Re: [PATCH 1/1] mm/gup_benchmark: fix MAP_HUGETLB case

[John Hubbard]

On 10/22/19 10:14 AM, Jerome Glisse wrote:
> On Mon, Oct 21, 2019 at 02:24:35PM -0700, John Hubbard wrote:
>> The MAP_HUGETLB ("-H" option) of gup_benchmark fails:
>>
>> $ sudo ./gup_benchmark -H
>> mmap: Invalid argument
>>
>> This is because gup_benchmark.c is passing in a file descriptor to
>> mmap(), but the fd came from opening up the /dev/zero file. This
>> confuses the mmap syscall implementation, which thinks that, if the
>> caller did not specify MAP_ANONYMOUS, then the file must be a huge
>> page file. So it attempts to verify that the file really is a huge
>> page file, as you can see here:
>>
>> ksys_mmap_pgoff()
>> {
>>     if (!(flags & MAP_ANONYMOUS)) {
>>         retval = -EINVAL;
>>         if (unlikely(flags & MAP_HUGETLB && !is_file_hugepages(file)))
>>             goto out_fput; /* THIS IS WHERE WE END UP */
>>
>>     else if (flags & MAP_HUGETLB) {
>>         ...proceed normally, /dev/zero is ok here...
>>
>> ...and of course is_file_hugepages() returns "false" for the /dev/zero
>> file.
>>
>> The problem is that the user space program, gup_benchmark.c, really just
>> wants anonymous memory here. The simplest way to get that is to pass
>> MAP_ANONYMOUS whenever MAP_HUGETLB is specified, so that's what this
>> patch does.
> 
> This looks wrong, MAP_HUGETLB should only be use to create vma
> for hugetlbfs. If you want anonymous private vma do not set the
> MAP_HUGETLB. If you want huge page inside your anonymous vma
> there is nothing to do at the mmap time, this is the job of the
> transparent huge page code (THP).
> 

Not the point. Please look more closely at ksys_mmap_pgoff(). You'll 
see that, since 2009 (and probably earlier; 2009 is just when Hugh Dickens 
moved it over from util.c), this routine has had full support for using
hugetlbfs automatically, via mmap.

It does that via hugetlb_file_setup():

unsigned long ksys_mmap_pgoff(unsigned long addr, unsigned long len,
			      unsigned long prot, unsigned long flags,
			      unsigned long fd, unsigned long pgoff)
{
...
	if (!(flags & MAP_ANONYMOUS)) {
...
	} else if (flags & MAP_HUGETLB) {
		struct user_struct *user = NULL;
		struct hstate *hs;

		hs = hstate_sizelog((flags >> MAP_HUGE_SHIFT) & MAP_HUGE_MASK);
		if (!hs)
			return -EINVAL;

		len = ALIGN(len, huge_page_size(hs));
		/*
		 * VM_NORESERVE is used because the reservations will be
		 * taken when vm_ops->mmap() is called
		 * A dummy user value is used because we are not locking
		 * memory so no accounting is necessary
		 */
		file = hugetlb_file_setup(HUGETLB_ANON_FILE, len,
				VM_NORESERVE,
				&user, HUGETLB_ANONHUGE_INODE,
				(flags >> MAP_HUGE_SHIFT) & MAP_HUGE_MASK);
		if (IS_ERR(file))
			return PTR_ERR(file);
	}
...


Also, there are 14 (!) other pre-existing examples of passing
MAP_HUGETLB | MAP_ANONYMOUS to mmap, so I'm not exactly the first one
to reach this understanding.


> NAK as misleading

Ouch. But I think I'm actually leading correctly, rather than misleading.
Can you prove me wrong? :)


thanks,

John Hubbard
NVIDIA