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To:     Zi Yan <ziy@nvidia.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Sandipan Das <sandipan@linux.ibm.com>,
        David Rientjes <rientjes@google.com>,
        Alex Shi <alex.shi@linux.alibaba.com>
References: <20210308152221.28555-1-zi.yan@sent.com>
 <79458c46-b4b9-332b-77f7-44371502cbeb@redhat.com>
 <AD0AFA61-8DCF-467D-B961-ACFA1D33828C@nvidia.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [PATCH] mm: huge_memory: a new debugfs interface for splitting
 THP tests.
Message-ID: <8039e1d7-3442-f133-f4f6-fe934f02122e@redhat.com>
Date:   Mon, 8 Mar 2021 19:11:02 +0100
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On 08.03.21 18:49, Zi Yan wrote:
> On 8 Mar 2021, at 11:17, David Hildenbrand wrote:
> 
>> On 08.03.21 16:22, Zi Yan wrote:
>>> From: Zi Yan <ziy@nvidia.com>
>>>
>>> By writing "<pid>,<vaddr_start>,<vaddr_end>" to
>>> <debugfs>/split_huge_pages_in_range_pid, THPs in the process with the
>>> given pid and virtual address range are split. It is used to test
>>> split_huge_page function. In addition, a selftest program is added to
>>> tools/testing/selftests/vm to utilize the interface by splitting
>>> PMD THPs and PTE-mapped THPs.
>>
>> Won't something like
>>
>> 1. MADV_HUGEPAGE
>>
>> 2. Access memory
>>
>> 3. MADV_NOHUGEPAGE
>>
>> Have a similar effect? What's the benefit of this?
> 
> Thanks for checking the patch.
> 
> No, MADV_NOHUGEPAGE just replaces VM_HUGEPAGE with VM_NOHUGEPAGE,
> nothing else will be done.

Ah, okay - maybe my memory was tricking me. There is some s390x KVM code 
that forces MADV_NOHUGEPAGE and force-splits everything.

I do wonder, though, if this functionality would be worth a proper user 
interface (e.g., madvise), though. There might be actual benefit in 
having this as a !debug interface.

I think you aware of the discussion in 
https://lkml.kernel.org/r/d098c392-273a-36a4-1a29-59731cdf5d3d@google.com


If there will be an interface to collapse a THP -- "this memory area is 
worth extra performance now by collapsing a THP if possible" -- it might 
also be helpful to have the opposite functionality -- "this memory area 
is not worth a THP, rather use that somehwere else".

MADV_HUGE_COLLAPSE vs. MADV_HUGE_SPLIT

Just a thought.

