Re: [PATCH 4/4] tools/testing/selftests: add VMA merge tests for KSM merge

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2025/5/19 16:51, Lorenzo Stoakes wrote:
> Add test to assert that we have now allowed merging of VMAs when KSM
> merging-by-default has been set by prctl(PR_SET_MEMORY_MERGE, ...).
> 
> We simply perform a trivial mapping of adjacent VMAs expecting a merge,
> however prior to recent changes implementing this mode earlier than before,
> these merges would not have succeeded.
> 
> Assert that we have fixed this!
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Tested-by: Chengming Zhou <chengming.zhou@linux.dev>

Thanks!

> ---
>   tools/testing/selftests/mm/merge.c | 78 ++++++++++++++++++++++++++++++
>   1 file changed, 78 insertions(+)
> 
> diff --git a/tools/testing/selftests/mm/merge.c b/tools/testing/selftests/mm/merge.c
> index c76646cdf6e6..2380a5a6a529 100644
> --- a/tools/testing/selftests/mm/merge.c
> +++ b/tools/testing/selftests/mm/merge.c
> @@ -2,10 +2,12 @@
>   
>   #define _GNU_SOURCE
>   #include "../kselftest_harness.h"
> +#include <linux/prctl.h>
>   #include <stdio.h>
>   #include <stdlib.h>
>   #include <unistd.h>
>   #include <sys/mman.h>
> +#include <sys/prctl.h>
>   #include <sys/wait.h>
>   #include "vm_util.h"
>   
> @@ -31,6 +33,11 @@ FIXTURE_TEARDOWN(merge)
>   {
>   	ASSERT_EQ(munmap(self->carveout, 12 * self->page_size), 0);
>   	ASSERT_EQ(close_procmap(&self->procmap), 0);
> +	/*
> +	 * Clear unconditionally, as some tests set this. It is no issue if this
> +	 * fails (KSM may be disabled for instance).
> +	 */
> +	prctl(PR_SET_MEMORY_MERGE, 0, 0, 0, 0);
>   }
>   
>   TEST_F(merge, mprotect_unfaulted_left)
> @@ -452,4 +459,75 @@ TEST_F(merge, forked_source_vma)
>   	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr2 + 5 * page_size);
>   }
>   
> +TEST_F(merge, ksm_merge)
> +{
> +	unsigned int page_size = self->page_size;
> +	char *carveout = self->carveout;
> +	struct procmap_fd *procmap = &self->procmap;
> +	char *ptr, *ptr2;
> +	int err;
> +
> +	/*
> +	 * Map two R/W immediately adjacent to one another, they should
> +	 * trivially merge:
> +	 *
> +	 * |-----------|-----------|
> +	 * |    R/W    |    R/W    |
> +	 * |-----------|-----------|
> +	 *      ptr         ptr2
> +	 */
> +
> +	ptr = mmap(&carveout[page_size], page_size, PROT_READ | PROT_WRITE,
> +		   MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> +	ASSERT_NE(ptr, MAP_FAILED);
> +	ptr2 = mmap(&carveout[2 * page_size], page_size,
> +		    PROT_READ | PROT_WRITE,
> +		    MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> +	ASSERT_NE(ptr2, MAP_FAILED);
> +	ASSERT_TRUE(find_vma_procmap(procmap, ptr));
> +	ASSERT_EQ(procmap->query.vma_start, (unsigned long)ptr);
> +	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 2 * page_size);
> +
> +	/* Unmap the second half of this merged VMA. */
> +	ASSERT_EQ(munmap(ptr2, page_size), 0);
> +
> +	/* OK, now enable global KSM merge. We clear this on test teardown. */
> +	err = prctl(PR_SET_MEMORY_MERGE, 1, 0, 0, 0);
> +	if (err == -1) {
> +		int errnum = errno;
> +
> +		/* Only non-failure case... */
> +		ASSERT_EQ(errnum, EINVAL);
> +		/* ...but indicates we should skip. */
> +		SKIP(return, "KSM memory merging not supported, skipping.");
> +	}
> +
> +	/*
> +	 * Now map a VMA adjacent to the existing that was just made
> +	 * VM_MERGEABLE, this should merge as well.
> +	 */
> +	ptr2 = mmap(&carveout[2 * page_size], page_size,
> +		    PROT_READ | PROT_WRITE,
> +		    MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> +	ASSERT_NE(ptr2, MAP_FAILED);
> +	ASSERT_TRUE(find_vma_procmap(procmap, ptr));
> +	ASSERT_EQ(procmap->query.vma_start, (unsigned long)ptr);
> +	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 2 * page_size);
> +
> +	/* Now this VMA altogether. */
> +	ASSERT_EQ(munmap(ptr, 2 * page_size), 0);
> +
> +	/* Try the same operation as before, asserting this also merges fine. */
> +	ptr = mmap(&carveout[page_size], page_size, PROT_READ | PROT_WRITE,
> +		   MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> +	ASSERT_NE(ptr, MAP_FAILED);
> +	ptr2 = mmap(&carveout[2 * page_size], page_size,
> +		    PROT_READ | PROT_WRITE,
> +		    MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
> +	ASSERT_NE(ptr2, MAP_FAILED);
> +	ASSERT_TRUE(find_vma_procmap(procmap, ptr));
> +	ASSERT_EQ(procmap->query.vma_start, (unsigned long)ptr);
> +	ASSERT_EQ(procmap->query.vma_end, (unsigned long)ptr + 2 * page_size);
> +}
> +
>   TEST_HARNESS_MAIN