[PATCH v4 02/17] mm/hugetlb: take src_mm->write_protect_seq in copy_hugetlb_page_range()

David Hildenbrand
2022-04-28T08:38:26Z

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 concurrent fork() with GUP-fast of hugetlb pages,
which will be relevant for further changes.

Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/mm.h | 4 ++++
mm/hugetlb.c | 8 ++++++--
2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9f44254af8ce..c3c862a2e533 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1576,6 +1576,8 @@ static inline bool page_maybe_dma_pinned(struct page *page)
/*
* 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_dma(struct vm_area_struct *vma,
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;

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3fc721789743..670117bbc9b4 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4717,6 +4717,8 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
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 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
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;
}
--
2.35.1