Re: [PATCH v2 5/6] mm: support batched checking of the young flag for MGLRU

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

On 2/27/26 10:44, Baolin Wang wrote:
> Use the batched helper test_and_clear_young_ptes_notify() to check and clear
> the young flag to improve the performance during large folio reclamation when
> MGLRU is enabled.
> 
> Meanwhile, we can also support batched checking the young and dirty flag
> when MGLRU walks the mm's pagetable to update the folios' generation
> counter. Since MGLRU also checks the PTE dirty bit, use folio_pte_batch_flags()
> with FPB_MERGE_YOUNG_DIRTY set to detect batches of PTEs for a large folio.
> 
> Then we can remove the ptep_test_and_clear_young_notify() since it has
> no users now.
> 
> Note that we also update the 'young' counter and 'mm_stats[MM_LEAF_YOUNG]' counter
> with the batched count in the lru_gen_look_around() and walk_pte_range(). However,
> the batched operations may inflate these two counters, because in a large folio not
> all PTEs may have been accessed. (Additionally, tracking how many PTEs have been
> accessed within a large folio is not very meaningful, since the mm core actually
> tracks access/dirty on a per-folio basis, not per page). The impact analysis is as
> follows:
> 
> 1. The 'mm_stats[MM_LEAF_YOUNG]' counter has no functional impact and is mainly for
> debugging.
> 
> 2. The 'young' counter is used to decide whether to place the current PMD entry into the
> bloom filters by suitable_to_scan() (so that next time we can check whether it has been
> accessed again), which may set the hash bit in the bloom filters for a PMD entry that
> hasn’t seen much access. However, bloom filters inherently allow some error, so this
> effect appears negligible.


Doesn't checkpatch complain about long lines in the patch description?

> 
> Reviewed-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---

...

> index a5f0a264ad56..a1b3967afe41 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1843,10 +1843,4 @@ static inline int pmdp_test_and_clear_young_notify(struct vm_area_struct *vma,
>  
>  #endif /* CONFIG_MMU_NOTIFIER */
>  
> -static inline int ptep_test_and_clear_young_notify(struct vm_area_struct *vma,
> -		unsigned long addr, pte_t *ptep)
> -{
> -	return test_and_clear_young_ptes_notify(vma, addr, ptep, 1);
> -}
> -
>  #endif	/* __MM_INTERNAL_H */
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 11cc6171344f..beb423f3e8ec 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -958,25 +958,21 @@ static bool folio_referenced_one(struct folio *folio,
>  			return false;
>  		}
>  
> +		if (pvmw.pte && folio_test_large(folio)) {
> +			const unsigned long end_addr = pmd_addr_end(address, vma->vm_end);
> +			const unsigned int max_nr = (end_addr - address) >> PAGE_SHIFT;
> +			pte_t pteval = ptep_get(pvmw.pte);
> +
> +			nr = folio_pte_batch(folio, pvmw.pte, pteval, max_nr);
> +			ptes += nr;

Could we move that "ptes += nr;" just before the "pra->mapcount -= nr;"?

Would make the whole thing look less weird (only incrementing "ptes" with large folios).

> +		}
> +
>  		if (lru_gen_enabled() && pvmw.pte) {
> -			if (lru_gen_look_around(&pvmw))
> +			if (lru_gen_look_around(&pvmw, nr))
>  				referenced++;
>  		} else if (pvmw.pte) {
> -			if (folio_test_large(folio)) {
> -				unsigned long end_addr = pmd_addr_end(address, vma->vm_end);
> -				unsigned int max_nr = (end_addr - address) >> PAGE_SHIFT;
> -				pte_t pteval = ptep_get(pvmw.pte);
> -
> -				nr = folio_pte_batch(folio, pvmw.pte,
> -						     pteval, max_nr);
> -			}
> -
> -			ptes += nr;
>  			if (clear_flush_young_ptes_notify(vma, address, pvmw.pte, nr))
>  				referenced++;
> -			/* Skip the batched PTEs */
> -			pvmw.pte += nr - 1;
> -			pvmw.address += (nr - 1) * PAGE_SIZE;
>  		} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
>  			if (pmdp_clear_flush_young_notify(vma, address,
>  						pvmw.pmd))
> @@ -995,6 +991,10 @@ static bool folio_referenced_one(struct folio *folio,
>  			page_vma_mapped_walk_done(&pvmw);
>  			break;
>  		}
> +
> +		/* Skip the batched PTEs */
> +		pvmw.pte += nr - 1;
> +		pvmw.address += (nr - 1) * PAGE_SIZE;
>  	}
>  
>  	if (referenced)
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 0a5622420987..7457b3c06fa3 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3474,6 +3474,7 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
>  	struct pglist_data *pgdat = lruvec_pgdat(walk->lruvec);
>  	DEFINE_MAX_SEQ(walk->lruvec);
>  	int gen = lru_gen_from_seq(max_seq);
> +	unsigned int nr;
>  	pmd_t pmdval;
>  
>  	pte = pte_offset_map_rw_nolock(args->mm, pmd, start & PMD_MASK, &pmdval, &ptl);
> @@ -3492,11 +3493,13 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
>  
>  	lazy_mmu_mode_enable();
>  restart:
> -	for (i = pte_index(start), addr = start; addr != end; i++, addr += PAGE_SIZE) {
> +	for (i = pte_index(start), addr = start; addr != end; i += nr, addr += nr * PAGE_SIZE) {
>  		unsigned long pfn;
>  		struct folio *folio;
> -		pte_t ptent = ptep_get(pte + i);
> +		pte_t *cur_pte = pte + i;
> +		pte_t ptent = ptep_get(cur_pte);
>  
> +		nr = 1;


