From: David Hildenbrand <david@redhat.com>
Subject: mm/hugetlb: take src_mm->write_protect_seq in copy_hugetlb_page_range()

Let's do it just like copy_page_range(), taking the seqlock and making
sure the mmap_lock is held in write mode.

This allows for add a VM_BUG_ON to page_needs_cow_for_dma() and properly
synchronizes cocnurrent fork() with GUP-fast of hugetlb pages, which will
be relevant for further changes.

Link: https://lkml.kernel.org/r/20220329160440.193848-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Don Dutile <ddutile@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Khalid Aziz <khalid.aziz@oracle.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Liang Zhang <zhangliang5@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Oded Gabbay <oded.gabbay@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/mm.h |    4 ++++
 mm/hugetlb.c       |    8 ++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

--- a/include/linux/mm.h~mm-hugetlb-take-src_mm-write_protect_seq-in-copy_hugetlb_page_range
+++ a/include/linux/mm.h
@@ -1576,6 +1576,8 @@ static inline bool page_maybe_dma_pinned
 /*
  * This should most likely only be called during fork() to see whether we
  * should break the cow immediately for a page on the src mm.
+ *
+ * The caller has to hold the PT lock and the vma->vm_mm->->write_protect_seq.
  */
 static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
 					  struct page *page)
@@ -1583,6 +1585,8 @@ static inline bool page_needs_cow_for_dm
 	if (!is_cow_mapping(vma->vm_flags))
 		return false;
 
+	VM_BUG_ON(!(raw_read_seqcount(&vma->vm_mm->write_protect_seq) & 1));
+
 	if (!test_bit(MMF_HAS_PINNED, &vma->vm_mm->flags))
 		return false;
 
--- a/mm/hugetlb.c~mm-hugetlb-take-src_mm-write_protect_seq-in-copy_hugetlb_page_range
+++ a/mm/hugetlb.c
@@ -4717,6 +4717,8 @@ int copy_hugetlb_page_range(struct mm_st
 					vma->vm_start,
 					vma->vm_end);
 		mmu_notifier_invalidate_range_start(&range);
+		mmap_assert_write_locked(src);
+		raw_write_seqcount_begin(&src->write_protect_seq);
 	} else {
 		/*
 		 * For shared mappings i_mmap_rwsem must be held to call
@@ -4849,10 +4851,12 @@ again:
 		spin_unlock(dst_ptl);
 	}
 
-	if (cow)
+	if (cow) {
+		raw_write_seqcount_end(&src->write_protect_seq);
 		mmu_notifier_invalidate_range_end(&range);
-	else
+	} else {
 		i_mmap_unlock_read(mapping);
+	}
 
 	return ret;
 }
_