Message-ID: <3EB06925-1695-46E9-88E4-B85B4F1DBE8A@nvidia.com>
Date:   Wed, 24 Mar 2021 15:55:58 -0400
From:   "Zi Yan" <ziy@...dia.com>
To:     "David Rientjes" <rientjes@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Shuah Khan" <shuah@...nel.org>,
        "John Hubbard" <jhubbard@...dia.com>,
        "Sandipan Das" <sandipan@...ux.ibm.com>,
        "David Hildenbrand" <david@...hat.com>,
        "Yang Shi" <shy828301@...il.com>,
        "Mika Penttila" <mika.penttila@...tfour.com>
Subject: Re: [PATCH v6 1/2] mm: huge_memory: a new debugfs interface for
 splitting THP tests.

On 24 Mar 2021, at 15:16, David Rientjes wrote:

> On Mon, 22 Mar 2021, Zi Yan wrote:
>
>> From: Zi Yan <ziy@...dia.com>
>>
>> We did not have a direct user interface of splitting the compound page
>> backing a THP and there is no need unless we want to expose the THP
>> implementation details to users. Make <debugfs>/split_huge_pages accept
>> a new command to do that.
>>
>> By writing "<pid>,<vaddr_start>,<vaddr_end>" to
>> <debugfs>/split_huge_pages, THPs within the given virtual address range
>> from the process with the given pid 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.
>>
>
> I'm curious if this is the only use case or whether you have additional
> use cases or extensions in mind?

At the moment, this is the only use case I have in mind. I am developing
1GB THP support and split huge pages to any lower order (this will be useful
when Matthew Wilcox’s large page in page cache is in) and find there is no
way of splitting specific THPs (the compound pages) from user space.
So I added this interface for testing, mostly for splitting one or two
specified THPs. A potential extension might be to add a new <order>
parameter to test my split_huge_page_to_any_order(), after it gets in.

>
> Specifically, I'm wondering if you are looking into more appropriately
> dividing the limited number of hugepages available on the system amongst
> the most latency sensitive processes?
>
> The set of hugepages available on the system can be limited by
> fragmentation.  We've found opportunities where latency sensitive
> processes would benefit from memory backed by thp, but they cannot be
> allocated at fault for this reason.  Yet, there are other processes that
> have memory backed by hugepages that may not be benefiting from them.
>
> Could this be useful to split a hugepage for a latency tolerant
> application, migrate the pages elsewhere, and make the now-free hugepage
> available for a latency sensitive application (through something like my
> MADV_COLLAPSE proposal?)

The idea sounds quite interesting and reasonable. In the scenario you described,
I just wonder if we want to do SPLIT + COLLAPSE or we can combine them
into a single swap of THP and base pages (potentially using some extra free
pages as tmp space). Because there is no guarantee that the split subpages
can all be migrated and make space for a new THP, or other allocations might
steal a free page from the split THP range, causing subsequence THP allocation
failure.

I am OK with exposing this via a proper user space interface and can prepare
a patch for it. I just want to know if there are other use cases of splitting
THPs (the compound pages).

>
> Couple questions inline.
>
>> This does not change the old behavior, i.e., writing 1 to the interface
>> to split all THPs in the system.
>>
>> Changelog:
>>
>> From v5:
>> 1. Skipped special VMAs and other fixes. (suggested by Yang Shi)
>>
>> From v4:
>> 1. Fixed the error code return issue, spotted by kernel test robot
>>    <lkp@...el.com>.
>>
>> From v3:
>> 1. Factored out split huge pages in the given pid code to a separate
>>    function.
>> 2. Added the missing put_page for not split pages.
>> 3. pr_debug -> pr_info, make reading results simpler.
>>
>> From v2:
>> 1. Reused existing <debugfs>/split_huge_pages interface. (suggested by
>>    Yang Shi)
>>
>> From v1:
>> 1. Removed unnecessary calling to vma_migratable, spotted by kernel test
>>    robot <lkp@...el.com>.
>> 2. Dropped the use of find_mm_struct and code it directly, since there
>>    is no need for the permission check in that function and the function
>>    is only available when migration is on.
>> 3. Added some comments in the selftest program to clarify how PTE-mapped
>>    THPs are formed.
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> Reviewed-by: Yang Shi <shy828301@...il.com>
>> ---
>>  mm/huge_memory.c                              | 151 ++++++++-
>>  tools/testing/selftests/vm/.gitignore         |   1 +
>>  tools/testing/selftests/vm/Makefile           |   1 +
>>  .../selftests/vm/split_huge_page_test.c       | 318 ++++++++++++++++++
>>  4 files changed, 464 insertions(+), 7 deletions(-)
>>  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 bff92dea5ab3..b653255a548e 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>
>> @@ -2922,16 +2923,14 @@ static struct shrinker deferred_split_shrinker = {
>>  };
>>
>>  #ifdef CONFIG_DEBUG_FS
>> -static int split_huge_pages_set(void *data, u64 val)
>> +static void split_huge_pages_all(void)
>>  {
>>  	struct zone *zone;
>>  	struct page *page;
>>  	unsigned long pfn, max_zone_pfn;
>>  	unsigned long total = 0, split = 0;
>>
>> -	if (val != 1)
>> -		return -EINVAL;
>> -
>> +	pr_info("Split all THPs\n");
>
> Is this necessary to print?
This interface accepts three different split commands, so I would like to explicitly
print what command is executed during my debugging.

