Re: [RFC PATCH 1/3] mm/compaction: skip isolate mlocked folios when compact_unevictable_allowed=0

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 6/4/26 04:38, Wandun Chen wrote:
> From: Wandun Chen <chenwandun@lixiang.com>
> 
> compact_unevictable_allowed is default 0 under PREEMPT_RT,
> isolate_migratepages_block() skips folios with PG_unevictable set.
> However, mlock_folio() sets PG_mlocked immediately but defers
> PG_unevictable to mlock_folio_batch(), result in a folio with
> PG_mlocked=1 but PG_unevictable=0. Compaction will isolate such a
> folio.
> 
> Fix by checking folio_test_mlocked() together with the existing
> folio_test_unevictable() check.
> 
> A similar issue has been reported by Alexander Krabler on a 6.12-rt
> aarch64 system. Vlastimil suggested to check the mlocked flag [1].
> 
> Reported-by: Alexander Krabler <Alexander.Krabler@kuka.com>
> Closes: https://lore.kernel.org/all/DU0PR01MB10385345F7153F334100981888259A@DU0PR01MB10385.eurprd01.prod.exchangelabs.com/
> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Wandun Chen <chenwandun@lixiang.com>
> Link: https://lore.kernel.org/all/33275585-f2db-4779-89f0-3ae24b455a67@suse.cz/ [1]

Well in that thread, Hugh doubted my suggestion and then it seems we didn't
concluded anything. Did you actually in practice observe the issue that
Alexander had, and that this patch fixed it, or is that theoretical?

> ---
>  mm/compaction.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index b776f35ad020..7e07b792bcb5 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1116,7 +1116,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  		is_unevictable = folio_test_unevictable(folio);
>  
>  		/* Compaction might skip unevictable pages but CMA takes them */
> -		if (!(mode & ISOLATE_UNEVICTABLE) && is_unevictable)
> +		if (!(mode & ISOLATE_UNEVICTABLE) &&
> +		    (is_unevictable || folio_test_mlocked(folio)))
>  			goto isolate_fail_put;
>  
>  		/*