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

[Wandun <chenwandun1@gmail.com>]

On 6/18/26 02:52, Vlastimil Babka (SUSE) wrote:
> 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?
> 
Yes, I wrote a test case that can reproduce it in a few second.

The test case contains 3 steps:
1. mlockall
2. mmap file(2GB) + trigger file write page fault;
3. during step 1, trigger compact via /proc/sys/vm/compact_memory


My reproduction environment is qemu with 4GB ram, 8 core, aarch64,
preempt_rt and includes the tracepoint in patch 02.
After running the reproduction program for a few seconds, the
following output appears.

repro-403     [004] ....1   101.270505: mm_compaction_isolate_folio: pfn=0x71e3a mode=0x0 flags=referenced|uptodate|mlocked
repro-403     [004] ....1   101.270507: mm_compaction_isolate_folio: pfn=0x71e3b mode=0x0 flags=referenced|uptodate|mlocked
repro-403     [004] ....1   101.270513: mm_compaction_isolate_folio: pfn=0x71e3c mode=0x0 flags=referenced|uptodate|mlocked
repro-403     [004] ....1   101.270515: mm_compaction_isolate_folio: pfn=0x71e3d mode=0x0 flags=uptodate|mlocked
repro-403     [004] ....1   101.270517: mm_compaction_isolate_folio: pfn=0x71e3e mode=0x0 flags=uptodate|mlocked
repro-403     [004] ....1   101.270520: mm_compaction_isolate_folio: pfn=0x71e3f mode=0x0 flags=uptodate|mlocked


Unfortunately, I recently found that there is still a bug in the
fix patch. Setting mlocked in the mlock_folio function could happen
even after the page is successfully isolated, so it still cannot
prevent migration. Because of this, I need to think more about how
to fix it.

Perhaps we should double-check whether the page is mlocked during
the actual migration phase.

What do you think of this best-effort approach?


Best regards,
Wandun





The full reproducer is as below:

/* gcc repro.c -o repro -lpthread */

#define _GNU_SOURCE
#include <fcntl.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>

#define PAGE_SIZE       4096
#define NR_PAGES        32
#define FILE_SIZE       (2ULL * 1024 * 1024 * 1024)

static void *worker_fn(void *arg)
{
	int fd = (long)arg;
	size_t len = (size_t)FILE_SIZE;
	char *p = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
	if (p == MAP_FAILED)
		return NULL;

	for (size_t off = 0; off + NR_PAGES * PAGE_SIZE <= len;
	     off += NR_PAGES * PAGE_SIZE) {
		for (int i = 0; i < NR_PAGES; i++)
			p[off + i * PAGE_SIZE] = 1;
		usleep(200);
	}

	munmap(p, len);
	return NULL;
}

static void *compact_fn(void *arg)
{
	(void)arg;
	int fd = open("/proc/sys/vm/compact_memory", O_WRONLY);
	if (fd < 0)
		return NULL;

	while (1) {
		if (write(fd, "1", 1) < 0) {}
		usleep(5000);
	}
}

int main(void)
{
	mlockall(MCL_CURRENT | MCL_FUTURE);

	int fd = open("./repro_largefile.dat", O_RDWR | O_CREAT, 0600);
	if (fd < 0)
		return 1;
	unlink("./repro_largefile.dat");
	if (ftruncate(fd, (off_t)FILE_SIZE) < 0)
		return 1;

	printf("repro_largefile: 1 worker, %d pages/batch, Ctrl-C to stop\n",
	       NR_PAGES);

	pthread_t compact, worker;
	pthread_create(&compact, NULL, compact_fn, NULL);
	pthread_create(&worker, NULL, worker_fn, (void *)(long)fd);

	pthread_join(worker, NULL);
	return 0;
}

>> ---
>>  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;
>>  
>>  		/*
>