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Message-ID: <13220ee2-d767-4133-9ef8-780fa165bbeb@lucifer.local>
Date: Wed, 13 Aug 2025 16:13:59 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, david@...hat.com,
linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, corbet@....net,
rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
hannes@...xchg.org, baohua@...nel.org, shakeel.butt@...ux.dev,
riel@...riel.com, ziy@...dia.com, laoar.shao@...il.com,
dev.jain@....com, baolin.wang@...ux.alibaba.com, npache@...hat.com,
Liam.Howlett@...cle.com, ryan.roberts@....com, vbabka@...e.cz,
jannh@...gle.com, Arnd Bergmann <arnd@...db.de>, sj@...nel.org,
linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
kernel-team@...a.com
Subject: Re: [PATCH v4 7/7] selftests: prctl: introduce tests for disabling
THPs except for madvise
On Wed, Aug 13, 2025 at 02:55:42PM +0100, Usama Arif wrote:
> The test will set the global system THP setting to never, madvise
> or always depending on the fixture variant and the 2M setting to
> inherit before it starts (and reset to original at teardown).
> The fixture setup will also test if PR_SET_THP_DISABLE prctl call can
> be made with PR_THP_DISABLE_EXCEPT_ADVISED and skip if it fails.
>
> This tests if the process can:
> - successfully get the policy to disable THPs expect for madvise.
> - get hugepages only on MADV_HUGE and MADV_COLLAPSE if the global policy
> is madvise/always and only with MADV_COLLAPSE if the global policy is
> never.
> - successfully reset the policy of the process.
> - after reset, only get hugepages with:
> - MADV_COLLAPSE when policy is set to never.
> - MADV_HUGE and MADV_COLLAPSE when policy is set to madvise.
> - always when policy is set to "always".
> - repeat the above tests in a forked process to make sure the policy is
> carried across forks.
>
> Test results:
> ./prctl_thp_disable
> TAP version 13
> 1..12
> ok 1 prctl_thp_disable_completely.never.nofork
> ok 2 prctl_thp_disable_completely.never.fork
> ok 3 prctl_thp_disable_completely.madvise.nofork
> ok 4 prctl_thp_disable_completely.madvise.fork
> ok 5 prctl_thp_disable_completely.always.nofork
> ok 6 prctl_thp_disable_completely.always.fork
> ok 7 prctl_thp_disable_except_madvise.never.nofork
> ok 8 prctl_thp_disable_except_madvise.never.fork
> ok 9 prctl_thp_disable_except_madvise.madvise.nofork
> ok 10 prctl_thp_disable_except_madvise.madvise.fork
> ok 11 prctl_thp_disable_except_madvise.always.nofork
> ok 12 prctl_thp_disable_except_madvise.always.fork
>
> Signed-off-by: Usama Arif <usamaarif642@...il.com>
I don't see any tests asserting VM_NOHUGEPAGE behaviour could you expand
this test to add that?
Other than that this is looking good.
Thanks!
