Hi,

I am currently exploring the various ways of allocating memory in huge
pages under Linux. I somehow can not wrap my head around the concept of
the HugeTLB 'filesystem'. Note that I'm not talking about transparent huge
pages - those are a whole different beast.

The Conventional Way
--------------------

The conventional wisdom (as e.g. presented in the Debian Wiki[0] or the
Kernel docs[1]) seems to be:

- Make sure set your kernel configuration correctly
- set various kernel parameters right
- mount a special filesystem (`hugetlbfs`) to some arbitrary directory,
say `/dev/hugepages/` (that seems to be the default on Fedora…)
- `mmap()` a file within that directory into your address space, i.e.,
something like:

```
int fd = open("/dev/hugepages/myfile, O_CREAT | O_RDWR, 0755);
void * addr = mmap(0, 10*1024*1024, (PROT_READ | PROT_WRITE), MAP_SHARED,
fd, 0);
```

… and if these two calls succeed, I should have `addr` pointing to 10 MB
of memory allocated in five 2 MB huge pages. Nice.

The Easy Way
------------

However, this seems awfully overcomplicated?

At least on Linux 5.15 the whole filesystem thing seems to be completely
unnecessary. I just tried this:

* kernel configured with HugeTLBfs
* kernel parameters set correctly (i.e., `vm.nr_hugepages > 0`)
* no `hugetlbfs` mounted anywhere

And then just do an `mmap` of anonymous memory:

```
void *addr = mmap(0, 10*1024*1024, (PROT_READ | PROT_WRITE),
(MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB), 0, 0);
```

This gives me 10 MB of memory allocated in huge pages (at least if I don't
fail at interpreting the flags in the page table).

Why the Filesystem?
-------------------

So my question is: Why the filesystem? Is it actually "necessary" to go
via the filesystem, as the various guides suggest, and my attempt above
was just lucky? Does the filesystem approach have other advantages (aside
from having a file which represents parts of your RAM, which seems like a
huge footgun…)? Or is this maybe just a remnant from some previous time,
when `MAP_ANONYMOUS | MAP_HUGETLB` was not allowed?

Thanks for any hints!

Lukas

P.S.: If you collect internet points, you can also anwser this question on
the Unix StackExchange at[2].

[0] https://wiki.debian.org/Hugepages#Enabling_HugeTlbPage

[1] https://www.kernel.org/doc/html/latest/admin-guide/mm/
hugetlbpage.html#using-huge-pages

[2] https://unix.stackexchange.com/questions/753039/linux-hugetlb-what-is-
the-advantage-of-the-filesystem-approach