On Wed, Jul 28, 2021 at 7:41 AM Mike Kravetz [off-list ref] wrote:

Any additional comments on these patches/this approach?

The first patch addressing this issue actually went into the 5.14 merge
window as commit 7118fc2906e2 ("hugetlb: address ref count racing in
prep_compound_gigantic_page").

All this code is very tricky and subtle.  It addresses potential issues
discovered by code analysis.  I do not believe the races have ever been
experienced in practice.  If anyone has suggestions for a simpler or
alternative approach, I would love to hear them.

Hi Mike,

I agree with you that this code is very tricky and subtle. I have looked
at this patch set. For me, I cannot figure out a better solution for this
race.

--
Thanks,
Muchun

--
Mike Kravetz


On 7/9/21 5:24 PM, Mike Kravetz wrote:

quoted

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=QNGEg@mail.gmail.com/ (local)

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 | 137 ++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 104 insertions(+), 33 deletions(-)