[v2,0/3] hugetlb: fix potential ref counting races

Message

Mike Kravetz Aug. 9, 2021, 6:48 p.m. UTC

When Muchun Song brought up a potential issue with hugetlb ref counting[1],
I started looking closer at the code.  hugetlbfs is the only code with it's
own specialized compound page destructor and taking special action when ref
counts drop to zero.  Potential races happen in this unique handling of ref
counts.  The following patches address these races when creating and
destroying hugetlb pages.

These potential races have likely existed since the creation of
hugetlbfs.  They certainly have been around for more than 10 years.
However, I am unaware of anyone actually hitting these races.  It is
VERY unlikely than anyone will actually hit these races, but they do
exist.

I could not think of an easy (or difficult) way to force these races.
Therefore, testing consisted of adding code to randomly increase ref
counts in strategic places.  In this way, I was able to exercise all the
race handling code paths.

[1] https://lore.kernel.org/linux-mm/CAMZfGtVMn3daKrJwZMaVOGOaJU+B4dS--x_oPmGQMD=c=[e-mail]/

v1 -> v2
- Update comment in prep_compound_gigantic_page as suggested by Muchun
- Add Muchun Reviewed-by: on patch 3.  Still an open question on whether
  the free_contig_range should be removed.

Mike Kravetz (3):
  hugetlb: simplify prep_compound_gigantic_page ref count racing code
  hugetlb: drop ref count earlier after page allocation
  hugetlb: before freeing hugetlb page set dtor to appropriate value

 mm/hugetlb.c | 139 +++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 106 insertions(+), 33 deletions(-)