Re: [PATCH 0/3] hugetlb: fix potential ref counting races

[Muchun Song]

On Wed, Jul 28, 2021 at 7:41 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> 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:
> > 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@xxxxxxxxxxxxxx/
> >
> > 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(-)
> >