While discussing the issue with huge_pte_offset [1], I remembered that there were more outstanding hugetlb races. These issues are: 1) For shared pmds, huge PTE pointers returned by huge_pte_alloc can become invalid via a call to huge_pmd_unshare by another thread. 2) hugetlbfs page faults can race with truncation causing invalid global reserve counts and state. A previous attempt was made to use i_mmap_rwsem in this manner as described at [2]. However, those patches were reverted starting with [3] due to locking issues. To effectively use i_mmap_rwsem to address the above issues it needs to be held (in read mode) during page fault processing. However, during fault processing we need to lock the page we will be adding. Lock ordering requires we take page lock before i_mmap_rwsem. Waiting until after taking the page lock is too late in the fault process for the synchronization we want to do. To address this lock ordering issue, the following patches change the lock ordering for hugetlb pages. This is not too invasive as hugetlbfs processing is done separate from core mm in many places. However, I don't really like this idea. Much ugliness is contained in the new routine hugetlb_page_mapping_lock_write() of patch 1. The only other way I can think of to address these issues is by catching all the races. After catching a race, cleanup, backout, retry ... etc, as needed. This can get really ugly, especially for huge page reservations. At one time, I started writing some of the reservation backout code for page faults and it got so ugly and complicated I went down the path of adding synchronization to avoid the races. Any other suggestions would be welcome. [1] https://lore.kernel.org/linux-mm/1582342427-230392-1-git-send-email-longpeng2@xxxxxxxxxx/ [2] https://lore.kernel.org/linux-mm/20181222223013.22193-1-mike.kravetz@xxxxxxxxxx/ [3] https://lore.kernel.org/linux-mm/20190103235452.29335-1-mike.kravetz@xxxxxxxxxx Mike Kravetz (2): hugetlbfs: use i_mmap_rwsem for more pmd sharing synchronization hugetlbfs: Use i_mmap_rwsem to address page fault/truncate race fs/hugetlbfs/inode.c | 34 +++++--- include/linux/fs.h | 5 ++ include/linux/hugetlb.h | 8 ++ mm/hugetlb.c | 175 +++++++++++++++++++++++++++++++++++----- mm/memory-failure.c | 29 ++++++- mm/migrate.c | 24 +++++- mm/rmap.c | 17 +++- mm/userfaultfd.c | 11 ++- 8 files changed, 264 insertions(+), 39 deletions(-) -- 2.24.1
