linux-kernel - Re: [RESEND PATCH v7 09/10] selftests/mm/cow: Generalize do_run_with

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Date:   Fri, 24 Nov 2023 18:48:50 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v7 09/10] selftests/mm/cow: Generalize
 do_run_with_thp() helper

On 22.11.23 17:29, Ryan Roberts wrote:
> do_run_with_thp() prepares (PMD-sized) THP memory into different states
> before running tests. With the introduction of small-sized THP, we would
> like to reuse this logic to also test those smaller THP sizes. So let's
> add a size parameter which tells the function what size THP it should
> operate on.
> 
> A separate commit will utilize this change to add new tests for
> small-sized THP, where available.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>   tools/testing/selftests/mm/cow.c | 146 +++++++++++++++++--------------
>   1 file changed, 79 insertions(+), 67 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/cow.c b/tools/testing/selftests/mm/cow.c
> index 7324ce5363c0..d03c453cfd5c 100644
> --- a/tools/testing/selftests/mm/cow.c
> +++ b/tools/testing/selftests/mm/cow.c
> @@ -32,7 +32,7 @@
> 
>   static size_t pagesize;
>   static int pagemap_fd;
> -static size_t thpsize;
> +static size_t pmdsize;
>   static int nr_hugetlbsizes;
>   static size_t hugetlbsizes[10];
>   static int gup_fd;
> @@ -734,14 +734,14 @@ enum thp_run {
>   	THP_RUN_PARTIAL_SHARED,
>   };
> 
> -static void do_run_with_thp(test_fn fn, enum thp_run thp_run)
> +static void do_run_with_thp(test_fn fn, enum thp_run thp_run, size_t size)

Nit: can we still call it "thpsize" in this function? That makes it 
clearer IMHO and avoids most renaming.

>   {
>   	char *mem, *mmap_mem, *tmp, *mremap_mem = MAP_FAILED;
> -	size_t size, mmap_size, mremap_size;
> +	size_t mmap_size, mremap_size;
>   	int ret;
> 
> -	/* For alignment purposes, we need twice the thp size. */
> -	mmap_size = 2 * thpsize;
> +	/* For alignment purposes, we need twice the requested size. */
> +	mmap_size = 2 * size;
>   	mmap_mem = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
>   			MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>   	if (mmap_mem == MAP_FAILED) {
> @@ -749,36 +749,40 @@ static void do_run_with_thp(test_fn fn, enum thp_run thp_run)
>   		return;
>   	}
> 
> -	/* We need a THP-aligned memory area. */
> -	mem = (char *)(((uintptr_t)mmap_mem + thpsize) & ~(thpsize - 1));
> +	/* We need to naturally align the memory area. */
> +	mem = (char *)(((uintptr_t)mmap_mem + size) & ~(size - 1));
> 
> -	ret = madvise(mem, thpsize, MADV_HUGEPAGE);
> +	ret = madvise(mem, size, MADV_HUGEPAGE);
>   	if (ret) {
>   		ksft_test_result_fail("MADV_HUGEPAGE failed\n");
>   		goto munmap;
>   	}
> 
>   	/*
> -	 * Try to populate a THP. Touch the first sub-page and test if we get
> -	 * another sub-page populated automatically.
> +	 * Try to populate a THP. Touch the first sub-page and test if
> +	 * we get the last sub-page populated automatically.
>   	 */
>   	mem[0] = 0;
> -	if (!pagemap_is_populated(pagemap_fd, mem + pagesize)) {
> +	if (!pagemap_is_populated(pagemap_fd, mem + size - pagesize)) {
>   		ksft_test_result_skip("Did not get a THP populated\n");
>   		goto munmap;
>   	}

Yes! I have a patch lying around here that does that same. :)

I guess there is no need to set MADV_NOHUGEPAGE on the remainder of the 
mmap'ed are:

Assume we want a 64KiB thp. We mmap'ed 128KiB. If we get a reasonably 
aligned area, we might populate a 128KiB THP.

But I assume the MADV_HUGEPAGE will in all configurations properly 
create a separate 64KiB VMA and we'll never get 128 KiB populated. So 
this should work reliably.

> -	memset(mem, 0, thpsize);
> +	memset(mem, 0, size);
> 
> -	size = thpsize;
>   	switch (thp_run) {
>   	case THP_RUN_PMD:
>   	case THP_RUN_PMD_SWAPOUT:
> +		if (size != pmdsize) {
> +			ksft_test_result_fail("test bug: can't PMD-map size\n");
> +			goto munmap;
> +		}

Maybe rather "assert()" because that's a real BUG in the test?

[...]

> +	pmdsize = read_pmd_pagesize();
> +	if (pmdsize)
> +		ksft_print_msg("[INFO] detected PMD-mapped THP size: %zu KiB\n",

Maybe simply: "detected PMD size". Zes, we read it via the THP 
interface, but that shouldn't matter much.

-- 
Cheers,

David / dhildenb