On Mon, 16 Aug 2021 15:49:45 -0700 Mike Kravetz [off-list ref] wrote:

This is a resend of PATCHes sent here [4].  There was some discussion
and interest when the RFC [5] was sent, but little after that.  The
resend is just a rebase of [4] to next-20210816 with a few typos in
commmit messages fixed.

Original Cover Letter
---------------------
The concurrent use of multiple hugetlb page sizes on a single system
is becoming more common.  One of the reasons is better TLB support for
gigantic page sizes on x86 hardware.  In addition, hugetlb pages are
being used to back VMs in hosting environments.

When using hugetlb pages to back VMs in such environments, it is
sometimes desirable to preallocate hugetlb pools.  This avoids the delay
and uncertainty of allocating hugetlb pages at VM startup.  In addition,
preallocating huge pages minimizes the issue of memory fragmentation that
increases the longer the system is up and running.

In such environments, a combination of larger and smaller hugetlb pages
are preallocated in anticipation of backing VMs of various sizes.  Over
time, the preallocated pool of smaller hugetlb pages may become
depleted while larger hugetlb pages still remain.  In such situations,
it may be desirable to convert larger hugetlb pages to smaller hugetlb
pages.

Converting larger to smaller hugetlb pages can be accomplished today by
first freeing the larger page to the buddy allocator and then allocating
the smaller pages.  However, there are two issues with this approach:
1) This process can take quite some time, especially if allocation of
   the smaller pages is not immediate and requires migration/compaction.
2) There is no guarantee that the total size of smaller pages allocated
   will match the size of the larger page which was freed.  This is
   because the area freed by the larger page could quickly be
   fragmented.

To address these issues, introduce the concept of hugetlb page demotion.
Demotion provides a means of 'in place' splitting a hugetlb page to
pages of a smaller size.  For example, on x86 one 1G page can be
demoted to 512 2M pages.  Page demotion is controlled via sysfs files.
- demote_size   Read only target page size for demotion

Should this be "write only"?  If not, I'm confused.

If "yes" then "write only" would be a misnomer - clearly this file is
readable (looks at demote_size_show()).

- demote        Writable number of hugetlb pages to be demoted

So how does this interface work?  Write the target size to
`demote_size', write the number of to-be-demoted larger pages to
`demote' and then the operation happens?

If so, how does one select which size pages should be selected for
the demotion?

And how does one know the operation has completed so the sysfs files
can be reloaded for another operation?

Only hugetlb pages which are free at the time of the request can be demoted.
Demotion does not add to the complexity surplus pages.  Demotion also honors
reserved huge pages.  Therefore, when a value is written to the sysfs demote
file, that value is only the maximum number of pages which will be demoted.
It is possible fewer will actually be demoted.

If demote_size is PAGESIZE, demote will simply free pages to the buddy
allocator.

Real world use cases
--------------------
There are groups today using hugetlb pages to back VMs on x86.  Their
use case is as described above.  They have experienced the issues with
performance and not necessarily getting the excepted number smaller huge

("expected")

pages after free/allocate cycle.

It seems odd to add the interfaces in patch 1 then document them in
patch 5.  Why not add-and-document in a single patch?