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Message-ID: <b9c72ab9-9687-4953-adfe-0a588a6dd0f7@redhat.com>
Date: Mon, 28 Jul 2025 17:06:38 +0200
From: David Hildenbrand <david@...hat.com>
To: Usama Arif <usamaarif642@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc: 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, lorenzo.stoakes@...cle.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 4/5] selftests: prctl: introduce tests for disabling THPs
completely
On 25.07.25 18:22, Usama Arif wrote:
> The test will set the global system THP setting to madvise and
> the 2M setting to inherit before it starts (and reset to original
> at teardown)
>
> This tests if the process can:
> - successfully set and get the policy to disable THPs completely.
> - never get a hugepage when the THPs are completely disabled,
> including with MADV_HUGE and MADV_COLLAPSE.
> - successfully reset the policy of the process.
> - get hugepages only on MADV_HUGE and MADV_COLLAPSE after reset.
> - repeat the above tests in a forked process to make sure
> the policy is carried across forks.
>
> Signed-off-by: Usama Arif <usamaarif642@...il.com>
> ---
> tools/testing/selftests/mm/.gitignore | 1 +
> tools/testing/selftests/mm/Makefile | 1 +
> .../testing/selftests/mm/prctl_thp_disable.c | 162 ++++++++++++++++++
> 3 files changed, 164 insertions(+)
> create mode 100644 tools/testing/selftests/mm/prctl_thp_disable.c
>
> diff --git a/tools/testing/selftests/mm/.gitignore b/tools/testing/selftests/mm/.gitignore
> index e7b23a8a05fe..eb023ea857b3 100644
> --- a/tools/testing/selftests/mm/.gitignore
> +++ b/tools/testing/selftests/mm/.gitignore
> @@ -58,3 +58,4 @@ pkey_sighandler_tests_32
> pkey_sighandler_tests_64
> guard-regions
> merge
> +prctl_thp_disable
> diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
> index d13b3cef2a2b..2bb8d3ebc17c 100644
> --- a/tools/testing/selftests/mm/Makefile
> +++ b/tools/testing/selftests/mm/Makefile
> @@ -86,6 +86,7 @@ TEST_GEN_FILES += on-fault-limit
> TEST_GEN_FILES += pagemap_ioctl
> TEST_GEN_FILES += pfnmap
> TEST_GEN_FILES += process_madv
> +TEST_GEN_FILES += prctl_thp_disable
> TEST_GEN_FILES += thuge-gen
> TEST_GEN_FILES += transhuge-stress
> TEST_GEN_FILES += uffd-stress
> diff --git a/tools/testing/selftests/mm/prctl_thp_disable.c b/tools/testing/selftests/mm/prctl_thp_disable.c
> new file mode 100644
> index 000000000000..52f7e6659b1f
> --- /dev/null
> +++ b/tools/testing/selftests/mm/prctl_thp_disable.c
> @@ -0,0 +1,162 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Basic tests for PR_GET/SET_THP_DISABLE prctl calls
> + *
> + * Author(s): Usama Arif <usamaarif642@...il.com>
> + */
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <unistd.h>
> +#include <sys/mman.h>
> +#include <sys/prctl.h>
> +#include <sys/wait.h>
> +
> +#include "../kselftest_harness.h"
> +#include "thp_settings.h"
> +#include "vm_util.h"
> +
> +#ifndef PR_THP_DISABLE_EXCEPT_ADVISED
> +#define PR_THP_DISABLE_EXCEPT_ADVISED (1 << 1)
> +#endif
Into patch #2 I guess.
> +
> +#define NR_HUGEPAGES 6
> +
> +static int sz2ord(size_t size, size_t pagesize)
> +{
> + return __builtin_ctzll(size / pagesize);
> +}
> +
> +enum madvise_buffer {
> + NONE,
> + HUGE,
> + COLLAPSE
> +};
Is that rather something like:
enum thp_collapse_type {
THP_COLLAPSE_NONE,
THP_COLLAPSE_MADV_HUGEPAGE, /* MADV_HUGEPAGE before access */
THP_COLLAPSE_MADV_COLLAPSE, /* MADV_COLLAPSE after access */
};
> +
> +/*
> + * Function to mmap a buffer, fault it in, madvise it appropriately (before
> + * page fault for MADV_HUGE, and after for MADV_COLLAPSE), and check if the
> + * mmap region is huge.
