linux-kernel - Re: [PATCH 2/2] selftests/mm: Add a new test for madv and hugetlb

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Message-ID: <026a75f627bc7ffa45ead3fd6ecb9da7b5bc2692.camel@surriel.com>
Date:   Wed, 04 Oct 2023 20:22:08 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Breno Leitao <leitao@...ian.org>, mike.kravetz@...cle.com,
        muchun.song@...ux.dev, akpm@...ux-foundation.org,
        Shuah Khan <shuah@...nel.org>
Cc:     linux-mm@...ck.org, open list <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH 2/2] selftests/mm: Add a new test for madv and hugetlb

On Wed, 2023-10-04 at 10:11 -0700, Breno Leitao wrote:
> 
> +char *huge_ptr;
> +
> +/* Touch the memory while it is being madvised() */
> +void *touch(void *unused)
> +{
> +       char *ptr = (char *)huge_ptr;
> +
> +       if (!ptr) {
> +               fprintf(stderr, "Failed to allocate memory\n");
> +               perror("");
> +       }

I'm not sure this error message makes a lot of sense
away from where the huge page gets allocated.

> 
> +       while (max--) {
> +               huge_ptr = mmap(NULL, MMAP_SIZE, PROT_READ |
> PROT_WRITE,
> +                               MAP_PRIVATE | MAP_ANONYMOUS |
> MAP_HUGETLB, -1, 0);
> +
> +               if ((unsigned long)huge_ptr == -1) {
> +                       perror("Failed to allocate\n");
> +                       continue;
> +               }

Should the test case just exit with an error here, when
the allocation fails?

Looping around when it cannot get memory seems pointless,
but telling the user that the allocation fails, when it
should clearly have succeeded could be useful.

This test case certainly seems to do the trick in showing
whether the race between MADV_DONTNEED and page faults
exists in a particular kernel.


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