On 2/17/21 12:46 PM, Peter Xu wrote: > Huge pmd sharing could bring problem to userfaultfd. The thing is that > userfaultfd is running its logic based on the special bits on page table > entries, however the huge pmd sharing could potentially share page table > entries for different address ranges. That could cause issues on either: > > - When sharing huge pmd page tables for an uffd write protected range, the > newly mapped huge pmd range will also be write protected unexpectedly, or, > > - When we try to write protect a range of huge pmd shared range, we'll first > do huge_pmd_unshare() in hugetlb_change_protection(), however that also > means the UFFDIO_WRITEPROTECT could be silently skipped for the shared > region, which could lead to data loss. > > Since at it, a few other things are done altogether: > > - Move want_pmd_share() from mm/hugetlb.c into linux/hugetlb.h, because > that's definitely something that arch code would like to use too > > - ARM64 currently directly check against CONFIG_ARCH_WANT_HUGE_PMD_SHARE when > trying to share huge pmd. Switch to the want_pmd_share() helper. > > Since at it, move vma_shareable() from huge_pmd_share() into want_pmd_share(). > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> > --- > arch/arm64/mm/hugetlbpage.c | 3 +-- > include/linux/hugetlb.h | 2 ++ > include/linux/userfaultfd_k.h | 9 +++++++++ > mm/hugetlb.c | 20 ++++++++++++++------ > 4 files changed, 26 insertions(+), 8 deletions(-) Thanks, Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> -- Mike Kravetz
