Re: [PATCH v2] selftests/mm: check MREMAP_DONTUNMAP mlock accounting

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Tue, Oct 06, 2026 at 08:39:01AM +0200, Jose A. Perez de Azpillaga wrote:
> MREMAP_DONTUNMAP keeps the source VMA in place, but clears its mlock flags
> while setting them on the destination VMA.  The "mm/mremap: fix two issues
> with MREMAP_DONTUNMAP" series fixed two cases where this leaked
> mm->locked_vm:
>
>  - an unfaulted mlock-on-fault VMA moved behind itself self-merges, so the
>    single resulting VMA loses the flags without the accounting being
>    dropped;
>
>  - a partial mremap() moves only part of the range, leaving the pages which
>    are not moved accounted as locked in a VMA whose flags were cleared.
>
> Add a test which mlock2()s a source mapping, moves all or part of it with
> MREMAP_DONTUNMAP, unmaps everything and checks that VmLck is back to its
> value from before, and run it for the self-merge case and for a partial
> move with and without MLOCK_ONFAULT.
>
> Verified on x86_64: the three cases fail on mm-unstable with the two fixes
> reverted and pass with them applied.
>
> Signed-off-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>

Thanks for writing tests for this. Looks generally OK to me, so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
> v2:
>  - Reduce the churn (David): fold the three cases into one parameterised
>    test and drop the child process per case, comparing VmLck against its
>    value before each case instead.
>  - Use the file's BUG_ON() for setup failures and vm_util's
>    check_for_pattern() to read VmLck, call mlock2() directly.
>  - Verify against mm-unstable with the two fixes reverted rather than an
>    older -rc, now that the series is applied.
>
>  tools/testing/selftests/mm/mremap_dontunmap.c | 63 ++++++++++++++++++-
>  1 file changed, 62 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/mm/mremap_dontunmap.c b/tools/testing/selftests/mm/mremap_dontunmap.c
> index 96ba537facf7..5ca33d9d42af 100644
> --- a/tools/testing/selftests/mm/mremap_dontunmap.c
> +++ b/tools/testing/selftests/mm/mremap_dontunmap.c
> @@ -7,6 +7,7 @@
>   */
>  #define _GNU_SOURCE
>  #include <sys/mman.h>
> +#include <sys/syscall.h>
>  #include <linux/mman.h>
>  #include <errno.h>
>  #include <stdio.h>
> @@ -15,6 +16,7 @@
>  #include <unistd.h>
>
>  #include "kselftest.h"
> +#include "vm_util.h"
>
>  unsigned long page_size;
>  char *page_buffer;
> @@ -335,6 +337,62 @@ static void mremap_dontunmap_partial_mapping_overwrite(void)
>  	ksft_test_result_pass("%s\n", __func__);
>  }
>
> +/* VmLck from /proc/self/status, which is mm->locked_vm in kB. */
> +static unsigned long locked_vm_kb(void)
> +{
> +	unsigned long kb;
> +	char buf[256];
> +	FILE *fp;
> +
> +	fp = fopen("/proc/self/status", "r");
> +	BUG_ON(!fp, "unable to open /proc/self/status");
> +	BUG_ON(!check_for_pattern(fp, "VmLck:", buf, sizeof(buf)) ||
> +	       sscanf(buf, "VmLck: %lu kB", &kb) != 1, "unable to read VmLck");
> +	fclose(fp);
> +
> +	return kb;
> +}
> +
> +/*
> + * MREMAP_DONTUNMAP clears the mlock flags of the source VMA and sets them on
> + * the destination, and mm->locked_vm has to follow.  mlock2() a source of
> + * num_pages and move its first move_pages to gap pages past its end:
> + *
> + *  |------ source ------|... gap ...|-- dest --|
> + *
> + * then unmap everything and check that VmLck is back where it started.  With
> + * no gap the destination is adjacent to the source and could merge with it,
> + * otherwise the gap is left PROT_NONE so that it cannot.
> + */
> +static void mremap_dontunmap_mlock(const char *desc, unsigned long num_pages,
> +				   unsigned long move_pages, unsigned long gap,
> +				   int mlock_flags)
> +{
> +	unsigned long size = (num_pages + gap + move_pages) * page_size;
> +	unsigned long locked = locked_vm_kb();
> +	void *source, *dest;
> +
> +	source = mmap(NULL, size, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +	BUG_ON(source == MAP_FAILED, "mmap");
> +	BUG_ON(mprotect(source, num_pages * page_size,
> +			PROT_READ | PROT_WRITE) == -1, "mprotect");
> +	dest = source + (num_pages + gap) * page_size;
> +
> +	if (syscall(__NR_mlock2, source, num_pages * page_size, mlock_flags)) {
> +		ksft_test_result_skip("%s: %s: mlock2: %s\n", __func__, desc,
> +				      strerror(errno));
> +		BUG_ON(munmap(source, size) == -1, "unable to unmap mappings");
> +		return;
> +	}
> +
> +	BUG_ON(mremap(source, move_pages * page_size, move_pages * page_size,
> +		      MREMAP_DONTUNMAP | MREMAP_MAYMOVE | MREMAP_FIXED,
> +		      dest) != dest, "mremap");
> +
> +	BUG_ON(munmap(source, size) == -1, "unable to unmap mappings");
> +	ksft_test_result(locked_vm_kb() == locked, "%s: %s\n", __func__, desc);
> +}
> +
>  int main(void)
>  {
>  	ksft_print_header();
> @@ -348,7 +406,7 @@ int main(void)
>  		ksft_finished();
>  	}
>
> -	ksft_set_plan(5);
> +	ksft_set_plan(8);
>
>  	// Keep a page sized buffer around for when we need it.
>  	page_buffer =
> @@ -361,6 +419,9 @@ int main(void)
>  	mremap_dontunmap_simple_fixed();
>  	mremap_dontunmap_partial_mapping();
>  	mremap_dontunmap_partial_mapping_overwrite();
> +	mremap_dontunmap_mlock("onfault self-merge", 1, 1, 0, MLOCK_ONFAULT);
> +	mremap_dontunmap_mlock("partial", 3, 2, 1, 0);
> +	mremap_dontunmap_mlock("onfault partial", 3, 2, 1, MLOCK_ONFAULT);
>
>  	BUG_ON(munmap(page_buffer, page_size) == -1,
>  	       "unable to unmap page buffer");
> --
> 2.55.0
>

--
Cheers, Lorenzo