> ---
> .../testing/selftests/mm/prctl_thp_disable.c | 107 ++++++++++++++++++
> 1 file changed, 107 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
> index 8845e9f414560..9bfed4598a1a6 100644
> --- a/tools/testing/selftests/mm/prctl_thp_disable.c
> +++ b/tools/testing/selftests/mm/prctl_thp_disable.c
> @@ -16,6 +16,10 @@
> #include "thp_settings.h"
> #include "vm_util.h"
>
> +#ifndef PR_THP_DISABLE_EXCEPT_ADVISED
> +#define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
> +#endif
> +
> enum thp_collapse_type {
> THP_COLLAPSE_NONE,
> THP_COLLAPSE_MADV_HUGEPAGE, /* MADV_HUGEPAGE before access */
> @@ -165,4 +169,107 @@ TEST_F(prctl_thp_disable_completely, fork)
> ASSERT_EQ(ret, 0);
> }
>
> +static void prctl_thp_disable_except_madvise_test(struct __test_metadata *const _metadata,
> + size_t pmdsize,
> + enum thp_enabled thp_policy)
> +{
> + ASSERT_EQ(prctl(PR_GET_THP_DISABLE, NULL, NULL, NULL, NULL), 3);
> +
> + /* tests after prctl overrides global policy */
> + ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_NONE, pmdsize), 0);
> +
> + ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize),
> + thp_policy == THP_NEVER ? 0 : 1);
> +
> + ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize), 1);
> +
> + /* Reset to global policy */
> + ASSERT_EQ(prctl(PR_SET_THP_DISABLE, 0, NULL, NULL, NULL), 0);
> +
> + /* tests after prctl is cleared, and only global policy is effective */
> + ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_NONE, pmdsize),
> + thp_policy == THP_ALWAYS ? 1 : 0);
> +
> + ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_HUGEPAGE, pmdsize),
> + thp_policy == THP_NEVER ? 0 : 1);
> +
> + ASSERT_EQ(test_mmap_thp(THP_COLLAPSE_MADV_COLLAPSE, pmdsize), 1);
> +}
> +
> +FIXTURE(prctl_thp_disable_except_madvise)
> +{
> + struct thp_settings settings;
> + size_t pmdsize;
> +};
> +
> +FIXTURE_VARIANT(prctl_thp_disable_except_madvise)
> +{
> + enum thp_enabled thp_policy;
> +};
> +
> +FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, never)
> +{
> + .thp_policy = THP_NEVER,
> +};
> +
> +FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, madvise)
> +{
> + .thp_policy = THP_MADVISE,
> +};
> +
> +FIXTURE_VARIANT_ADD(prctl_thp_disable_except_madvise, always)
> +{
> + .thp_policy = THP_ALWAYS,
> +};
again the kselftest_harness stuff is really useful!
> +
> +FIXTURE_SETUP(prctl_thp_disable_except_madvise)
> +{
> + if (!thp_available())
> + SKIP(return, "Transparent Hugepages not available\n");
> +
> + self->pmdsize = read_pmd_pagesize();
> + if (!self->pmdsize)
> + SKIP(return, "Unable to read PMD size\n");
> +
> + if (prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, NULL, NULL))
> + SKIP(return, "Unable to set PR_THP_DISABLE_EXCEPT_ADVISED\n");
This should be a test fail I think, as the only ways this could fail are
invalid flags, or failure to obtain an mmap write lock.
> +
> + thp_save_settings();
> + thp_read_settings(&self->settings);
> + self->settings.thp_enabled = variant->thp_policy;
> + self->settings.hugepages[sz2ord(self->pmdsize, getpagesize())].enabled = THP_INHERIT;
> + thp_write_settings(&self->settings);
> +}
> +
> +FIXTURE_TEARDOWN(prctl_thp_disable_except_madvise)
> +{
> + thp_restore_settings();
> +}
> +
> +TEST_F(prctl_thp_disable_except_madvise, nofork)
> +{
> + prctl_thp_disable_except_madvise_test(_metadata, self->pmdsize, variant->thp_policy);
> +}
> +
> +TEST_F(prctl_thp_disable_except_madvise, fork)
> +{
> + int ret = 0;
> + pid_t pid;
> +
> + /* Make sure prctl changes are carried across fork */
> + pid = fork();
> + ASSERT_GE(pid, 0);
> +
> + if (!pid)
> + prctl_thp_disable_except_madvise_test(_metadata, self->pmdsize,
> + variant->thp_policy);
> +
> + wait(&ret);
> + if (WIFEXITED(ret))
> + ret = WEXITSTATUS(ret);
> + else
> + ret = -EINVAL;
> + ASSERT_EQ(ret, 0);
> +}
> +
> TEST_HARNESS_MAIN
> --
> 2.47.3
>
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