Re: [PATCHv9 03/14] mm/page_alloc: Fake unaccepted memory

David Hildenbrand
2023-04-03T14:44:58Z

On 30.03.23 13:49, Kirill A. Shutemov wrote:
> For testing purposes, it is useful to fake unaccepted memory in the
> system. It helps to understand unaccepted memory overhead to the page
> allocator.
>
> The patch allows to treat memory above the specified physical memory
> address as unaccepted.
>
> The change only fakes unaccepted memory for page allocator. Memblock is
> not affected.
>
> It also assumes that arch-provided accept_memory() on already accepted
> memory is a nop.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> mm/page_alloc.c | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d62fcb2f28bd..509a93b7e5af 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7213,6 +7213,8 @@ static DEFINE_STATIC_KEY_FALSE(zones_with_unaccepted_pages);
>
> static bool lazy_accept = true;
>
> +static unsigned long fake_unaccepted_start = -1UL;
> +
> static int __init accept_memory_parse(char *p)
> {
> if (!strcmp(p, "lazy")) {
> @@ -7227,11 +7229,30 @@ static int __init accept_memory_parse(char *p)
> }
> early_param("accept_memory", accept_memory_parse);
>
> +static int __init fake_unaccepted_start_parse(char *p)
> +{
> + if (!p)
> + return -EINVAL;
> +
> + fake_unaccepted_start = memparse(p, &p);
> +
> + if (*p != '\0') {
> + fake_unaccepted_start = -1UL;
> + return -EINVAL;
> + }
> +
> + return 0;
> +}
> +early_param("fake_unaccepted_start", fake_unaccepted_start_parse);
> +
> static bool page_contains_unaccepted(struct page *page, unsigned int order)
> {
> phys_addr_t start = page_to_phys(page);
> phys_addr_t end = start + (PAGE_SIZE << order);
>
> + if (start >= fake_unaccepted_start)
> + return true;
> +
> return range_contains_unaccepted_memory(start, end);
> }
>

The "unpleasant" thing about this is, that page_contains_unaccepted()
could not be used for sanity checks because the result is static.

For example, something like

if (page_contains_unaccepted(page, 0))
accept_memory(page, 0);
BUG_ON(!page_contains_unaccepted(page, 0));

Would work on real hardware, however, not for the fake variant.

--
Thanks,

David / dhildenb