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(-) -- 2.31.1
