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

[Mike Kravetz]

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.
-- 
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(-)
>