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

["David Hildenbrand (Arm)" <david@kernel.org>]

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.

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;
 	while (true) {
 		cond_resched();
 
@@ -2563,13 +2574,12 @@ static void khugepaged_do_scan(struct collapse_control *cc)
 			pass_through_head++;
 		if (khugepaged_has_work() &&
 		    pass_through_head < 2)
-			progress += khugepaged_scan_mm_slot(pages - progress,
-							    &result, cc);
+			khugepaged_scan_mm_slot(progress_max, &result, cc);
 		else
-			progress = pages;
+			cc->progress = progress_max;
 		spin_unlock(&khugepaged_mm_lock);
 
-		if (progress >= pages)
+		if (cc->progress >= progress_max)
 			break;
 
 		if (result == SCAN_ALLOC_HUGE_PAGE_FAIL) {
-- 
2.43.0


-- 
Cheers,

David