[PATCH 19/21] userfaultfd: remap_pages: UFFDIO_REMAP preparation

From: Andrea Arcangeli <aarcange@redhat.com>
To: qemu-devel@nongnu.org, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-api@vger.kernel.org,
	Android Kernel Team <kernel-team@android.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	Pavel Emelyanov <xemul@parallels.com>,
	Sanidhya Kashyap <sanidhya.gatech@gmail.com>,
	zhang.zhanghailiang@huawei.com,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andres Lagar-Cavilla <andreslc@google.com>,
	Dave Hansen <dave@sr71.net>, Paolo Bonzini <pbonzini@redhat.com>,
	Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>,
	Andy Lutomirski <luto@amacapital.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Sasha Levin <sasha.levin@oracle.com>,
	Hugh Dickins <hughd@google.com>,
	Peter Feiner <pfeiner@google.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Christopher Covington <cov@codeaurora.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Robert Love <rlove@google.com>,
	Dmitry Adamushko <dmitry.adamushko@gmail.com>,
	Neil Brown <neilb@suse.de>, Mike Hommey <mh@glandium.org>,
	Taras Glek <tglek@mozilla.com>, Jan Kara <jack@suse.cz>,
	KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
	Michel Lespinasse <walken@google.com>,
	Minchan Kim <minchan@kernel.org>,
	Keith Packard <keithp@keithp.com>,
	"Huangpeng (Peter)" <peter.huangpeng@huawei.com>,
	Anthony Liguori <anthony@codemonkey.ws>,
	Stefan Hajnoczi <stefanha@gmail.com>,
	Wenchao Xia <wenchaoqemu@gmail.com>,
	Andrew Jones <drjones@redhat.com>,
	Juan Quintela <quintela@redhat.com>
Subject: [PATCH 19/21] userfaultfd: remap_pages: UFFDIO_REMAP preparation
Date: Thu,  5 Mar 2015 18:18:02 +0100	[thread overview]
Message-ID: <1425575884-2574-20-git-send-email-aarcange@redhat.com> (raw)
In-Reply-To: <1425575884-2574-1-git-send-email-aarcange@redhat.com>

remap_pages is the lowlevel mm helper needed to implement
UFFDIO_REMAP.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 include/linux/userfaultfd_k.h |  17 ++
 mm/huge_memory.c              | 120 ++++++++++
 mm/userfaultfd.c              | 526 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 663 insertions(+)

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 587480a..3c39a4f 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -36,6 +36,23 @@ extern ssize_t mfill_zeropage(struct mm_struct *dst_mm,
 			      unsigned long dst_start,
 			      unsigned long len);
 
+/* remap_pages */
+extern void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2);
+extern void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2);
+extern ssize_t remap_pages(struct mm_struct *dst_mm,
+			   struct mm_struct *src_mm,
+			   unsigned long dst_start,
+			   unsigned long src_start,
+			   unsigned long len, __u64 flags);
+extern int remap_pages_huge_pmd(struct mm_struct *dst_mm,
+				struct mm_struct *src_mm,
+				pmd_t *dst_pmd, pmd_t *src_pmd,
+				pmd_t dst_pmdval,
+				struct vm_area_struct *dst_vma,
+				struct vm_area_struct *src_vma,
+				unsigned long dst_addr,
+				unsigned long src_addr);
+
 /* mm helpers */
 static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
 					struct vm_userfaultfd_ctx vm_ctx)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1e25cb3..08c8afc 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1531,6 +1531,124 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 	return ret;
 }
 
+#ifdef CONFIG_USERFAULTFD
+/*
+ * The PT lock for src_pmd and the mmap_sem for reading are held by
+ * the caller, but it must return after releasing the
+ * page_table_lock. We're guaranteed the src_pmd is a pmd_trans_huge
+ * until the PT lock of the src_pmd is released. Just move the page
+ * from src_pmd to dst_pmd if possible. Return zero if succeeded in
+ * moving the page, -EAGAIN if it needs to be repeated by the caller,
+ * or other errors in case of failure.
