hugetlb: fix vma lock handling during split vma and range unmapping

Patch series "hugetlb: fixes for new vma lock series".

In review of the series "hugetlb: Use new vma lock for huge pmd sharing
synchronization", Miaohe Lin pointed out two key issues:

1) There is a race in the routine hugetlb_unmap_file_folio when locks
   are dropped and reacquired in the correct order [1].

2) With the switch to using vma lock for fault/truncate synchronization,
   we need to make sure lock exists for all VM_MAYSHARE vmas, not just
   vmas capable of pmd sharing.

These two issues are addressed here.  In addition, having a vma lock
present in all VM_MAYSHARE vmas, uncovered some issues around vma
splitting.  Those are also addressed.

[1] https://lore.kernel.org/linux-mm/[email protected]/


This patch (of 3):

The hugetlb vma lock hangs off the vm_private_data field and is specific
to the vma.  When vm_area_dup() is called as part of vma splitting, the
vma lock pointer is copied to the new vma.  This will result in issues
such as double freeing of the structure.  Update the hugetlb open vm_ops
to allocate a new vma lock for the new vma.

The routine __unmap_hugepage_range_final unconditionally unset VM_MAYSHARE
to prevent subsequent pmd sharing.  hugetlb_vma_lock_free attempted to
anticipate this by checking both VM_MAYSHARE and VM_SHARED.  However, if
only VM_MAYSHARE was set we would miss the free.  With the introduction of
the vma lock, a vma can not participate in pmd sharing if vm_private_data
is NULL.  Instead of clearing VM_MAYSHARE in __unmap_hugepage_range_final,
free the vma lock to prevent sharing.  Also, update the sharing code to
make sure vma lock is indeed a condition for pmd sharing. 
hugetlb_vma_lock_free can then key off VM_MAYSHARE and not miss any vmas.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: "hugetlb: add vma based lock for pmd sharing"
Signed-off-by: Mike Kravetz <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: James Houghton <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Prakash Sangappa <[email protected]>
Cc: Sven Schnelle <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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