> 
> Without this, we do not have a way of splitting a specific THP
> (the compound page) via any user interface for debugging.
> We can only write to <debugfs>/split_huge_pages to split all THPs
> in the system, which has an unwanted effect on the whole system
> and can overwhelm us with a lot of information. This new debugfs
> interface provides a more precise method.
> 
>>>
>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>> ---
>>>    mm/huge_memory.c                              |  98 ++++++
>>>    mm/internal.h                                 |   1 +
>>>    mm/migrate.c                                  |   2 +-
>>>    tools/testing/selftests/vm/.gitignore         |   1 +
>>>    tools/testing/selftests/vm/Makefile           |   1 +
>>>    .../selftests/vm/split_huge_page_test.c       | 318 ++++++++++++++++++
>>>    6 files changed, 420 insertions(+), 1 deletion(-)
>>>    create mode 100644 tools/testing/selftests/vm/split_huge_page_test.c
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 395c75111d33..818172f887bf 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -7,6 +7,7 @@
>>>     #include <linux/mm.h>
>>>    #include <linux/sched.h>
>>> +#include <linux/sched/mm.h>
>>>    #include <linux/sched/coredump.h>
>>>    #include <linux/sched/numa_balancing.h>
>>>    #include <linux/highmem.h>
>>> @@ -2971,10 +2972,107 @@ static int split_huge_pages_set(void *data, u64 val)
>>>    DEFINE_DEBUGFS_ATTRIBUTE(split_huge_pages_fops, NULL, split_huge_pages_set,
>>>    		"%llu\n");
>>>   +static ssize_t split_huge_pages_in_range_pid_write(struct file *file,
>>> +		const char __user *buf, size_t count, loff_t *ppops)
>>> +{
>>> +	static DEFINE_MUTEX(mutex);
>>> +	ssize_t ret;
>>> +	char input_buf[80]; /* hold pid, start_vaddr, end_vaddr */
>>> +	int pid;
>>> +	unsigned long vaddr_start, vaddr_end, addr;
>>> +	nodemask_t task_nodes;
>>> +	struct mm_struct *mm;
>>> +	unsigned long total = 0, split = 0;
>>> +
>>> +	ret = mutex_lock_interruptible(&mutex);
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	ret = -EFAULT;
>>> +
>>> +	memset(input_buf, 0, 80);
>>> +	if (copy_from_user(input_buf, buf, min_t(size_t, count, 80)))
>>> +		goto out;
>>> +
>>> +	input_buf[79] = '\0';
>>> +	ret = sscanf(input_buf, "%d,0x%lx,0x%lx", &pid, &vaddr_start, &vaddr_end);
>>> +	if (ret != 3) {
>>> +		ret = -EINVAL;
>>> +		goto out;
>>> +	}
>>> +	vaddr_start &= PAGE_MASK;
>>> +	vaddr_end &= PAGE_MASK;
>>> +
>>> +	ret = strlen(input_buf);
>>> +	pr_debug("split huge pages in pid: %d, vaddr: [%lx - %lx]\n",
>>> +		 pid, vaddr_start, vaddr_end);
>>> +
>>> +	mm = find_mm_struct(pid, &task_nodes);
>>> +	if (IS_ERR(mm)) {
>>> +		ret = -EINVAL;
>>> +		goto out;
>>> +	}
>>> +
>>> +	mmap_read_lock(mm);
>>> +	/*
>>> +	 * always increase addr by PAGE_SIZE, since we could have a PTE page
>>> +	 * table filled with PTE-mapped THPs, each of which is distinct.
>>> +	 */
>>> +	for (addr = vaddr_start; addr < vaddr_end; addr += PAGE_SIZE) {
>>> +		struct vm_area_struct *vma = find_vma(mm, addr);
>>> +		unsigned int follflags;
>>> +		struct page *page;
>>> +
>>> +		if (!vma || addr < vma->vm_start || !vma_migratable(vma))
>>> +			break;
>>> +
>>> +		/* FOLL_DUMP to ignore special (like zero) pages */
>>> +		follflags = FOLL_GET | FOLL_DUMP;
>>> +		page = follow_page(vma, addr, follflags);
>>> +
>>> +		if (IS_ERR(page))
>>> +			break;
>>> +		if (!page)
>>> +			break;
>>> +
>>> +		if (!is_transparent_hugepage(page))
>>> +			continue;
>>> +
>>> +		total++;
>>> +		if (!can_split_huge_page(compound_head(page), NULL))
>>> +			continue;
>>> +
>>> +		if (!trylock_page(page))
>>> +			continue;
>>> +
>>> +		if (!split_huge_page(page))
>>> +			split++;
>>> +
>>> +		unlock_page(page);
>>> +		put_page(page);
>>> +	}
>>> +	mmap_read_unlock(mm);
>>> +	mmput(mm);
>>> +
>>> +	pr_debug("%lu of %lu THP split\n", split, total);