+ */
+int remap_pages_huge_pmd(struct mm_struct *dst_mm,
+			 struct mm_struct *src_mm,
+			 pmd_t *dst_pmd, pmd_t *src_pmd,
+			 pmd_t dst_pmdval,
+			 struct vm_area_struct *dst_vma,
+			 struct vm_area_struct *src_vma,
+			 unsigned long dst_addr,
+			 unsigned long src_addr)
+{
+	pmd_t _dst_pmd, src_pmdval;
+	struct page *src_page;
+	struct anon_vma *src_anon_vma, *dst_anon_vma;
+	spinlock_t *src_ptl, *dst_ptl;
+	pgtable_t pgtable;
+
+	src_pmdval = *src_pmd;
+	src_ptl = pmd_lockptr(src_mm, src_pmd);
+
+	BUG_ON(!pmd_trans_huge(src_pmdval));
+	BUG_ON(pmd_trans_splitting(src_pmdval));
+	BUG_ON(!pmd_none(dst_pmdval));
+	BUG_ON(!spin_is_locked(src_ptl));
+	BUG_ON(!rwsem_is_locked(&src_mm->mmap_sem));
+	BUG_ON(!rwsem_is_locked(&dst_mm->mmap_sem));
+
+	src_page = pmd_page(src_pmdval);
+	BUG_ON(!PageHead(src_page));
+	BUG_ON(!PageAnon(src_page));
+	if (unlikely(page_mapcount(src_page) != 1)) {
+		spin_unlock(src_ptl);
+		return -EBUSY;
+	}
+
+	get_page(src_page);
+	spin_unlock(src_ptl);
+
+	mmu_notifier_invalidate_range_start(src_mm, src_addr,
+					    src_addr + HPAGE_PMD_SIZE);
+
+	/* block all concurrent rmap walks */
+	lock_page(src_page);
+
+	/*
+	 * split_huge_page walks the anon_vma chain without the page
+	 * lock. Serialize against it with the anon_vma lock, the page
+	 * lock is not enough.
+	 */
+	src_anon_vma = page_get_anon_vma(src_page);
+	if (!src_anon_vma) {
+		unlock_page(src_page);
+		put_page(src_page);
+		mmu_notifier_invalidate_range_end(src_mm, src_addr,
+						  src_addr + HPAGE_PMD_SIZE);
+		return -EAGAIN;
+	}
+	anon_vma_lock_write(src_anon_vma);
+
+	dst_ptl = pmd_lockptr(dst_mm, dst_pmd);
+	double_pt_lock(src_ptl, dst_ptl);
+	if (unlikely(!pmd_same(*src_pmd, src_pmdval) ||
+		     !pmd_same(*dst_pmd, dst_pmdval) ||
+		     page_mapcount(src_page) != 1)) {
+		double_pt_unlock(src_ptl, dst_ptl);
+		anon_vma_unlock_write(src_anon_vma);
+		put_anon_vma(src_anon_vma);
+		unlock_page(src_page);
+		put_page(src_page);
+		mmu_notifier_invalidate_range_end(src_mm, src_addr,
+						  src_addr + HPAGE_PMD_SIZE);
+		return -EAGAIN;
+	}
+
+	BUG_ON(!PageHead(src_page));
+	BUG_ON(!PageAnon(src_page));
+	/* the PT lock is enough to keep the page pinned now */
+	put_page(src_page);
+
+	dst_anon_vma = (void *) dst_vma->anon_vma + PAGE_MAPPING_ANON;
+	ACCESS_ONCE(src_page->mapping) = (struct address_space *) dst_anon_vma;
+	ACCESS_ONCE(src_page->index) = linear_page_index(dst_vma, dst_addr);
+
+	if (!pmd_same(pmdp_clear_flush(src_vma, src_addr, src_pmd),
+		      src_pmdval))
+		BUG();
+	_dst_pmd = mk_huge_pmd(src_page, dst_vma->vm_page_prot);
+	_dst_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
+	set_pmd_at(dst_mm, dst_addr, dst_pmd, _dst_pmd);
+
+	pgtable = pgtable_trans_huge_withdraw(src_mm, src_pmd);
+	pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
+	if (dst_mm != src_mm) {
+		add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
+		add_mm_counter(src_mm, MM_ANONPAGES, -HPAGE_PMD_NR);
+	}
+	double_pt_unlock(src_ptl, dst_ptl);
+
+	anon_vma_unlock_write(src_anon_vma);
+	put_anon_vma(src_anon_vma);
+
+	/* unblock rmap walks */
+	unlock_page(src_page);
+
+	mmu_notifier_invalidate_range_end(src_mm, src_addr,
+					  src_addr + HPAGE_PMD_SIZE);
+	return 0;
+}
+#endif /* CONFIG_USERFAULTFD */
+
 /*
  * Returns 1 if a given pmd maps a stable (not under splitting) thp.
