Re: [PATCH RFC 09/15] mm/mglru: frequency guided workingset promotion (MGLRU-FG)

[Barry Song]

On Tue, Aug 4, 2026 at 3:47 AM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@xxxxxxxxxx> wrote:
>
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> Complement MGLRU's eviction-time tier-PID protection with access-time
> frequency-guided promotion.  Introduce a unified set of helpers built based
> on referenced (access) count of a folio.
>
> Each access increments a folio's referenced count stored in folio flags
> (refs), refs still mappes to a logarithmic tier just like before, but with
> more formal bit definitions, a few special thresholds are introduced:
> LRU_REFS_REFERENCED (1), LRU_REFS_WORKINGSET (2), LRU_REFS_PROTECTED (3),
> and LRU_REFS_MAX(7). When it reaches certain threshold, the folio is
> promoted proactively instead of wait for the PID controller to kick in.
>
> Also simplify MGLRU's usage of PG_workingset and PG_referenced, now
> these 2 flags are purely used as the lower 2 bit of refs for MGLRU. This
> doesn't effect classical LRU in any way. This will actually simplify and
> make MGLRU's certain metric reading more accurate, and reduced MGLRU's
> original tier / referenced count bit by one since only one extra bit is
> now needed to record a max referenced count of 7 (previously 2 extra bits
> are needed). This changes make sense because MGLRU doesn't have demotion
> so these 2 flags are never separately useful for MGLRU.
>
> This addresses several shortcomings of the old model:
[...]

> + * Update the folio's lru refs indicator without taking the folio lock,
> + * isolation, or lruvec lock. Used by both page table access (@is_fault=true)
> + * and by file access (@is_fault=false).
> + */
> +int folio_inc_lru_refs(struct folio *folio, bool is_fault, bool is_exec)
> +{


This is really hard to follow, especially with call sites like:

folio_inc_lru_refs(folio, true, false);
folio_inc_lru_refs(folio, false, false);

The reader needs to remember what the first true and the second
false represent, which makes the call sites quite opaque.

Could we have something like this instead?

#define LRU_REFS_FAULT BIT(0)
#define LRU_REFS_EXEC BIT(1)

folio_inc_lru_refs(folio, LRU_REFS_FAULT);
folio_inc_lru_refs(folio, 0);
folio_inc_lru_refs(folio, LRU_REFS_EXEC);
folio_inc_lru_refs(folio, LRU_REFS_EXEC | LRU_REFS_FAULT);

BTW