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

Nico Pache
2025-01-10T21:49:22Z

On Fri, Jan 10, 2025 at 2:06 AM Dev Jain <dev.jain@arm.com> wrote:
>
>
>
> 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.

collapse_huge_page expects the mmap_lock to already be taken, and it
returns with it unlocked. If we are collapsing multiple mTHPs under
the same PMD, then I think we need to reacquire the lock before
calling unlock on it.

>
> > /*
> > * 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);
> > }
>