  * Returns -1 if it maps a thp under splitting. Returns 0 otherwise.
@@ -2484,6 +2602,8 @@ static void collapse_huge_page(struct mm_struct *mm,
 	 * Prevent all access to pagetables with the exception of
 	 * gup_fast later hanlded by the ptep_clear_flush and the VM
 	 * handled by the anon_vma lock + PG_lock.
+	 *
+	 * remap_pages is prevented to race as well thanks to the mmap_sem.
 	 */
 	down_write(&mm->mmap_sem);
 	if (unlikely(khugepaged_test_exit(mm)))
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 3f4c0ef..49521af 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -265,3 +265,529 @@ ssize_t mfill_zeropage(struct mm_struct *dst_mm, unsigned long start,
 {
 	return __mcopy_atomic(dst_mm, start, 0, len, true);
 }
+
+void double_pt_lock(spinlock_t *ptl1,
+		    spinlock_t *ptl2)
+	__acquires(ptl1)
+	__acquires(ptl2)
+{
+	spinlock_t *ptl_tmp;
+
+	if (ptl1 > ptl2) {
+		/* exchange ptl1 and ptl2 */
+		ptl_tmp = ptl1;
+		ptl1 = ptl2;
+		ptl2 = ptl_tmp;
+	}
+	/* lock in virtual address order to avoid lock inversion */
+	spin_lock(ptl1);
+	if (ptl1 != ptl2)
+		spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING);
+	else
+		__acquire(ptl2);
+}
+
+void double_pt_unlock(spinlock_t *ptl1,
+		      spinlock_t *ptl2)
+	__releases(ptl1)
+	__releases(ptl2)
+{
+	spin_unlock(ptl1);
+	if (ptl1 != ptl2)
+		spin_unlock(ptl2);
+	else
+		__release(ptl2);
+}
+
+/*
+ * The mmap_sem for reading is held by the caller. Just move the page
+ * from src_pmd to dst_pmd if possible, and return true if succeeded
+ * in moving the page.
+ */
+static int remap_pages_pte(struct mm_struct *dst_mm,
+			   struct mm_struct *src_mm,
+			   pte_t *dst_pte, pte_t *src_pte, pmd_t *src_pmd,
+			   struct vm_area_struct *dst_vma,
+			   struct vm_area_struct *src_vma,
+			   unsigned long dst_addr,
+			   unsigned long src_addr,
+			   spinlock_t *dst_ptl,
+			   spinlock_t *src_ptl,
+			   __u64 mode)
+{
+	struct page *src_page;
+	swp_entry_t entry;
+	pte_t orig_src_pte, orig_dst_pte;
+	struct anon_vma *src_anon_vma, *dst_anon_vma;
+
+	spin_lock(dst_ptl);
+	orig_dst_pte = *dst_pte;
+	spin_unlock(dst_ptl);
+	if (!pte_none(orig_dst_pte))
+		return -EEXIST;
+
+	spin_lock(src_ptl);
+	orig_src_pte = *src_pte;
+	spin_unlock(src_ptl);
+	if (pte_none(orig_src_pte)) {
+		if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES))
+			return -ENOENT;
+		else
+			/* nothing to do to remap an hole */
+			return 0;
+	}
+
+	if (pte_present(orig_src_pte)) {
+		/*
+		 * Pin the page while holding the lock to be sure the
+		 * page isn't freed under us
+		 */
+		spin_lock(src_ptl);
+		if (!pte_same(orig_src_pte, *src_pte)) {
+			spin_unlock(src_ptl);
+			return -EAGAIN;
+		}
+		src_page = vm_normal_page(src_vma, src_addr, orig_src_pte);
+		if (!src_page || !PageAnon(src_page) ||
+		    page_mapcount(src_page) != 1) {
+			spin_unlock(src_ptl);
+			return -EBUSY;
+		}
+
+		get_page(src_page);
+		spin_unlock(src_ptl);
+
+		/* block all concurrent rmap walks */
+		lock_page(src_page);
+
+		/*
+		 * page_referenced_anon walks the anon_vma chain
+		 * without the page lock. Serialize against it with
+		 * the anon_vma lock, the page lock is not enough.
