Re: [PATCH] mm: mglru: promote mapped executable folios after first usage

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 7/16/26 3:44 AM, Andrew Morton wrote:
> On Wed, 15 Jul 2026 17:00:19 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
> 
>> I did a quick test with the same test case on my x86 machine, and I
>> still see an improvement in sys time.
>>
>> base			patched
>> 1152.249s		993.235s
> 
> Nice.  Sashiko asked a thing:
> 	https://sashiko.dev/#/patchset/4b921ed528c483e13c9e22d1ae44ba58b4a15b0b.1784096432.git.baolin.wang@linux.alibaba.com

Thanks.

Forward Sashiko's comments:
"
This is a pre-existing issue, but does this miss promoting mapped
executable file folios during direct reclaim?

......

This means current->reclaim_state->mm_walk is NULL. When 
shrink_folio_list() calls folio_referenced(), it invokes 
lru_gen_look_around() which calls walk_update_folio() with a NULL walk. 
Since walk is NULL, we fall back to lru_gen_set_refs() here, which 
bypasses the newly patched folio_update_gen():

mm/vmscan.c:lru_gen_set_refs() {
	if (!folio_test_referenced(folio) && !folio_test_workingset(folio)) {
		set_mask_bits(&folio->flags.f, LRU_REFS_MASK, BIT(PG_referenced));
		return false;
	}
	...
}

Because lru_gen_set_refs() lacks the new VMA_EXEC_BIT promotion logic, 
it merely sets PG_referenced without updating the folio's generation. 
When those adjacent folios are subsequently isolated by 
shrink_folio_list(), their PTE young bits have already been cleared, 
causing folio_check_references() to return FOLIOREF_RECLAIM and evict 
them. Will we need a similar fix for lru_gen_set_refs() to handle direct 
reclaim paths?
"

During aging, MGLRU always sets up a valid mm_walk to walk the page 
table access flags in try_to_inc_max_seq().

When aging is not needed, it is indeed possible to fall into 
lru_gen_set_refs() and miss promoting mapped exec folios after first 
usage. Although I think the impact is limited, because after the current 
patch, accessed mapped exec folios rarely get a chance to enter the 
eviction path. Anyway, I'll add promotion logic for lru_gen_set_refs().