>>> +out:
>>> +	mutex_unlock(&mutex);
>>> +	return ret;
>>> +
>>> +}
>>> +
>>> +static const struct file_operations split_huge_pages_in_range_pid_fops = {
>>> +	.owner	 = THIS_MODULE,
>>> +	.write	 = split_huge_pages_in_range_pid_write,
>>> +	.llseek  = no_llseek,
>>> +};
>>> +
>>>    static int __init split_huge_pages_debugfs(void)
>>>    {
>>>    	debugfs_create_file("split_huge_pages", 0200, NULL, NULL,
>>>    			    &split_huge_pages_fops);
>>> +	debugfs_create_file("split_huge_pages_in_range_pid", 0200, NULL, NULL,
>>> +			    &split_huge_pages_in_range_pid_fops);
>>>    	return 0;
>>>    }
>>>    late_initcall(split_huge_pages_debugfs);
>>> diff --git a/mm/internal.h b/mm/internal.h
>>> index 9902648f2206..1659d00100ef 100644
>>> --- a/mm/internal.h
>>> +++ b/mm/internal.h
>>> @@ -623,4 +623,5 @@ struct migration_target_control {
>>>    	gfp_t gfp_mask;
>>>    };
>>>   +struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes);
>>>    #endif	/* __MM_INTERNAL_H */
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 62b81d5257aa..ce5f213debb2 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1913,7 +1913,7 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
>>>    	return nr_pages ? -EFAULT : 0;
>>>    }
>>>   -static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
>>> +struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
>>>    {
>>>    	struct task_struct *task;
>>>    	struct mm_struct *mm;
>>> diff --git a/tools/testing/selftests/vm/.gitignore b/tools/testing/selftests/vm/.gitignore
>>> index 9a35c3f6a557..1f651e85ed60 100644
>>> --- a/tools/testing/selftests/vm/.gitignore
>>> +++ b/tools/testing/selftests/vm/.gitignore
>>> @@ -22,3 +22,4 @@ map_fixed_noreplace
>>>    write_to_hugetlbfs
>>>    hmm-tests
>>>    local_config.*
>>> +split_huge_page_test
>>> diff --git a/tools/testing/selftests/vm/Makefile b/tools/testing/selftests/vm/Makefile
>>> index d42115e4284d..4cbc91d6869f 100644
>>> --- a/tools/testing/selftests/vm/Makefile
>>> +++ b/tools/testing/selftests/vm/Makefile
>>> @@ -42,6 +42,7 @@ TEST_GEN_FILES += on-fault-limit
>>>    TEST_GEN_FILES += thuge-gen
>>>    TEST_GEN_FILES += transhuge-stress
>>>    TEST_GEN_FILES += userfaultfd
>>> +TEST_GEN_FILES += split_huge_page_test
>>>     ifeq ($(MACHINE),x86_64)
>>>    CAN_BUILD_I386 := $(shell ./../x86/check_cc.sh $(CC) ../x86/trivial_32bit_program.c -m32)
>>> diff --git a/tools/testing/selftests/vm/split_huge_page_test.c b/tools/testing/selftests/vm/split_huge_page_test.c
>>> new file mode 100644
>>> index 000000000000..8ea8000fda62
>>> --- /dev/null
>>> +++ b/tools/testing/selftests/vm/split_huge_page_test.c
>>> @@ -0,0 +1,318 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +/*
>>> + * A test of splitting PMD THPs and PTE-mapped THPs from a specified virtual
>>> + * address range in a process via <debugfs>/split_huge_pages_in_range_pid
>>> + * interface.
>>> + */
>>> +
>>> +#define _GNU_SOURCE
>>> +#include <stdio.h>
>>> +#include <stdlib.h>
>>> +#include "numa.h"
>>> +#include <unistd.h>
>>> +#include <errno.h>
>>> +#include <inttypes.h>
>>> +#include <string.h>
>>> +#include <sys/types.h>
>>> +#include <sys/stat.h>
>>> +#include <fcntl.h>
>>> +#include <sys/mman.h>
>>> +#include <sys/time.h>
>>> +#include <sys/wait.h>
>>> +#include <malloc.h>
>>> +#include <stdbool.h>
>>> +
>>> +uint64_t pagesize;
>>> +unsigned int pageshift;
>>> +uint64_t pmd_pagesize;
>>> +
>>> +#define PMD_SIZE_PATH "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
>>> +#define SPLIT_DEBUGFS "/sys/kernel/debug/split_huge_pages_in_range_pid"
>>> +#define SMAP_PATH "/proc/self/smaps"
>>> +#define INPUT_MAX 80
>>> +
>>> +#define PFN_MASK     ((1UL<<55)-1)
>>> +#define KPF_THP      (1UL<<22)
>>> +
>>> +int is_backed_by_thp(char *vaddr, int pagemap_file, int kpageflags_file)
>>> +{
>>> +	uint64_t paddr;