+		 */
+		src_anon_vma = page_get_anon_vma(src_page);
+		if (!src_anon_vma) {
+			/* page was unmapped from under us */
+			unlock_page(src_page);
+			put_page(src_page);
+			return -EAGAIN;
+		}
+		anon_vma_lock_write(src_anon_vma);
+
+		double_pt_lock(dst_ptl, src_ptl);
+
+		if (!pte_same(*src_pte, orig_src_pte) ||
+		    !pte_same(*dst_pte, orig_dst_pte) ||
+		    page_mapcount(src_page) != 1) {
+			double_pt_unlock(dst_ptl, src_ptl);
+			anon_vma_unlock_write(src_anon_vma);
+			put_anon_vma(src_anon_vma);
+			unlock_page(src_page);
+			put_page(src_page);
+			return -EAGAIN;
+		}
+
+		BUG_ON(!PageAnon(src_page));
+		/* the PT lock is enough to keep the page pinned now */
+		put_page(src_page);
+
+		dst_anon_vma = (void *) dst_vma->anon_vma + PAGE_MAPPING_ANON;
+		ACCESS_ONCE(src_page->mapping) = ((struct address_space *)
+						  dst_anon_vma);
+		ACCESS_ONCE(src_page->index) = linear_page_index(dst_vma,
+								 dst_addr);
+
+		if (!pte_same(ptep_clear_flush(src_vma, src_addr, src_pte),
+			      orig_src_pte))
+			BUG();
+
+		orig_dst_pte = mk_pte(src_page, dst_vma->vm_page_prot);
+		orig_dst_pte = maybe_mkwrite(pte_mkdirty(orig_dst_pte),
+					     dst_vma);
+
+		set_pte_at(dst_mm, dst_addr, dst_pte, orig_dst_pte);
+
+		if (dst_mm != src_mm) {
+			inc_mm_counter(dst_mm, MM_ANONPAGES);
+			dec_mm_counter(src_mm, MM_ANONPAGES);
+		}
+
+		double_pt_unlock(dst_ptl, src_ptl);
+
+		anon_vma_unlock_write(src_anon_vma);
+		put_anon_vma(src_anon_vma);
+
+		/* unblock rmap walks */
+		unlock_page(src_page);
+
+		mmu_notifier_invalidate_page(src_mm, src_addr);
+	} else {
+		entry = pte_to_swp_entry(orig_src_pte);
+		if (non_swap_entry(entry)) {
+			if (is_migration_entry(entry)) {
+				migration_entry_wait(src_mm, src_pmd,
+						     src_addr);
+				return -EAGAIN;
+			}
+			return -EFAULT;
+		}
+
+		if (swp_entry_swapcount(entry) != 1)
+			return -EBUSY;
+
+		double_pt_lock(dst_ptl, src_ptl);
+
+		if (!pte_same(*src_pte, orig_src_pte) ||
+		    !pte_same(*dst_pte, orig_dst_pte) ||
+		    swp_entry_swapcount(entry) != 1) {
+			double_pt_unlock(dst_ptl, src_ptl);
+			return -EAGAIN;
+		}
+
+		if (pte_val(ptep_get_and_clear(src_mm, src_addr, src_pte)) !=
+		    pte_val(orig_src_pte))
+			BUG();
+		set_pte_at(dst_mm, dst_addr, dst_pte, orig_src_pte);
+
+		if (dst_mm != src_mm) {
+			inc_mm_counter(dst_mm, MM_ANONPAGES);
+			dec_mm_counter(src_mm, MM_ANONPAGES);
+		}
+
+		double_pt_unlock(dst_ptl, src_ptl);
+	}
+
+	return 0;
+}
+
+/**
+ * remap_pages - remap arbitrary anonymous pages of an existing vma
+ * @dst_start: start of the destination virtual memory range
+ * @src_start: start of the source virtual memory range
+ * @len: length of the virtual memory range
+ *
+ * remap_pages() remaps arbitrary anonymous pages atomically in zero
+ * copy. It only works on non shared anonymous pages because those can
+ * be relocated without generating non linear anon_vmas in the rmap
+ * code.