Looking at this again, we should get rid of "i" completely and instead
* rename pte to start_pte
* Add a new pte, which we increment in the loop

So we end up with something like

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 54cf4924d223..150cbb2253b9 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3486,9 +3486,8 @@ static void walk_update_folio(struct lru_gen_mm_walk *walk, struct folio *folio,
 static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
                           struct mm_walk *args)
 {
-       int i;
        bool dirty;
-       pte_t *pte;
+       pte_t *start_pte, pte;
        spinlock_t *ptl;
        unsigned long addr;
        int total = 0;
@@ -3499,29 +3498,31 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
        struct pglist_data *pgdat = lruvec_pgdat(walk->lruvec);
        DEFINE_MAX_SEQ(walk->lruvec);
        int gen = lru_gen_from_seq(max_seq);
+       unsigned int nr;
        pmd_t pmdval;
 
-       pte = pte_offset_map_rw_nolock(args->mm, pmd, start & PMD_MASK, &pmdval, &ptl);
-       if (!pte)
+       start_pte = pte_offset_map_rw_nolock(args->mm, pmd, start & PMD_MASK, &pmdval, &ptl);
+       if (!start_pte)
                return false;
 
        if (!spin_trylock(ptl)) {
-               pte_unmap(pte);
+               pte_unmap(start_pte);
                return true;
        }
 
        if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmd)))) {
-               pte_unmap_unlock(pte, ptl);
+               pte_unmap_unlock(start_pte, ptl);
                return false;
        }
 
        lazy_mmu_mode_enable();
 restart:
-       for (i = pte_index(start), addr = start; addr != end; i++, addr += PAGE_SIZE) {
+       for (addr = start, pte = start_pte; addr != end; addr += nr * PAGE_SIZE, pte += nr) {
                unsigned long pfn;
                struct folio *folio;
-               pte_t ptent = ptep_get(pte + i);
+               pte_t ptent = ptep_get(pte);
 
+               nr = 1;
                total++;
                walk->mm_stats[MM_LEAF_TOTAL]++;
 
@@ -3533,7 +3534,16 @@ static bool walk_pte_range(pmd_t *pmd, unsigned long start, unsigned long end,
                if (!folio)
                        continue;


...


>  	walk_update_folio(walk, last, gen, dirty);
> @@ -4166,7 +4178,7 @@ static void lru_gen_age_node(struct pglist_data *pgdat, struct scan_control *sc)
>   * the PTE table to the Bloom filter. This forms a feedback loop between the
>   * eviction and the aging.
>   */
> -bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
> +bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw, unsigned int nr)
>  {
>  	int i;
>  	bool dirty;
> @@ -4184,12 +4196,13 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
>  	struct lruvec *lruvec;
>  	struct lru_gen_mm_state *mm_state;
>  	unsigned long max_seq;
> +	pte_t *cur_pte;
>  	int gen;
>  
>  	lockdep_assert_held(pvmw->ptl);
>  	VM_WARN_ON_ONCE_FOLIO(folio_test_lru(folio), folio);
>  
> -	if (!ptep_test_and_clear_young_notify(vma, addr, pte))
> +	if (!test_and_clear_young_ptes_notify(vma, addr, pte, nr))
>  		return false;
>  
>  	if (spin_is_contended(pvmw->ptl))
> @@ -4229,10 +4242,12 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
>  
>  	pte -= (addr - start) / PAGE_SIZE;
>  
> -	for (i = 0, addr = start; addr != end; i++, addr += PAGE_SIZE) {
> +	for (i = 0, addr = start, cur_pte = pte; addr != end;
> +	     i += nr, cur_pte += nr, addr += nr * PAGE_SIZE) {
>  		unsigned long pfn;
> -		pte_t ptent = ptep_get(pte + i);
> +		pte_t ptent = ptep_get(cur_pte);
>  
> +		nr = 1;

Can't you just use pte and increment that, right?

"pte" is not used afterwards.

-- 
Cheers,

David