linux-kernel - Re: [PATCH v1 2/2] mm/migrate: move NUMA hinting fault folio isolation + checks under PTL

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Message-ID: <8f85c31a-e603-4578-bf49-136dae0d4b69@redhat.com>
Date: Wed, 26 Jun 2024 18:22:26 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Donet Tom <donettom@...ux.ibm.com>
Subject: Re: [PATCH v1 2/2] mm/migrate: move NUMA hinting fault folio
 isolation + checks under PTL

On 20.06.24 23:29, David Hildenbrand wrote:
> Currently we always take a folio reference even if migration will not
> even be tried or isolation failed, requiring us to grab+drop an additional
> reference.
> 
> Further, we end up calling folio_likely_mapped_shared() while the folio
> might have already been unmapped, because after we dropped the PTL, that
> can easily happen. We want to stop touching mapcounts and friends from
> such context, and only call folio_likely_mapped_shared() while the folio
> is still mapped: mapcount information is pretty much stale and unreliable
> otherwise.
> 
> So let's move checks into numamigrate_isolate_folio(), rename that
> function to migrate_misplaced_folio_prepare(), and call that function
> from callsites where we call migrate_misplaced_folio(), but still with
> the PTL held.
> 
> We can now stop taking temporary folio references, and really only take
> a reference if folio isolation succeeded. Doing the
> folio_likely_mapped_shared() + golio isolation under PT lock is now similar
> to how we handle MADV_PAGEOUT.
> 
> While at it, combine the folio_is_file_lru() checks.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---

Donet just reported an issue. I suspect this fixes it -- in any case, this is
the right thing to do.

 From 0833b9896e98c8d88c521609c811a220d14a2e7e Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@...hat.com>
Date: Wed, 26 Jun 2024 18:14:44 +0200
Subject: [PATCH] Fixup: mm/migrate: move NUMA hinting fault folio isolation +
  checks under PTL

Donet reports an issue during NUMA migration we haven't seen previously:

	[   71.422804] list_del corruption, c00c00000061b3c8->next is
	LIST_POISON1 (5deadbeef0000100)
	[   71.422839] ------------[ cut here ]------------
	[   71.422843] kernel BUG at lib/list_debug.c:56!
	[   71.422850] Oops: Exception in kernel mode, sig: 5 [#1]

We forgot to convert one "return 0;" to return an error instead from
migrate_misplaced_folio_prepare() in case the target node is nearly
full.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
  mm/migrate.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 8beedbb42a93..9ed43c1eea5e 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2564,7 +2564,7 @@ int migrate_misplaced_folio_prepare(struct folio *folio,
  		int z;
  
  		if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING))
-			return 0;
+			return -EAGAIN;
  		for (z = pgdat->nr_zones - 1; z >= 0; z--) {
  			if (managed_zone(pgdat->node_zones + z))
  				break;

base-commit: 4b17ce353e02b47b00e2fe87b862f09e8b9a47e6
-- 
2.45.2


-- 
Cheers,

David / dhildenb