+ *
+ * It is the ideal mechanism to handle userspace page faults. Normally
+ * the destination vma will have VM_USERFAULT set with
+ * madvise(MADV_USERFAULT) while the source vma will have VM_DONTCOPY
+ * set with madvise(MADV_DONTFORK).
+ *
+ * The thread receiving the page during the userland page fault
+ * (MADV_USERFAULT) will receive the faulting page in the source vma
+ * through the network, storage or any other I/O device (MADV_DONTFORK
+ * in the source vma avoids remap_pages() to fail with -EBUSY if the
+ * process forks before remap_pages() is called), then it will call
+ * remap_pages() to map the page in the faulting address in the
+ * destination vma.
+ *
+ * This userfaultfd command works purely via pagetables, so it's the
+ * most efficient way to move physical non shared anonymous pages
+ * across different virtual addresses. Unlike mremap()/mmap()/munmap()
+ * it does not create any new vmas. The mapping in the destination
+ * address is atomic.
+ *
+ * It only works if the vma protection bits are identical from the
+ * source and destination vma.
+ *
+ * It can remap non shared anonymous pages within the same vma too.
+ *
+ * If the source virtual memory range has any unmapped holes, or if
+ * the destination virtual memory range is not a whole unmapped hole,
+ * remap_pages() will fail respectively with -ENOENT or -EEXIST. This
+ * provides a very strict behavior to avoid any chance of memory
+ * corruption going unnoticed if there are userland race
+ * conditions. Only one thread should resolve the userland page fault
+ * at any given time for any given faulting address. This means that
+ * if two threads try to both call remap_pages() on the same
+ * destination address at the same time, the second thread will get an
+ * explicit error from this command.
+ *
+ * The command retval will return "len" is succesful. The command
+ * however can be interrupted by fatal signals or errors. If
+ * interrupted it will return the number of bytes successfully
+ * remapped before the interruption if any, or the negative error if
+ * none. It will never return zero. Either it will return an error or
+ * an amount of bytes successfully moved. If the retval reports a
+ * "short" remap, the remap_pages() command should be repeated by
+ * userland with src+retval, dst+reval, len-retval if it wants to know
+ * about the error that interrupted it.
+ *
+ * The UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES flag can be specified to
+ * prevent -ENOENT errors to materialize if there are holes in the
+ * source virtual range that is being remapped. The holes will be
+ * accounted as successfully remapped in the retval of the
+ * command. This is mostly useful to remap hugepage naturally aligned
+ * virtual regions without knowing if there are transparent hugepage
+ * in the regions or not, but preventing the risk of having to split
+ * the hugepmd during the remap.
+ *
+ * If there's any rmap walk that is taking the anon_vma locks without
+ * first obtaining the page lock (for example split_huge_page and
+ * page_referenced_anon), they will have to verify if the
+ * page->mapping has changed after taking the anon_vma lock. If it
+ * changed they should release the lock and retry obtaining a new
+ * anon_vma, because it means the anon_vma was changed by
+ * remap_pages() before the lock could be obtained. This is the only
+ * additional complexity added to the rmap code to provide this
+ * anonymous page remapping functionality.
+ */
+ssize_t remap_pages(struct mm_struct *dst_mm, struct mm_struct *src_mm,
+		    unsigned long dst_start, unsigned long src_start,
+		    unsigned long len, __u64 mode)
+{
+	struct vm_area_struct *src_vma, *dst_vma;
+	long err = -EINVAL;
+	pmd_t *src_pmd, *dst_pmd;
+	pte_t *src_pte, *dst_pte;
+	spinlock_t *dst_ptl, *src_ptl;
+	unsigned long src_addr, dst_addr;
+	int thp_aligned = -1;
+	ssize_t moved = 0;
+
+	/*
+	 * Sanitize the command parameters:
+	 */
+	BUG_ON(src_start & ~PAGE_MASK);
+	BUG_ON(dst_start & ~PAGE_MASK);
+	BUG_ON(len & ~PAGE_MASK);
+
+	/* Does the address range wrap, or is the span zero-sized? */
+	BUG_ON(src_start + len <= src_start);
+	BUG_ON(dst_start + len <= dst_start);
+
+	/*
+	 * Because these are read sempahores there's no risk of lock
+	 * inversion.
