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

[Jerome Glisse <jglisse@redhat.com>]

On Tue, Oct 22, 2019 at 11:41:57AM -0700, John Hubbard wrote:
> 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? :)

So i was misslead by the file descriptor, passing a file descriptor and
asking for anonymous always bugs me. But yeah the _linux_ kernel is happy
to ignore the file argument if you set the anonymous flag. I guess the
rules of passing -1 for fd when anonymous is just engrave in my brain.

Also i thought that the file was an argument of the test and thus that
for huge you needed to pass a hugetlbfs' file.

Anyway my mistake, you are right, you can pass a file and ask for anonymous
and hugetlb at the same time.

Reviewed-by: Jérôme Glisse <jglisse@redhat.com>