collapse_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. A minimum collapse order of 2 is used as this is
> > the lowest order supported by anon memory.
> >
> > 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@...hat.com>
>
> Any reason this should not be squashed into the actual mTHP collapse patch?
I wanted to keep them seperate to conceptually separate the bitmap and
collapse, but given youre the second person to point this out, I went
ahead and squashed them and their commit messages to be one commit.
>
> In particular
>
> a) The locking changes look weird without the bigger context
>
> b) The compiler complains about unused functions
>
> > ---
> > 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 ff6120463745..0f957711a117 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
> > +#define KHUGEPAGED_MIN_MTHP_NR (1<<KHUGEPAGED_MIN_MTHP_ORDER)
>
> "1 << "
Ah there is a mix of these ("1<<" vs "1 << ") being used across the
kernel. Fixed it thank you.
>
> > +#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 ee54e3c1db4e..59b2431ca616 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,
> > };
> >
> > /**
> > @@ -838,10 +855,6 @@ static void khugepaged_alloc_sleep(void)
> > remove_wait_queue(&khugepaged_wait, &wait);
> > }
> >
> > -struct collapse_control khugepaged_collapse_control = {
> > - .is_khugepaged = true,
> > -};
> > -
> > static bool collapse_scan_abort(int nid, struct collapse_control *cc)
> > {
> > int i;
> > @@ -1115,7 +1128,8 @@ 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)
>
> Indent broken.
Fixed!
>
> > {
> > LIST_HEAD(compound_pagelist);
> > pmd_t *pmd, _pmd;
> > @@ -1134,8 +1148,12 @@ 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)
> > @@ -1272,12 +1290,72 @@ 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 collapse_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
>
>
> /* */
>
> Same applies to the other instances.
Thank you! Fixed all instances
>
> --
> Cheers,
>
> David / dhildenb
>
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