>>> +	uint64_t page_flags;
>>> +
>>> +	if (pagemap_file) {
>>> +		pread(pagemap_file, &paddr, sizeof(paddr),
>>> +			((long)vaddr >> pageshift) * sizeof(paddr));
>>> +
>>> +		if (kpageflags_file) {
>>> +			pread(kpageflags_file, &page_flags, sizeof(page_flags),
>>> +				(paddr & PFN_MASK) * sizeof(page_flags));
>>> +
>>> +			return !!(page_flags & KPF_THP);
>>> +		}
>>> +	}
>>> +	return 0;
>>> +}
>>> +
>>> +
>>> +static uint64_t read_pmd_pagesize(void)
>>> +{
>>> +	int fd;
>>> +	char buf[20];
>>> +	ssize_t num_read;
>>> +
>>> +	fd = open(PMD_SIZE_PATH, O_RDONLY);
>>> +	if (fd == -1) {
>>> +		perror("Open hpage_pmd_size failed");
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +	num_read = read(fd, buf, 19);
>>> +	if (num_read < 1) {
>>> +		close(fd);
>>> +		perror("Read hpage_pmd_size failed");
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +	buf[num_read] = '\0';
>>> +	close(fd);
>>> +
>>> +	return strtoul(buf, NULL, 10);
>>> +}
>>> +
>>> +static int write_file(const char *path, const char *buf, size_t buflen)
>>> +{
>>> +	int fd;
>>> +	ssize_t numwritten;
>>> +
>>> +	fd = open(path, O_WRONLY);
>>> +	if (fd == -1)
>>> +		return 0;
>>> +
>>> +	numwritten = write(fd, buf, buflen - 1);
>>> +	close(fd);
>>> +	if (numwritten < 1)
>>> +		return 0;
>>> +
>>> +	return (unsigned int) numwritten;
>>> +}
>>> +
>>> +static void write_debugfs(int pid, uint64_t vaddr_start, uint64_t vaddr_end)
>>> +{
>>> +	char input[INPUT_MAX];
>>> +	int ret;
>>> +
>>> +	ret = snprintf(input, INPUT_MAX, "%d,0x%lx,0x%lx", pid, vaddr_start,
>>> +			vaddr_end);
>>> +	if (ret >= INPUT_MAX) {
>>> +		printf("%s: Debugfs input is too long\n", __func__);
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +	if (!write_file(SPLIT_DEBUGFS, input, ret + 1)) {
>>> +		perror(SPLIT_DEBUGFS);
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +}
>>> +
>>> +#define MAX_LINE_LENGTH 500
>>> +
>>> +static bool check_for_pattern(FILE *fp, const char *pattern, char *buf)
>>> +{
>>> +	while (fgets(buf, MAX_LINE_LENGTH, fp) != NULL) {
>>> +		if (!strncmp(buf, pattern, strlen(pattern)))
>>> +			return true;
>>> +	}
>>> +	return false;
>>> +}
>>> +
>>> +static uint64_t check_huge(void *addr)
>>> +{
>>> +	uint64_t thp = 0;
>>> +	int ret;
>>> +	FILE *fp;
>>> +	char buffer[MAX_LINE_LENGTH];
>>> +	char addr_pattern[MAX_LINE_LENGTH];
>>> +
>>> +	ret = snprintf(addr_pattern, MAX_LINE_LENGTH, "%08lx-",
>>> +		       (unsigned long) addr);
>>> +	if (ret >= MAX_LINE_LENGTH) {
>>> +		printf("%s: Pattern is too long\n", __func__);
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +
>>> +	fp = fopen(SMAP_PATH, "r");
>>> +	if (!fp) {
>>> +		printf("%s: Failed to open file %s\n", __func__, SMAP_PATH);
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +	if (!check_for_pattern(fp, addr_pattern, buffer))
>>> +		goto err_out;
>>> +
>>> +	/*
>>> +	 * Fetch the AnonHugePages: in the same block and check the number of
>>> +	 * hugepages.
>>> +	 */
>>> +	if (!check_for_pattern(fp, "AnonHugePages:", buffer))
>>> +		goto err_out;
>>> +
>>> +	if (sscanf(buffer, "AnonHugePages:%10ld kB", &thp) != 1) {
>>> +		printf("Reading smap error\n");
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +err_out:
>>> +	fclose(fp);
>>> +	return thp;
>>> +}
>>> +
>>> +void split_pmd_thp(void)
>>> +{
>>> +	char *one_page;
>>> +	size_t len = 4 * pmd_pagesize;
>>> +	uint64_t thp_size;
>>> +	size_t i;
>>> +
>>> +	one_page = memalign(pmd_pagesize, len);
>>> +
>>> +	madvise(one_page, len, MADV_HUGEPAGE);
>>> +
>>> +	for (i = 0; i < len; i++)
>>> +		one_page[i] = (char)i;
>>> +
>>> +	thp_size = check_huge(one_page);
>>> +	if (!thp_size) {
>>> +		printf("No THP is allocatd");
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +	/* split all possible huge pages */
>>> +	write_debugfs(getpid(), (uint64_t)one_page, (uint64_t)one_page + len);
