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

[Wandun <chenwandun1@gmail.com>]

On 6/18/26 02:57, Vlastimil Babka (SUSE) wrote:
> On 6/4/26 04:38, Wandun Chen wrote:
>> From: Wandun Chen <chenwandun@lixiang.com>
>>
>> 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@suse.cz>
>> Signed-off-by: Wandun Chen <chenwandun@lixiang.com>
>> Link: https://lore.kernel.org/all/25ba0d77-eb61-4efc-b2fc-73878cbd85c1@suse.cz/ [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:

Yes, It is the case.

> https://lore.kernel.org/all/CAPTztWZpnX1j8-7yeppVUsxE=O9hbVeqricDjZt8_pnN7a-kBQ@mail.gmail.com/

I missed this important information. Thanks for pointing it out, Vlastimil.

Best regards,
Wandun

> 
>> ---
>>  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) ?
>