On 6/22/26 17:55, Vlastimil Babka (SUSE) wrote:

On 6/18/26 13:43, Wandun wrote:

quoted

On 6/18/26 02:52, Vlastimil Babka (SUSE) wrote:

quoted

On 6/4/26 04:38, Wandun Chen wrote:

quoted

From: Wandun Chen <redacted>

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 <redacted>
Closes: https://lore.kernel.org/all/DU0PR01MB10385345F7153F334100981888259A@DU0PR01MB10385.eurprd01.prod.exchangelabs.com/ (local)
Suggested-by: Vlastimil Babka <redacted>
Signed-off-by: Wandun Chen <redacted>
Link: https://lore.kernel.org/all/33275585-f2db-4779-89f0-3ae24b455a67@suse.cz/ (local) [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.

Ah, nice.

quoted

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.

So IIUC the isolation+migration might be started between the folio is
allocated, and mlocked? In that case the check during migration could still

Yes, in that case it still be racy, it is not a good idea to check page flags.

be racy, and if the page is isolated, it's already bad for the RT process.

IIUC, more accurately, the migration entry in the page talbe is real a bad for
RT process, because isolate page doesn't modify the page table, so memory
access continues as usual, therefore a new idea occur.

S1. In the mlock[all] syscall, if mlock_vma_pages_range hit a migration entry,
    then, it should wait for the migration to complete.

S2. During the unmap phase of memory migration, prevent a page from being unmapped
    if the page's associated vma is markd with VM_LOCKED, similar to how reclaim is
    disabled for pages in a VM_LOCKED vma(try_to_unmap_one).


For a page handled during the mlock[all] syscall:
  - if migration has been already finished, there is noting to do;
  - if migration is in progress and the migration etnry is already filled, we
    wait (S1)
  - if the page is in-fight, going to be isolated/migrated, S2 prevents the unmap.

For a page handled during a page fault: VM_LOCKED is already set on the vma,
so S2 guarantees it will not be unmapped, hence no migration entry.


Thanks a lot for the detailed feedback, Vlastimil.

Best regards,
Wandun

So this would only be a short-term problem after the mlockall, but we don't
have a way for the RT process to know the moment it's all settled, right?

Yes, some pages may have been isolated and will do migration.

Probably the proper solution would be for mlock[all]() itself to wait for an
isolated page, and only continue once it knows it can't be isolated anymore.
This might howver would go against some of the folio batching optimizations?

quoted

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;
}

quoted

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---
 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;
 
 		/*