Re: [PATCH v3] mm/hugetlb: fix max-only subpool accounting on alloc_hugetlb_folio failure

[Karl Mehltretter <kmehltretter@gmail.com>]

On Tue, Apr 28, 2026, Zhao Li wrote:
> Mounts with min_hpages != -1 are left unchanged for now.

In case it helps with that follow-up, here is a small reproducer for the
min_size case. It needs no hugetlb cgroup.

The pool has two huge pages and no overcommit, and the mount is
size=8M,min_size=4M. A test maps a 4-page file MAP_SHARED | MAP_NORESERVE,
touches the first N pages, then unmaps and removes the file. Five rounds
on one mount:

  N   faults ok / SIGBUS   statfs free / blocks   HugePages_Rsvd
  -                                               2 (after mount)
  2   2 / 0                4 / 4                  2
  2   2 / 0                4 / 4                  2
  4   2 / 2                2 / 4                  0
  2   2 / 0                2 / 4                  0
  2   2 / 0                2 / 4                  0

The two SIGBUS faults keep their used_hpages charge. Half of the quota
stays gone with no file on the mount. used_hpages then no longer drops
below min_hpages, so hugepage_subpool_put_pages() never restores
rsv_hpages, and the mount has lost its minimum reservation for good.

The out_subpool_put: path skips this case on purpose, as the commit
message says.

Same result on v7.3-rc4 (f0100363d8c3), on v7.3-rc4 with this patch, and
on next-20260922. That tree also has 6f8b22e85eac ("mm/hugetlb: fix subpool
minimum reservation rollback"), which fixes the hugetlb_reserve_pages()
error path. This is the fault path. Tested under QEMU x86_64, one CPU.

Thanks,
Karl

#include <fcntl.h>
#include <setjmp.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/statfs.h>
#include <unistd.h>

#define HPAGE (2UL << 20)

static sigjmp_buf env;

static void on_sigbus(int sig)
{
	siglongjmp(env, 1);
}

/* usage: t <hugetlbfs dir> <pages to touch, at most 4> */
int main(int argc, char **argv)
{
	int n = atoi(argv[2]), fd, i;
	volatile int ok = 0, bus = 0;
	struct statfs st;
	char path[256];
	char *p;

	signal(SIGBUS, on_sigbus);
	snprintf(path, sizeof(path), "%s/f", argv[1]);
	fd = open(path, O_CREAT | O_RDWR, 0600);
	if (fd < 0 || ftruncate(fd, 4 * HPAGE))
		return 1;
	p = mmap(NULL, 4 * HPAGE, PROT_READ | PROT_WRITE,
		 MAP_SHARED | MAP_NORESERVE, fd, 0);
	if (p == MAP_FAILED)
		return 1;
	for (i = 0; i < n; i++) {
		if (!sigsetjmp(env, 1)) {
			p[i * HPAGE] = 1;
			ok++;
		} else {
			bus++;
		}
	}
	munmap(p, 4 * HPAGE);
	close(fd);
	unlink(path);
	statfs(argv[1], &st);
	printf("ok=%d sigbus=%d free=%ld/%ld\n", ok, bus,
	       (long)st.f_bfree, (long)st.f_blocks);
	return 0;
}

  echo 0 > /proc/sys/vm/nr_overcommit_hugepages
  echo 2 > /proc/sys/vm/nr_hugepages
  mount -t hugetlbfs -o size=8M,min_size=4M none /mnt/h
  for n in 2 2 4 2 2; do ./t /mnt/h $n; grep HugePages_Rsvd /proc/meminfo; done