linux-kernel - [PATCH RFC v2 0/1] hugetlbfs: Use i_mmap

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Date:   Tue, 23 Oct 2018 21:50:52 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Prakash Sangappa <prakash.sangappa@...cle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: [PATCH RFC v2 0/1] hugetlbfs: Use i_mmap_rwsem for pmd share and fault/trunc

This patch addresses issues with page fault/truncation synchronization.
The first issue was noticed as a negative hugetlb reserved page counts
during DB development testing.  Code inspection revealed that the most
likely cause were races with truncate and page faults.  In fact, I could
write a not too complicated program to cause the races and recreate the
issue.

A more dangerous issue exists when you introduce huge pmd sharing to
page fault/truncate races.  The fist thing that happens in huge page
fault processing is a call to huge_pte_alloc to get a ptep.  Suppose
that ptep points to a shared pmd.  Now, another thread could perform
a truncate and unmap everyone mapping the file.  huge_pmd_unshare can
be called for the mapping on which the first thread is operating.
huge_pmd_unshare can clear pud pointing to the pmd.  After this, the
ptep points to another task's page table or worse.  This leads to bad
things such as incorrect page map/reference counts or invaid memory
references.

Fix this all by modifying the usage of i_mmap_rwsem to cover
fault/truncate races as well as handling of shared pmds

Mike Kravetz (1):
  hugetlbfs: use i_mmap_rwsem for pmd sharing and truncate/fault sync

 fs/hugetlbfs/inode.c | 21 ++++++++++----
 mm/hugetlb.c         | 65 +++++++++++++++++++++++++++++++++-----------
 mm/rmap.c            | 10 +++++++
 mm/userfaultfd.c     | 11 ++++++--
 4 files changed, 84 insertions(+), 23 deletions(-)

-- 
2.17.2