Re: [RFC PATCH v0] mm/vmscan: Add readahead LRU to improve readahead file page reclamation efficiency

[zhongjinji <zhongjinji@honor.com>]

> 1. Problem Background
> In Android systems, a significant challenge arises during application 
> startup when a large number of private application files are read.
> Approximately 90% of these file pages are loaded into memory via readahead.
> However, about 85% of these pre-read pages are reclaimed without ever being
> accessed, which means only around 15% of the pre-read pages are effectively
> utilized. This results in wasted memory, as unaccessed file pages consume
> valuable memory space, leading to memory thrashing and unnecessary I/O 
> reads.
>  
> 2. Solution Proposal
> Introduce a Readahead LRU to track pages brought in via readahead. During
> memory reclamation, prioritize scanning this LRU to reclaim pages that
> have not been accessed recently. For pages in the Readahead LRU that are
> accessed, move them back to the inactive_file LRU to await subsequent
> reclamation.
>  
> 3. Benefits Data
> In tests involving the cold start of 30 applications:
>   Memory Reclamation Efficiency: The slowpath process saw a reduction of
>   over 30%.

Did you enable MGLRU? If you did, I guess "do not active page" and a separate
LRU would have the same effect, but I didn't find any benefits.

diff --git a/mm/swap.c b/mm/swap.c
index 3632dd061beb..9e87996abbc9 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -504,7 +504,8 @@ 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))
+           lru_gen_in_fault() && !(current->flags & PF_MEMALLOC) &&
+               !folio_test_readahead_lru(folio))
                folio_set_active(folio);

        folio_batch_add_and_move(folio, lru_add, false);

> 4. Current Issues
> The refault metric for file pages has significantly degraded, increasing
> by about 100%. This is primarily because pages are reclaimed too quickly,
> without sufficient aging.
>  
> 5. Next Steps
> When calculating reclamation propensity, adjust the intensity of
> reclamation from the Readahead LRU. This ensures aging and reclamation
> efficiency while allowing adequate aging time.
> 
> Signed-off-by: Lei Liu <liulei.rjpt@vivo.com>