>
>>  	for_each_populated_zone(zone) {
>>  		max_zone_pfn = zone_end_pfn(zone);
>>  		for (pfn = zone->zone_start_pfn; pfn < max_zone_pfn; pfn++) {
>> @@ -2959,11 +2958,149 @@ static int split_huge_pages_set(void *data, u64 val)
>>  	}
>>
>>  	pr_info("%lu of %lu THP split\n", split, total);
>> +}
>>
>> -	return 0;
>> +static inline bool vma_not_suitable_for_thp_split(struct vm_area_struct *vma)
>> +{
>> +	return vma_is_special_huge(vma) || (vma->vm_flags & VM_IO) ||
>> +		    is_vm_hugetlb_page(vma);
>>  }
>> -DEFINE_DEBUGFS_ATTRIBUTE(split_huge_pages_fops, NULL, split_huge_pages_set,
>> -		"%llu\n");
>> +
>> +static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
>> +				unsigned long vaddr_end)
>> +{
>> +	int ret = 0;
>> +	struct task_struct *task;
>> +	struct mm_struct *mm;
>> +	unsigned long total = 0, split = 0;
>> +	unsigned long addr;
>> +
>> +	vaddr_start &= PAGE_MASK;
>> +	vaddr_end &= PAGE_MASK;
>> +
>> +	/* Find the task_struct from pid */
>> +	rcu_read_lock();
>> +	task = find_task_by_vpid(pid);
>> +	if (!task) {
>> +		rcu_read_unlock();
>> +		ret = -ESRCH;
>> +		goto out;
>> +	}
>> +	get_task_struct(task);
>> +	rcu_read_unlock();
>> +
>> +	/* Find the mm_struct */
>> +	mm = get_task_mm(task);
>> +	put_task_struct(task);
>> +
>> +	if (!mm) {
>> +		ret = -EINVAL;
>> +		goto out;
>> +	}
>> +
>> +	pr_info("Split huge pages in pid: %d, vaddr: [0x%lx - 0x%lx]\n",
>> +		 pid, vaddr_start, vaddr_end);
>
> Hmm, does this need to be in the kernel log?

Since this is a debugfs interface, I expect it is only used for debugging,
which should be rare. pr_info would save us from the additional enabling
dynamic debug printing.

>
>> +
>> +	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)
>> +			break;
>> +
>
> Should there be a cond_resched() right here?

I did not expect we are going to use this to split a lot of pages.
But I will add one here in case someone uses it that way.

>
>> +		/* skip special VMA and hugetlb VMA */
>> +		if (vma_not_suitable_for_thp_split(vma)) {
>> +			addr = vma->vm_end;
>> +			continue;
>> +		}
>> +
>> +		/* FOLL_DUMP to ignore special (like zero) pages */
>> +		follflags = FOLL_GET | FOLL_DUMP;
>> +		page = follow_page(vma, addr, follflags);
>> +
>> +		if (IS_ERR(page))
>> +			continue;
>> +		if (!page)
>> +			continue;
>> +
>> +		if (!is_transparent_hugepage(page))
>> +			goto next;
>> +
>> +		total++;
>> +		if (!can_split_huge_page(compound_head(page), NULL))
>> +			goto next;
>> +
>> +		if (!trylock_page(page))
>> +			goto next;
>> +
>> +		if (!split_huge_page(page))
>> +			split++;
>> +
>> +		unlock_page(page);
>> +next:
>> +		put_page(page);
>> +	}
>> +	mmap_read_unlock(mm);
>> +	mmput(mm);
>> +
>> +	pr_info("%lu of %lu THP split\n", split, total);
>> +
>> +out:
>> +	return ret;
>> +}
>> +
>> +#define MAX_INPUT_BUF_SZ 255
>> +
>> +static ssize_t split_huge_pages_write(struct file *file, const char __user *buf,
>> +				size_t count, loff_t *ppops)
>> +{
>> +	static DEFINE_MUTEX(split_debug_mutex);
>> +	ssize_t ret;
>> +	char input_buf[MAX_INPUT_BUF_SZ]; /* hold pid, start_vaddr, end_vaddr */
>> +	int pid;
>> +	unsigned long vaddr_start, vaddr_end;
>> +
>> +	ret = mutex_lock_interruptible(&split_debug_mutex);
>> +	if (ret)
>> +		return ret;
>> +
>> +	ret = -EFAULT;
>> +
>> +	memset(input_buf, 0, MAX_INPUT_BUF_SZ);
>> +	if (copy_from_user(input_buf, buf, min_t(size_t, count, MAX_INPUT_BUF_SZ)))
>> +		goto out;
>> +
>> +	input_buf[MAX_INPUT_BUF_SZ - 1] = '\0';
>> +	ret = sscanf(input_buf, "%d,0x%lx,0x%lx", &pid, &vaddr_start, &vaddr_end);
>> +	if (ret == 1 && pid == 1) {
>> +		split_huge_pages_all();
>> +		ret = strlen(input_buf);
>> +		goto out;
>> +	} else if (ret != 3) {
>> +		ret = -EINVAL;
>> +		goto out;
>> +	}
>> +
>> +	ret = split_huge_pages_pid(pid, vaddr_start, vaddr_end);
>> +	if (!ret)
>> +		ret = strlen(input_buf);
>> +out:
>> +	mutex_unlock(&split_debug_mutex);
>> +	return ret;
>> +
>> +}
>> +
>> +static const struct file_operations split_huge_pages_fops = {
>> +	.owner	 = THIS_MODULE,
>> +	.write	 = split_huge_pages_write,
>> +	.llseek  = no_llseek,
>> +};
>>
>>  static int __init split_huge_pages_debugfs(void)
>>  {


—
Best Regards,
Yan Zi

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