>>> +
>>> +	for (i = 0; i < len; i++)
>>> +		if (one_page[i] != (char)i) {
>>> +			printf("%ld byte corrupted\n", i);
>>> +			exit(EXIT_FAILURE);
>>> +		}
>>> +
>>> +
>>> +	thp_size = check_huge(one_page);
>>> +	if (thp_size) {
>>> +		printf("Still %ld kB AnonHugePages not split\n", thp_size);
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +	printf("Split huge pages successful\n");
>>> +	free(one_page);
>>> +}
>>> +
>>> +void split_pte_mapped_thp(void)
>>> +{
>>> +	char *one_page, *pte_mapped, *pte_mapped2;
>>> +	size_t len = 4 * pmd_pagesize;
>>> +	uint64_t thp_size;
>>> +	size_t i;
>>> +	const char *pagemap_template = "/proc/%d/pagemap";
>>> +	const char *kpageflags_proc = "/proc/kpageflags";
>>> +	char pagemap_proc[255];
>>> +	int pagemap_fd;
>>> +	int kpageflags_fd;
>>> +
>>> +	if (snprintf(pagemap_proc, 255, pagemap_template, getpid()) < 0) {
>>> +		perror("get pagemap proc error");
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +	pagemap_fd = open(pagemap_proc, O_RDONLY);
>>> +
>>> +	if (pagemap_fd == -1) {
>>> +		perror("read pagemap:");
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +	kpageflags_fd = open(kpageflags_proc, O_RDONLY);
>>> +
>>> +	if (kpageflags_fd == -1) {
>>> +		perror("read kpageflags:");
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +	one_page = mmap((void *)(1UL << 30), len, PROT_READ | PROT_WRITE,
>>> +			MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>>> +
>>> +	madvise(one_page, len, MADV_HUGEPAGE);
>>> +
>>> +	for (i = 0; i < len; i++)
>>> +		one_page[i] = (char)i;
>>> +
>>> +	thp_size = check_huge(one_page);
>>> +	if (!thp_size) {
>>> +		printf("No THP is allocatd");
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +	pte_mapped = mremap(one_page, pagesize, pagesize, MREMAP_MAYMOVE);
>>> +
>>> +	for (i = 1; i < 4; i++) {
>>> +		pte_mapped2 = mremap(one_page + pmd_pagesize * i + pagesize * i,
>>> +				     pagesize, pagesize,
>>> +				     MREMAP_MAYMOVE|MREMAP_FIXED,
>>> +				     pte_mapped + pagesize * i);
>>> +		if (pte_mapped2 == (char *)-1) {
>>> +			perror("mremap failed");
>>> +			exit(EXIT_FAILURE);
>>> +		}
>>> +	}
>>> +
>>> +	/* smap does not show THPs after mremap, use kpageflags instead */
>>> +	thp_size = 0;
>>> +	for (i = 0; i < pagesize * 4; i++)
>>> +		if (i % pagesize == 0 &&
>>> +		    is_backed_by_thp(&pte_mapped[i], pagemap_fd, kpageflags_fd))
>>> +			thp_size++;
>>> +
>>> +	if (thp_size != 4) {
>>> +		printf("Some THPs are missing during mremap\n");
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +	/* split all possible huge pages */
>>> +	write_debugfs(getpid(), (uint64_t)pte_mapped,
>>> +		      (uint64_t)pte_mapped + pagesize * 4);
>>> +
>>> +	/* smap does not show THPs after mremap, use kpageflags instead */
>>> +	thp_size = 0;
>>> +	for (i = 0; i < pagesize * 4; i++) {
>>> +		if (pte_mapped[i] != (char)i) {
>>> +			printf("%ld byte corrupted\n", i);
>>> +			exit(EXIT_FAILURE);
>>> +		}
>>> +		if (i % pagesize == 0 &&
>>> +		    is_backed_by_thp(&pte_mapped[i], pagemap_fd, kpageflags_fd))
>>> +			thp_size++;
>>> +	}
>>> +
>>> +	if (thp_size) {
>>> +		printf("Still %ld THPs not split\n", thp_size);
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +	printf("Split PTE-mapped huge pages successful\n");
>>> +	munmap(one_page, len);
>>> +	close(pagemap_fd);
>>> +	close(kpageflags_fd);
>>> +}
>>> +
>>> +int main(int argc, char **argv)
>>> +{
>>> +	if (geteuid() != 0) {
>>> +		printf("Please run the benchmark as root\n");
>>> +		exit(EXIT_FAILURE);
>>> +	}
>>> +
>>> +	pagesize = getpagesize();
>>> +	pageshift = ffs(pagesize) - 1;
>>> +	pmd_pagesize = read_pmd_pagesize();
>>> +
>>> +	split_pmd_thp();
>>> +	split_pte_mapped_thp();
>>> +
>>> +	return 0;
>>> +}
>>>
>>
>>
>> -- 
>> Thanks,
>>
>> David / dhildenb
> 
> 
> —
> Best Regards,
> Yan Zi
> 


-- 
Thanks,

David / dhildenb