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

[Usama Arif <usamaarif642@gmail.com>]

On 22/08/2024 06:43, Johannes Weiner wrote:
> On Wed, Aug 21, 2024 at 03:53:21PM -0400, Usama Arif wrote:
>> 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>
> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> 
> I think this is the right move for now, until there is a robust
> solution for the original issue.
> 
> But hould b7108d66318abf3e060c7839eabcba52e9461568 be reverted along
> with it? From its changelog:
> 
>     No observable issue without this patch on MGLRU, but logically it make
>     sense to skip the CMA page reclaim when those pages can't be satisfied for
>     the current allocation context.
> 
> Presumably it has the same risk reward profile as it does on
> conventional reclaim, with long skip runs while holding the
> lruvec->lock.

Yes makes sense to remove it from there a well, Just doing it in a single commit below:

From 9ad9fb73edf2c04ef932d128fc2729dfd8391c0c 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 and
b7108d66318abf3e060c7839eabcba52e9461568.

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].

Remove skipping CMA for MGLRU as well, as it was introduced in
sort_folio for the same resaon as 5da226dbfce3a2f44978c2c7cf88166e69a6788b.

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

Link: https://lkml.kernel.org/r/9060a32d-b2d7-48c0-8626-1db535653c54@gmail.com
Fixes: 5da226dbfce3 ("mm: skip CMA pages when they are not available")
Signed-off-by: Usama Arif <usamaarif642@gmail.com>
Cc: Bharata B Rao <bharata@amd.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zhaoyang Huang <huangzhaoyang@gmail.com>
Cc: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/vmscan.c | 24 ++----------------------
 1 file changed, 2 insertions(+), 22 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index cfa839284b92..bd489c1af228 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1604,25 +1604,6 @@ 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.
  *
@@ -1669,8 +1650,7 @@ 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;
@@ -4320,7 +4300,7 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
 	}
 
 	/* ineligible */
-	if (zone > sc->reclaim_idx || skip_cma(folio, sc)) {
+	if (zone > sc->reclaim_idx) {
 		gen = folio_inc_gen(lruvec, folio, false);
 		list_move_tail(&folio->lru, &lrugen->folios[gen][type][zone]);
 		return true;
-- 
2.43.5