> + * returns:
> + * 0 if test doesn't give hugepage
> + * 1 if test gives a hugepage
> + * -1 if mmap fails
> + */
> +static int test_mmap_thp(enum madvise_buffer madvise_buf, size_t pmdsize)
> +{
> + int ret;
> + int buf_size = NR_HUGEPAGES * pmdsize;
> +
> + char *buffer = (char *)mmap(NULL, buf_size, PROT_READ | PROT_WRITE,
> + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
Can we get rid of NR_HUGEPAGES and just use a single one, aligning in a bigger area? This is similar to what we do in cow.c
/* For alignment purposes, we need twice the thp size. */
mmap_size = 2 * pmdsize;
mmap_area = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (mmap_area == MAP_FAILED)
return -errno; /* todo, document that above */
buffer = (char *)(((uintptr_t)mmap_area + pmdsize) & ~(pmdsize - 1));
...
ret = check_huge_anon(buffer, NR_HUGEPAGES, pmdsize);
...
munmap(mmap_area, mmap_size);
> + if (buffer == MAP_FAILED)
> + return -1;
> +
> + if (madvise_buf == HUGE)
> + madvise(buffer, buf_size, MADV_HUGEPAGE);
> +
> + /* Ensure memory is allocated */
> + memset(buffer, 1, buf_size);
> +
> + if (madvise_buf == COLLAPSE)
> + madvise(buffer, buf_size, MADV_COLLAPSE);
> +
> + ret = check_huge_anon(buffer, NR_HUGEPAGES, pmdsize);
> + munmap(buffer, buf_size);
> + return ret;
> +}
Empty line missing :)
> +FIXTURE(prctl_thp_disable_completely)
> +{
> + struct thp_settings settings;
> + size_t pmdsize;
> +};
> +
> +FIXTURE_SETUP(prctl_thp_disable_completely)
> +{
> + if (!thp_is_enabled())
> + SKIP(return, "Transparent Hugepages not available\n");
Heh, not completely correct. enabled != available.
Do we want a thp_available() that is essentially the first part of thp_is_enabled() ?
> +
> + self->pmdsize = read_pmd_pagesize();
> + if (!self->pmdsize)
> + SKIP(return, "Unable to read PMD size\n");
> +
> + thp_read_settings(&self->settings);
> + self->settings.thp_enabled = THP_MADVISE;
> + self->settings.hugepages[sz2ord(self->pmdsize, getpagesize())].enabled = THP_INHERIT;
> + thp_save_settings();
> + thp_push_settings(&self->settings);
push without pop, should that be alarming? :)
Can we just use thp_write_settings()? (not sure why that push/pop is required ... is it?)
> +}
> +
> +FIXTURE_TEARDOWN(prctl_thp_disable_completely)
> +{> + thp_restore_settings();
> +}
> +
> +/* prctl_thp_disable_except_madvise fixture sets system THP setting to madvise */
> +static void prctl_thp_disable_completely(struct __test_metadata *const _metadata,
> + size_t pmdsize)
> +{
> + int res = 0;
> +
> + res = prctl(PR_GET_THP_DISABLE, NULL, NULL, NULL, NULL);
> + ASSERT_EQ(res, 1);
> +
> + /* global = madvise, process = never, we shouldn't get HPs even with madvise */
s/HPs/THPs/
> + res = test_mmap_thp(NONE, pmdsize);
> + ASSERT_EQ(res, 0);
> +
> + res = test_mmap_thp(HUGE, pmdsize);
> + ASSERT_EQ(res, 0);
> +
> + res = test_mmap_thp(COLLAPSE, pmdsize);
> + ASSERT_EQ(res, 0);
> +
> + /* Reset to system policy */
> + res = prctl(PR_SET_THP_DISABLE, 0, NULL, NULL, NULL);
> + ASSERT_EQ(res, 0);
> +
> + /* global = madvise */
> + res = test_mmap_thp(NONE, pmdsize);
> + ASSERT_EQ(res, 0);
> +
> + res = test_mmap_thp(HUGE, pmdsize);
> + ASSERT_EQ(res, 1);
> +
> + res = test_mmap_thp(COLLAPSE, pmdsize);
> + ASSERT_EQ(res, 1);
Makes me wonder: should we test for global=always and global=always?
(or simply for all possible values, including global=never if easily possible?)
At least testing with global=always should exercise more possible paths
than global=always (esp., test_mmap_thp(NONE, pmdsize) which would
never apply in madvise mode).
--
Cheers,
David / dhildenb
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