Re: [PATCH] mm: khugepaged: simplify scanning progress in pmd

[Vernon Yang <vernon2gm@gmail.com>]

On Thu, Feb 26, 2026 at 09:42:41AM +0100, David Hildenbrand (Arm) wrote:
> On 2/26/26 09:13, Vernon Yang wrote:
> > From: Vernon Yang <yanglincheng@kylinos.cn>
> >
> > Placing "cur_progress" inside "struct collapse_control" makes the
> > overall code simpler, there also coincidentally has a 4-bytes hole,
> > as shown below:
> >
> > struct collapse_control {
> >         bool                       is_khugepaged;        /*     0     1 */
> >         /* XXX 3 bytes hole, try to pack */
> >         u32                        node_load[64];        /*     4   256 */
> >         /* XXX 4 bytes hole, try to pack */
> >         /* --- cacheline 4 boundary (256 bytes) was 8 bytes ago --- */
> >         nodemask_t                 alloc_nmask;          /*   264     8 */
> >
> >         /* size: 272, cachelines: 5, members: 3 */
> >         /* sum members: 265, holes: 2, sum holes: 7 */
> >         /* last cacheline: 16 bytes */
> > };
> >
> > Also rename "cur_progress" to "progress_in_pmd", make it clearer.
> >
> > No function changes.
> >
> > Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> > ---
>
> We should really squash that into the original patch and revisit it.

Yes, squashing this patch into the original patch makes the git commit
log clearer.

> Because I think we might be able to do even better.
>
> Could we just switch to "cc->progress" like so (modified original patch):
>
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4d7baf220ad9..5d67e5e60ece 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -68,7 +68,10 @@ enum scan_result {
>  static struct task_struct *khugepaged_thread __read_mostly;
>  static DEFINE_MUTEX(khugepaged_mutex);
>
> -/* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */
> +/*
> + * default scan 8*HPAGE_PMD_NR ptes, pte_mapped_hugepage, pmd_mapped,
> + * no_pte_table or vmas every 10 second.
> + */
>  static unsigned int khugepaged_pages_to_scan __read_mostly;
>  static unsigned int khugepaged_pages_collapsed;
>  static unsigned int khugepaged_full_scans;
> @@ -100,6 +103,9 @@ struct collapse_control {
>  	/* Num pages scanned per node */
>  	u32 node_load[MAX_NUMNODES];
>
> +	/* Num pages scanned (see khugepaged_pages_to_scan).  */
> +	unsigned int progress;
> +
>  	/* nodemask for allocation fallback */
>  	nodemask_t alloc_nmask;
>  };
> @@ -1247,19 +1253,24 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm,
>  	VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);
>
>  	result = find_pmd_or_thp_or_none(mm, start_addr, &pmd);
> -	if (result != SCAN_SUCCEED)
> +	if (result != SCAN_SUCCEED) {
> +		cc->progress++;
>  		goto out;
> +	}
>
>  	memset(cc->node_load, 0, sizeof(cc->node_load));
>  	nodes_clear(cc->alloc_nmask);
>  	pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl);
>  	if (!pte) {
> +		cc->progress++;
>  		result = SCAN_NO_PTE_TABLE;
>  		goto out;
>  	}
>
>  	for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
>  	     _pte++, addr += PAGE_SIZE) {
> +		cc->progress++;
> +
>  		pte_t pteval = ptep_get(_pte);
>  		if (pte_none_or_zero(pteval)) {
>  			++none_or_zero;
> @@ -2370,6 +2381,10 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
>  		}
>  	}
>  	rcu_read_unlock();
> +	if (result == SCAN_PTE_MAPPED_HUGEPAGE)
> +		cc->progress++;
> +	else
> +		cc->progress += HPAGE_PMD_NR;
>
>  	if (result == SCAN_SUCCEED) {
>  		if (cc->is_khugepaged &&
> @@ -2385,8 +2400,8 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
>  	return result;
>  }
>
> -static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result *result,
> -					    struct collapse_control *cc)
> +static void khugepaged_scan_mm_slot(unsigned int progress_max,
> +		enum scan_result *result, struct collapse_control *cc)
>  	__releases(&khugepaged_mm_lock)
>  	__acquires(&khugepaged_mm_lock)
>  {
> @@ -2394,9 +2409,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
>  	struct mm_slot *slot;
>  	struct mm_struct *mm;
>  	struct vm_area_struct *vma;
> -	int progress = 0;
>
> -	VM_BUG_ON(!pages);
>  	lockdep_assert_held(&khugepaged_mm_lock);
>  	*result = SCAN_FAIL;
>
> @@ -2419,7 +2432,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
>  	if (unlikely(!mmap_read_trylock(mm)))
>  		goto breakouterloop_mmap_lock;
>
> -	progress++;
> +	cc->progress++;
>  	if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
>  		goto breakouterloop;
>
> @@ -2429,17 +2442,17 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
>
>  		cond_resched();
>  		if (unlikely(hpage_collapse_test_exit_or_disable(mm))) {
> -			progress++;
> +			cc->progress++;
>  			break;
>  		}
>  		if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_KHUGEPAGED, PMD_ORDER)) {
> -			progress++;
> +			cc->progress++;
>  			continue;
>  		}
>  		hstart = round_up(vma->vm_start, HPAGE_PMD_SIZE);
>  		hend = round_down(vma->vm_end, HPAGE_PMD_SIZE);
>  		if (khugepaged_scan.address > hend) {
> -			progress++;
> +			cc->progress++;
>  			continue;
>  		}
>  		if (khugepaged_scan.address < hstart)
> @@ -2486,7 +2499,6 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
>
>  			/* move to next address */
>  			khugepaged_scan.address += HPAGE_PMD_SIZE;
> -			progress += HPAGE_PMD_NR;
>  			if (!mmap_locked)
>  				/*
>  				 * We released mmap_lock so break loop.  Note
> @@ -2496,7 +2508,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
>  				 * correct result back to caller.
>  				 */
>  				goto breakouterloop_mmap_lock;
> -			if (progress >= pages)
> +			if (cc->progress >= progress_max)
>  				goto breakouterloop;
>  		}
>  	}
> @@ -2527,9 +2539,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
>  		collect_mm_slot(slot);
>  	}
>
> -	trace_mm_khugepaged_scan(mm, progress, khugepaged_scan.mm_slot == NULL);
> -
> -	return progress;
> +	trace_mm_khugepaged_scan(mm, cc->progress, khugepaged_scan.mm_slot == NULL);
>  }
>
>  static int khugepaged_has_work(void)
> @@ -2545,13 +2555,14 @@ static int khugepaged_wait_event(void)
>
>  static void khugepaged_do_scan(struct collapse_control *cc)
>  {
> -	unsigned int progress = 0, pass_through_head = 0;
> -	unsigned int pages = READ_ONCE(khugepaged_pages_to_scan);
> +	const unsigned int progress_max = READ_ONCE(khugepaged_pages_to_scan);
> +	unsigned int pass_through_head = 0;
>  	bool wait = true;
>  	enum scan_result result = SCAN_SUCCEED;
>
>  	lru_add_drain_all();
>
> +	cc->progress = 0;

madvise(MADV_COLLAPSE) is missing the initialization of "cc->progress"
because "cc = kmalloc_obj(*cc)". We need to manually initialize it in
madvise_collapse().

LGTM, I will submit v2 with this patch and squashed into the original
patch.

--
Cheers,
Vernon