Re: [PATCH] mm/mglru: Use folio_mark_accessed to replace folio_set_active in PF

[Andrew Morton]

On Sat, 18 Apr 2026 20:02:33 +0800 "Barry Song (Xiaomi)" <baohua@xxxxxxxxxx> wrote:

> MGLRU gives high priority to folios mapped in page tables.
> As a result, folio_set_active() is invoked for all folios
> read during page faults. In practice, however, readahead
> can bring in many folios that are never accessed via page
> tables.
> 
> A previous attempt by Lei Liu proposed introducing a separate
> LRU for readahead[1] to make readahead pages easier to reclaim,
> but that approach is likely over-engineered.
> 
> Before commit 4d5d14a01e2c ("mm/mglru: rework workingset
> protection"), folios with PG_active were always placed in
> the youngest generation, leading to over-protection and
> increased refaults. After that commit, PG_active folios
> are placed in the second youngest generation, which is
> still too optimistic given the presence of readahead. In
> contrast, the classic active/inactive scheme is more
> conservative.
> 
> This patch switches to folio_mark_accessed(). If
> folio_check_references() later detects referenced PTEs,
> the folio will be promoted based on the reference flag
> set by folio_mark_accessed().
> 
> The following uses a simple model to demonstrate why the current
> code is not ideal. It runs fio-3.42 in a memcg, reading a file in a
> strided pattern—4KB every 64KB—to simulate prefaulted pages that may
> not be accessed.

Are you able to suggest any workloads which might regress?  And test
for those?

> Without the patch, we observed 12883855 file refaults and a very low
> bandwidth of 58.5 MiB/s, because prefaulted but unused pages occupy
> hot positions, continuously pushing out the real working set and
> causing incorrect reclaim. With the patch, we observed 0 refaults
> and bandwidth increased to 5078 MiB/s.

Wow.  And that isn't a crazy workload.

> For those who want to try the model on x86, you will need the
> following in arch/x86/include/asm/pgtable.h.
> 
>  #define arch_wants_old_prefaulted_pte arch_wants_old_prefaulted_pte
>  static inline bool arch_wants_old_prefaulted_pte(void)
>  {
>  	return true;
>  }

Can you propose a patch?  We can at least toss it in there for testing
while we think about it.

> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -512,7 +512,7 @@ void folio_add_lru(struct folio *folio)
>  	/* see the comment in lru_gen_folio_seq() */
>  	if (lru_gen_enabled() && !folio_test_unevictable(folio) &&
>  	    lru_gen_in_fault() && !(current->flags & PF_MEMALLOC))
> -		folio_set_active(folio);
> +		folio_mark_accessed(folio);
>  
>  	folio_batch_add_and_move(folio, lru_add);
>  }

lol, I was expecting something larger ;)