Re: [PATCH] Revert "mm: skip CMA pages when they are not available"

[Usama Arif <usamaarif642@gmail.com>]

On 21/08/2024 15:35, Usama Arif wrote:
> This reverts commit 5da226dbfce3a2f44978c2c7cf88166e69a6788b.
> 
> lruvec->lru_lock is highly contended and is held when calling
> isolate_lru_folios. If the lru has a large number of CMA folios
> consecutively, while the allocation type requested is not
> MIGRATE_MOVABLE, isolate_lru_folios can hold the lock for a very long
> time while it skips those. For FIO workload, ~150million order=0
> folios were skipped to isolate a few ZONE_DMA folios [1].
> This can cause lockups [1] and high memory pressure for extended periods
> of time [2].
> 
> [1] https://lore.kernel.org/all/CAOUHufbkhMZYz20aM_3rHZ3OcK4m2puji2FGpUpn_-DevGk3Kg@mail.gmail.com/
> [2] https://lore.kernel.org/all/ZrssOrcJIDy8hacI@gmail.com/
> 
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> ---
>  mm/vmscan.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 5dc96a843466..78ad4a141409 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1690,8 +1690,7 @@ static unsigned long isolate_lru_folios(unsigned long nr_to_scan,
>  		nr_pages = folio_nr_pages(folio);
>  		total_scan += nr_pages;
>  
> -		if (folio_zonenum(folio) > sc->reclaim_idx ||
> -				skip_cma(folio, sc)) {
> +		if (folio_zonenum(folio) > sc->reclaim_idx) {
>  			nr_skipped[folio_zonenum(folio)] += nr_pages;
>  			move_to = &folios_skipped;
>  			goto move;


Actually, I didn't build test this with other config options before sending this patch.
skip_cma is only neeeded with MGLRU after this patch, we need to move the definition of
the function to be guarded under CONFIG_LRU_GEN, so that we don't get 
‘skip_cma’ defined but not used error. Below patch should be good:


From 1aae7f04a5cb203ea2c3ede7973dd9eddbbd7a8b Mon Sep 17 00:00:00 2001
From: Usama Arif <usamaarif642@gmail.com>
Date: Wed, 21 Aug 2024 20:26:07 +0100
Subject: [PATCH] Revert "mm: skip CMA pages when they are not available"

This reverts commit 5da226dbfce3a2f44978c2c7cf88166e69a6788b.

lruvec->lru_lock is highly contended and is held when calling
isolate_lru_folios. If the lru has a large number of CMA folios
consecutively, while the allocation type requested is not
MIGRATE_MOVABLE, isolate_lru_folios can hold the lock for a very long
time while it skips those. For FIO workload, ~150million order=0
folios were skipped to isolate a few ZONE_DMA folios [1].
This can cause lockups [1] and high memory pressure for extended periods
of time [2].

[1] https://lore.kernel.org/all/CAOUHufbkhMZYz20aM_3rHZ3OcK4m2puji2FGpUpn_-DevGk3Kg@mail.gmail.com/
[2] https://lore.kernel.org/all/ZrssOrcJIDy8hacI@gmail.com/

Signed-off-by: Usama Arif <usamaarif642@gmail.com>
---
 mm/vmscan.c | 41 ++++++++++++++++++++---------------------
 1 file changed, 20 insertions(+), 21 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5dc96a843466..53372f726a1b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1625,25 +1625,6 @@ static __always_inline void update_lru_sizes(struct lruvec *lruvec,
 
 }
 
-#ifdef CONFIG_CMA
-/*
- * It is waste of effort to scan and reclaim CMA pages if it is not available
- * for current allocation context. Kswapd can not be enrolled as it can not
- * distinguish this scenario by using sc->gfp_mask = GFP_KERNEL
- */
-static bool skip_cma(struct folio *folio, struct scan_control *sc)
-{
-	return !current_is_kswapd() &&
-			gfp_migratetype(sc->gfp_mask) != MIGRATE_MOVABLE &&
-			folio_migratetype(folio) == MIGRATE_CMA;
-}
-#else
-static bool skip_cma(struct folio *folio, struct scan_control *sc)
-{
-	return false;
-}
-#endif
-
 /*
  * Isolating page from the lruvec to fill in @dst list by nr_to_scan times.
  *
@@ -1690,8 +1671,7 @@ static unsigned long isolate_lru_folios(unsigned long nr_to_scan,
 		nr_pages = folio_nr_pages(folio);
 		total_scan += nr_pages;
 
-		if (folio_zonenum(folio) > sc->reclaim_idx ||
-				skip_cma(folio, sc)) {
+		if (folio_zonenum(folio) > sc->reclaim_idx) {
 			nr_skipped[folio_zonenum(folio)] += nr_pages;
 			move_to = &folios_skipped;
 			goto move;
@@ -4297,6 +4277,25 @@ void lru_gen_soft_reclaim(struct mem_cgroup *memcg, int nid)
 
 #endif /* CONFIG_MEMCG */
 
+#ifdef CONFIG_CMA
+/*
+ * It is waste of effort to scan and reclaim CMA pages if it is not available
+ * for current allocation context. Kswapd can not be enrolled as it can not
+ * distinguish this scenario by using sc->gfp_mask = GFP_KERNEL
+ */
+static bool skip_cma(struct folio *folio, struct scan_control *sc)
+{
+	return !current_is_kswapd() &&
+			gfp_migratetype(sc->gfp_mask) != MIGRATE_MOVABLE &&
+			folio_migratetype(folio) == MIGRATE_CMA;
+}
+#else
+static bool skip_cma(struct folio *folio, struct scan_control *sc)
+{
+	return false;
+}
+#endif
+
 /******************************************************************************
  *                          the eviction
  ******************************************************************************/
-- 
2.43.5