Re: [PATCH v4 5/5] ksm: add mremap selftests for ksm_rmap_walk

[David Hildenbrand (Arm)]

On 5/16/26 04:30, xu.xin16@xxxxxxxxxx wrote:
>>> --- a/tools/testing/selftests/mm/rmap.c
>>> +++ b/tools/testing/selftests/mm/rmap.c
>>> @@ -430,4 +430,83 @@ TEST_F(migrate, ksm)
>>>  	propagate_children(_metadata, data);
>>>  }
>>>
>>> +static void prepare_two_pages(struct global_data *data)
>>> +{
>>> +	/* Allocate exactly 2 pages for the test */
>>> +	data->mapsize = 2 * getpagesize();
>>> +	data->region = mmap(NULL, data->mapsize, PROT_READ | PROT_WRITE,
>>> +			    MAP_PRIVATE | MAP_ANON, -1, 0);
>>> +	if (data->region == MAP_FAILED)
>>> +		ksft_exit_fail_perror("mmap failed");
>>> +
>>> +	/* Fill both pages with identical content to encourage KSM merging */
>>> +	memset(data->region, 0x77, data->mapsize);
>>> +}
>>> +
>>> +static int mremap_merge_and_migrate(struct global_data *data)
>>> +{
>>> +	int ret, pagemap_fd;
>>> +	void *old_region = data->region;
>>> +	unsigned long page_sz = getpagesize();
>>> +
>>> +	/*
>>> +	 * Mremap the second page to the first page's location (FIXED).
>>> +	 * This effectively overwrites the first page, leaving the second page
>>> +	 * unmapped. The physical page originally at the second page is now
>>> +	 * mapped at the first page's virtual address.
>>> +	 */
>>
>>
>> But if there is only a single page, why should it get merged by KSM?
> 
> This test runs in multi-threads, So there are not only one page from this thread but
> also many other pages which are mergeable in all other threads.

At least the test did not reveal the problem in patch #3 :)

Can't we just use a single VMA (e.g., 32 pages), fill it with pages of the same
content, let it merge, then mremap half of it, and then migrate a page etc?



-- 
Cheers,

David