Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   David Hildenbrand <david@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Nadav Amit <namit@vmware.com>, Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        Liang Zhang <zhangliang5@huawei.com>,
        Pedro Gomes <pedrodemargomes@gmail.com>,
        Oded Gabbay <oded.gabbay@gmail.com>, linux-mm@kvack.org,
        David Hildenbrand <david@redhat.com>
Subject: [PATCH v4 02/17] mm/hugetlb: take src_mm->write_protect_seq in copy_hugetlb_page_range()
Date:   Thu, 28 Apr 2022 10:34:26 +0200
Message-Id: <20220428083441.37290-3-david@redhat.com>
In-Reply-To: <20220428083441.37290-1-david@redhat.com>
References: <20220428083441.37290-1-david@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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