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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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.

> +#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?

> +			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,