+	 */
+	down_read(&dst_mm->mmap_sem);
+	if (dst_mm != src_mm)
+		down_read(&src_mm->mmap_sem);
+
+	/*
+	 * Make sure the vma is not shared, that the src and dst remap
+	 * ranges are both valid and fully within a single existing
+	 * vma.
+	 */
+	src_vma = find_vma(src_mm, src_start);
+	if (!src_vma || (src_vma->vm_flags & VM_SHARED))
+		goto out;
+	if (src_start < src_vma->vm_start ||
+	    src_start + len > src_vma->vm_end)
+		goto out;
+
+	dst_vma = find_vma(dst_mm, dst_start);
+	if (!dst_vma || (dst_vma->vm_flags & VM_SHARED))
+		goto out;
+	if (dst_start < dst_vma->vm_start ||
+	    dst_start + len > dst_vma->vm_end)
+		goto out;
+
+	if (pgprot_val(src_vma->vm_page_prot) !=
+	    pgprot_val(dst_vma->vm_page_prot))
+		goto out;
+
+	/* only allow remapping if both are mlocked or both aren't */
+	if ((src_vma->vm_flags & VM_LOCKED) ^ (dst_vma->vm_flags & VM_LOCKED))
+		goto out;
+
+	/*
+	 * Be strict and only allow remap_pages if either the src or
+	 * dst range is registered in the userfaultfd to prevent
+	 * userland errors going unnoticed. As far as the VM
+	 * consistency is concerned, it would be perfectly safe to
+	 * remove this check, but there's no useful usage for
+	 * remap_pages ouside of userfaultfd registered ranges. This
+	 * is after all why it is an ioctl belonging to the
+	 * userfaultfd and not a syscall.
+	 *
+	 * Allow both vmas to be registered in the userfaultfd, just
+	 * in case somebody finds a way to make such a case useful.
+	 * Normally only one of the two vmas would be registered in
+	 * the userfaultfd.
+	 */
+	if (!dst_vma->vm_userfaultfd_ctx.ctx &&
+	    !src_vma->vm_userfaultfd_ctx.ctx)
+		goto out;
+
+	/*
+	 * FIXME: only allow remapping across anonymous vmas,
+	 * tmpfs should be added.
+	 */
+	if (src_vma->vm_ops || dst_vma->vm_ops)
+		goto out;
+
+	/*
+	 * Ensure the dst_vma has a anon_vma or this page
+	 * would get a NULL anon_vma when moved in the
+	 * dst_vma.
+	 */
+	err = -ENOMEM;
+	if (unlikely(anon_vma_prepare(dst_vma)))
+		goto out;
+
+	for (src_addr = src_start, dst_addr = dst_start;
+	     src_addr < src_start + len; ) {
+		spinlock_t *ptl;
+		pmd_t dst_pmdval;
+		BUG_ON(dst_addr >= dst_start + len);
+		src_pmd = mm_find_pmd(src_mm, src_addr);
+		if (unlikely(!src_pmd)) {
+			if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES)) {
+				err = -ENOENT;
+				break;
+			} else {
+				src_pmd = mm_alloc_pmd(src_mm, src_addr);
+				if (unlikely(!src_pmd)) {
+					err = -ENOMEM;
+					break;
+				}
+			}
+		}
+		dst_pmd = mm_alloc_pmd(dst_mm, dst_addr);
+		if (unlikely(!dst_pmd)) {
+			err = -ENOMEM;
+			break;
+		}
+
+		dst_pmdval = pmd_read_atomic(dst_pmd);
+		/*
+		 * If the dst_pmd is mapped as THP don't
+		 * override it and just be strict.
+		 */
+		if (unlikely(pmd_trans_huge(dst_pmdval))) {
+			err = -EEXIST;
+			break;
+		}
+		if (pmd_trans_huge_lock(src_pmd, src_vma, &ptl) == 1) {
+			/*
+			 * Check if we can move the pmd without
+			 * splitting it. First check the address
+			 * alignment to be the same in src/dst.  These
+			 * checks don't actually need the PT lock but
+			 * it's good to do it here to optimize this
+			 * block away at build time if
+			 * CONFIG_TRANSPARENT_HUGEPAGE is not set.
