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(-)
>
