Re: [RFC PATCH 3/3] mm/compaction: respect compact_unevictable_allowed in alloc_contig path

[Vlastimil Babka (SUSE)]

On 6/4/26 04:38, Wandun Chen wrote:
> From: Wandun Chen <chenwandun@xxxxxxxxxxx>
> 
> vm.compact_unevictable_allowed=0 is used to prevent compacting
> unevictable pages. However, isolate_migratepages_range() passes
> ISOLATE_UNEVICTABLE regardless of this sysctl, so the setting
> has no effect in the alloc_contig path.
> 
> Fix it by:
>   - Keep ISOLATE_UNEVICTABLE for CMA allocation, discussed in [1].
>   - Honour sysctl_compact_unevictable_allowed for non-CMA allocation.
> 
> Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Wandun Chen <chenwandun@xxxxxxxxxxx>
> Link: https://lore.kernel.org/all/25ba0d77-eb61-4efc-b2fc-73878cbd85c1@xxxxxxx/ [1]

There was also the "Ideally by not having mlock'd pages in CMA areas at
all." part. Is it the case? It was more elaborated here:
https://lore.kernel.org/all/CAPTztWZpnX1j8-7yeppVUsxE=O9hbVeqricDjZt8_pnN7a-kBQ@xxxxxxxxxxxxxx/

> ---
>  include/linux/compaction.h | 6 ++++++
>  mm/compaction.c            | 9 +++++++--
>  mm/internal.h              | 1 +
>  mm/page_alloc.c            | 2 ++
>  4 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index f29ef0653546..04e60f65b976 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -106,6 +106,7 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
>  extern void __meminit kcompactd_run(int nid);
>  extern void __meminit kcompactd_stop(int nid);
>  extern void wakeup_kcompactd(pg_data_t *pgdat, int order, int highest_zoneidx);
> +extern bool compaction_allow_unevictable(void);
>  
>  #else
>  static inline void reset_isolation_suitable(pg_data_t *pgdat)
> @@ -131,6 +132,11 @@ static inline void wakeup_kcompactd(pg_data_t *pgdat,
>  {
>  }
>  
> +static inline bool compaction_allow_unevictable(void)
> +{
> +	return true;
> +}
> +
>  #endif /* CONFIG_COMPACTION */
>  
>  struct node;
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 007d5e00a8ae..a10acb273454 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1341,6 +1341,7 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
>  							unsigned long end_pfn)
>  {
>  	unsigned long pfn, block_start_pfn, block_end_pfn;
> +	isolate_mode_t mode = cc->allow_unevictable ? ISOLATE_UNEVICTABLE : 0;
>  	int ret = 0;
>  
>  	/* Scan block by block. First and last block may be incomplete */
> @@ -1360,8 +1361,7 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
>  					block_end_pfn, cc->zone))
>  			continue;
>  
> -		ret = isolate_migratepages_block(cc, pfn, block_end_pfn,
> -						 ISOLATE_UNEVICTABLE);
> +		ret = isolate_migratepages_block(cc, pfn, block_end_pfn, mode);
>  
>  		if (ret)
>  			break;
> @@ -1902,6 +1902,11 @@ typedef enum {
>   * compactable pages.
>   */
>  static int sysctl_compact_unevictable_allowed __read_mostly = CONFIG_COMPACT_UNEVICTABLE_DEFAULT;
> +
> +bool compaction_allow_unevictable(void)
> +{
> +	return sysctl_compact_unevictable_allowed;
> +}
>  /*
>   * Tunable for proactive compaction. It determines how
>   * aggressively the kernel should compact memory in the
> diff --git a/mm/internal.h b/mm/internal.h
> index 181e79f1d6a2..163f9d6b37f3 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1052,6 +1052,7 @@ struct compact_control {
>  					 * ensure forward progress.
>  					 */
>  	bool alloc_contig;		/* alloc_contig_range allocation */
> +	bool allow_unevictable;		/* Allow isolation of unevictable folios */
>  };
>  
>  /*
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 81a9d4d1e6c0..1cf9d4a3b14c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7118,6 +7118,8 @@ int alloc_contig_frozen_range_noprof(unsigned long start, unsigned long end,
>  		.ignore_skip_hint = true,
>  		.no_set_skip_hint = true,
>  		.alloc_contig = true,
> +		.allow_unevictable = !!(alloc_flags & ACR_FLAGS_CMA) ||
> +					     compaction_allow_unevictable(),
>  	};
>  	INIT_LIST_HEAD(&cc.migratepages);
>  	enum pb_isolate_mode mode = (alloc_flags & ACR_FLAGS_CMA) ?