+			 */
+			if (thp_aligned == -1)
+				thp_aligned = ((src_addr & ~HPAGE_PMD_MASK) ==
+					       (dst_addr & ~HPAGE_PMD_MASK));
+			if (!thp_aligned || (src_addr & ~HPAGE_PMD_MASK) ||
+			    !pmd_none(dst_pmdval) ||
+			    src_start + len - src_addr < HPAGE_PMD_SIZE) {
+				spin_unlock(ptl);
+				/* Fall through */
+				split_huge_page_pmd(src_vma, src_addr,
+						    src_pmd);
+			} else {
+				BUG_ON(dst_addr & ~HPAGE_PMD_MASK);
+				err = remap_pages_huge_pmd(dst_mm,
+							   src_mm,
+							   dst_pmd,
+							   src_pmd,
+							   dst_pmdval,
+							   dst_vma,
+							   src_vma,
+							   dst_addr,
+							   src_addr);
+				cond_resched();
+
+				if (!err) {
+					dst_addr += HPAGE_PMD_SIZE;
+					src_addr += HPAGE_PMD_SIZE;
+					moved += HPAGE_PMD_SIZE;
+				}
+
+				if ((!err || err == -EAGAIN) &&
+				    fatal_signal_pending(current))
+					err = -EINTR;
+
+				if (err && err != -EAGAIN)
+					break;
+
+				continue;
+			}
+		}
+
+		if (pmd_none(*src_pmd)) {
+			if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES)) {
+				err = -ENOENT;
+				break;
+			} else {
+				if (unlikely(__pte_alloc(src_mm, src_vma,
+							 src_pmd, src_addr))) {
+					err = -ENOMEM;
+					break;
+				}
+			}
+		}
+
+		/*
+		 * We held the mmap_sem for reading so MADV_DONTNEED
+		 * can zap transparent huge pages under us, or the
+		 * transparent huge page fault can establish new
+		 * transparent huge pages under us.
+		 */
+		if (unlikely(pmd_trans_unstable(src_pmd))) {
+			err = -EFAULT;
+			break;
+		}
+
+		if (unlikely(pmd_none(dst_pmdval)) &&
+		    unlikely(__pte_alloc(dst_mm, dst_vma, dst_pmd,
+					 dst_addr))) {
+			err = -ENOMEM;
+			break;
+		}
+		/* If an huge pmd materialized from under us fail */
+		if (unlikely(pmd_trans_huge(*dst_pmd))) {
+			err = -EFAULT;
+			break;
+		}
+
+		BUG_ON(pmd_none(*dst_pmd));
+		BUG_ON(pmd_none(*src_pmd));
+		BUG_ON(pmd_trans_huge(*dst_pmd));
+		BUG_ON(pmd_trans_huge(*src_pmd));
+
+		dst_pte = pte_offset_map(dst_pmd, dst_addr);
+		src_pte = pte_offset_map(src_pmd, src_addr);
+		dst_ptl = pte_lockptr(dst_mm, dst_pmd);
+		src_ptl = pte_lockptr(src_mm, src_pmd);
+
+		err = remap_pages_pte(dst_mm, src_mm,
+				      dst_pte, src_pte, src_pmd,
+				      dst_vma, src_vma,
+				      dst_addr, src_addr,
+				      dst_ptl, src_ptl, mode);
+
+		pte_unmap(dst_pte);
+		pte_unmap(src_pte);
+		cond_resched();
+
+		if (!err) {
+			dst_addr += PAGE_SIZE;
+			src_addr += PAGE_SIZE;
+			moved += PAGE_SIZE;
+		}
+
+		if ((!err || err == -EAGAIN) &&
+		    fatal_signal_pending(current))
+			err = -EINTR;
+
+		if (err && err != -EAGAIN)
+			break;
+	}
+
+out:
+	up_read(&dst_mm->mmap_sem);
+	if (dst_mm != src_mm)
+		up_read(&src_mm->mmap_sem);
+	BUG_ON(moved < 0);
+	BUG_ON(err > 0);
+	BUG_ON(!moved && !err);
+	return moved ? moved : err;
+}
