Re: [PATCH 2/3] ksm: perform a range-walk in break_ksm

[David Hildenbrand]

On 28.10.25 14:19, Pedro Demarchi Gomes wrote:
> Make break_ksm() receive an address range and change
> break_ksm_pmd_entry() to perform a range-walk and return the address of
> the first ksm page found.
>
> This change allows break_ksm() to skip unmapped regions instead of
> iterating every page address. When unmerging large sparse VMAs, this
> significantly reduces runtime, as confirmed by benchmark test (see
> cover letter).

Instead of referencing the cover letter, directly include the data here.

> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@xxxxxxxxx>
> ---
>   mm/ksm.c | 88 +++++++++++++++++++++++++++++++-------------------------
>   1 file changed, 49 insertions(+), 39 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 2a9a7fd4c777..1d1ef0554c7c 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -607,34 +607,54 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
>   	return atomic_read(&mm->mm_users) == 0;
>   }
>   
> -static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
> +struct break_ksm_arg {
> +	unsigned long addr;
> +};

You can avoid that by simply passing a pointer to addr.

> +
> +static int break_ksm_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned long end,
>   			struct mm_walk *walk)
>   {
> -	struct page *page = NULL;
> +	struct page *page;
>   	spinlock_t *ptl;
> -	pte_t *pte;
> -	pte_t ptent;
> -	int ret;
> +	pte_t *start_ptep = NULL, *ptep, pte;

Is there a need to initialize start_ptep?

> +	int ret = 0;
> +	struct mm_struct *mm = walk->mm;
> +	struct break_ksm_arg *private = (struct break_ksm_arg *) walk->private;

Prefer reverse xmas tree:

struct break_ksm_arg *private = (struct break_ksm_arg *) walk->private;
struct mm_struct *mm = walk->mm;
...

With the break_ksm_arg simplification you'd had

unsigned long *found_addr = (unsigned long *)walk->private;


>   
> -	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> -	if (!pte)
> +	if (ksm_test_exit(walk->mm))
>   		return 0;
> -	ptent = ptep_get(pte);
> -	if (pte_present(ptent)) {
> -		page = vm_normal_page(walk->vma, addr, ptent);
> -	} else if (!pte_none(ptent)) {
> -		swp_entry_t entry = pte_to_swp_entry(ptent);
>   
> -		/*
> -		 * As KSM pages remain KSM pages until freed, no need to wait
> -		 * here for migration to end.
> -		 */
> -		if (is_migration_entry(entry))
> -			page = pfn_swap_entry_to_page(entry);
> +	if (signal_pending(current))
> +		return -ERESTARTSYS;

I assume that's not a problem for the other callsites that wouldn't 
check this before.

> +
> +	start_ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +	if (!start_ptep)
> +		return 0;
> +
> +	for (ptep = start_ptep; addr < end; ptep++, addr += PAGE_SIZE) {

Best to declare pte and folio (see last mail) here in the loop:

	pte_t pte = ptep_get(ptep);
	struct folio *folio = NULL;

> +		pte = ptep_get(ptep);
> +		page = NULL;
> +		if (pte_present(pte)) {
> +			page = vm_normal_page(walk->vma, addr, pte);
> +		} else if (!pte_none(pte)) {
> +			swp_entry_t entry = pte_to_swp_entry(pte);
> +
> +			/*
> +			 * As KSM pages remain KSM pages until freed, no need to wait
> +			 * here for migration to end.
> +			 */
> +			if (is_migration_entry(entry))
> +				page = pfn_swap_entry_to_page(entry);
> +		}
> +		/* return 1 if the page is an normal ksm page or KSM-placed zero page */
> +		ret = (page && folio_test_ksm(page_folio(page))) || is_ksm_zero_pte(pte);
> +		if (ret) {
> +			private->addr = addr;
> +			goto out_unlock;
> +		}

I suggest you call "ret" "found" instead.

>   	}
> -	/* return 1 if the page is an normal ksm page or KSM-placed zero page */
> -	ret = (page && folio_test_ksm(page_folio(page))) || is_ksm_zero_pte(ptent);
> -	pte_unmap_unlock(pte, ptl);
> +out_unlock:
> +	pte_unmap_unlock(ptep, ptl);
>   	return ret;
>   }
>   
> @@ -661,9 +681,11 @@ static const struct mm_walk_ops break_ksm_lock_vma_ops = {
>    * of the process that owns 'vma'.  We also do not want to enforce
>    * protection keys here anyway.
>    */
> -static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_vma)
> +static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
> +		unsigned long end, bool lock_vma)
>   {
>   	vm_fault_t ret = 0;
> +	struct break_ksm_arg break_ksm_arg;
>   	const struct mm_walk_ops *ops = lock_vma ?
>   				&break_ksm_lock_vma_ops : &break_ksm_ops;
>   
> @@ -671,11 +693,10 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_v
>   		int ksm_page;
>   
>   		cond_resched();
> -		ksm_page = walk_page_range_vma(vma, addr, addr + 1, ops, NULL);
> -		if (WARN_ON_ONCE(ksm_page < 0))
> +		ksm_page = walk_page_range_vma(vma, addr, end, ops, &break_ksm_arg);
> +		if (ksm_page <= 0)
>   			return ksm_page;
> -		if (!ksm_page)
> -			return 0;
> +		addr = break_ksm_arg.addr;
>   		ret = handle_mm_fault(vma, addr,
>   				      FAULT_FLAG_UNSHARE | FAULT_FLAG_REMOTE,
>   				      NULL);
> @@ -761,7 +782,7 @@ static void break_cow(struct ksm_rmap_item *rmap_item)
>   	mmap_read_lock(mm);
>   	vma = find_mergeable_vma(mm, addr);
>   	if (vma)
> -		break_ksm(vma, addr, false);
> +		break_ksm(vma, addr, addr + 1, false);

Better to use addr + PAGE_SIZE

--
Cheers

David / dhildenb