Re: [RFC Patch 2/2] selftests/mm: assert rmap behave as expected

[Wei Yang <richard.weiyang@gmail.com>]

On Fri, Jul 11, 2025 at 05:39:39PM +0200, David Hildenbrand wrote:
>On 04.06.25 10:34, Wei Yang wrote:
>> On Wed, Jun 04, 2025 at 08:21:45AM +0000, Wei Yang wrote:
>> [...]
>> > +int try_to_move_page(char *region)
>> > +{
>> > +	int ret;
>> > +	int node;
>> > +	int status = 0;
>> > +
>> > +	ksft_print_msg("worker %d move_pages of content: %.15s\n", getpid(), region);
>> 
>> One thing confused me here.
>> 
>> If I don't access region here, the following move_pages() would report
>> -ENOENT occationally. The reason is do_pages_stat_array() ->
>> folio_walk_start() returns NULL.
>
>Right, the pages were not faulted in. The man page mentions that as
>
>"-ENOENT: The page is not present."
>

Thanks I see the man page, but from the code point of view, I don't follow it.

The move_pages() return -ENOENT in a child process, and the child duplicate it
memory space during fork().

dup_mmap()
    for_each_vma()
        copy_page_range()
	...
            copy_pte_range() -> copy_present_ptes()
                __copy_present_ptes()
                    set_ptes()

Even we map the range by MAP_SHARED, we don't need to wrprotect it. 
But we still set_ptes() in the child process page table.


So it looks the child has prepared the page table and not need to fault in to
setup it.

Do I miss something?

>-- 
>Cheers,
>
>David / dhildenb

-- 
Wei Yang
Help you, Help me