userfaultfd: UFFDIO_MOVE uABI

Implement the uABI of UFFDIO_MOVE ioctl.
UFFDIO_COPY performs ~20% better than UFFDIO_MOVE when the application
needs pages to be allocated [1]. However, with UFFDIO_MOVE, if pages are
available (in userspace) for recycling, as is usually the case in heap
compaction algorithms, then we can avoid the page allocation and memcpy
(done by UFFDIO_COPY). Also, since the pages are recycled in the
userspace, we avoid the need to release (via madvise) the pages back to
the kernel [2].

We see over 40% reduction (on a Google pixel 6 device) in the compacting
thread's completion time by using UFFDIO_MOVE vs.  UFFDIO_COPY.  This was
measured using a benchmark that emulates a heap compaction implementation
using userfaultfd (to allow concurrent accesses by application threads). 
More details of the usecase are explained in [2].  Furthermore,
UFFDIO_MOVE enables moving swapped-out pages without touching them within
the same vma.  Today, it can only be done by mremap, however it forces
splitting the vma.

[1] https://lore.kernel.org/all/1425575884-2574-1-git-send-email-aarcange@redhat.com/
[2] https://lore.kernel.org/linux-mm/CA+EESO4uO84SSnBhArH4HvLNhaUQ5nZKNKXqxRCyjniNVjp0Aw@mail.gmail.com/

Update for the ioctl_userfaultfd(2)  manpage:

   UFFDIO_MOVE
       (Since Linux xxx)  Move a continuous memory chunk into the
       userfault registered range and optionally wake up the blocked
       thread. The source and destination addresses and the number of
       bytes to move are specified by the src, dst, and len fields of
       the uffdio_move structure pointed to by argp:

           struct uffdio_move {
               __u64 dst;    /* Destination of move */
               __u64 src;    /* Source of move */
               __u64 len;    /* Number of bytes to move */
               __u64 mode;   /* Flags controlling behavior of move */
               __s64 move;   /* Number of bytes moved, or negated error */
           };

       The following value may be bitwise ORed in mode to change the
       behavior of the UFFDIO_MOVE operation:

       UFFDIO_MOVE_MODE_DONTWAKE
              Do not wake up the thread that waits for page-fault
              resolution

       UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES
              Allow holes in the source virtual range that is being moved.
              When not specified, the holes will result in ENOENT error.
              When specified, the holes will be accounted as successfully
              moved memory. This is mostly useful to move hugepage aligned
              virtual regions without knowing if there are transparent
              hugepages in the regions or not, but preventing the risk of
              having to split the hugepage during the operation.

       The move field is used by the kernel to return the number of
       bytes that was actually moved, or an error (a negated errno-
       style value).  If the value returned in move doesn't match the
       value that was specified in len, the operation fails with the
       error EAGAIN.  The move field is output-only; it is not read by
       the UFFDIO_MOVE operation.

       The operation may fail for various reasons. Usually, remapping of
       pages that are not exclusive to the given process fail; once KSM
       might deduplicate pages or fork() COW-shares pages during fork()
       with child processes, they are no longer exclusive. Further, the
       kernel might only perform lightweight checks for detecting whether
       the pages are exclusive, and return -EBUSY in case that check fails.
       To make the operation more likely to succeed, KSM should be
       disabled, fork() should be avoided or MADV_DONTFORK should be
       configured for the source VMA before fork().

       This ioctl(2) operation returns 0 on success.  In this case, the
       entire area was moved.  On error, -1 is returned and errno is
       set to indicate the error.  Possible errors include:

       EAGAIN The number of bytes moved (i.e., the value returned in
              the move field) does not equal the value that was
              specified in the len field.

       EINVAL Either dst or len was not a multiple of the system page
              size, or the range specified by src and len or dst and len
              was invalid.

       EINVAL An invalid bit was specified in the mode field.

       ENOENT
              The source virtual memory range has unmapped holes and
              UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES is not set.

       EEXIST
              The destination virtual memory range is fully or partially
              mapped.

       EBUSY
              The pages in the source virtual memory range are either
              pinned or not exclusive to the process. The kernel might
              only perform lightweight checks for detecting whether the
              pages are exclusive. To make the operation more likely to
              succeed, KSM should be disabled, fork() should be avoided
              or MADV_DONTFORK should be configured for the source virtual
              memory area before fork().

       ENOMEM Allocating memory needed for the operation failed.

       ESRCH
              The target process has exited at the time of a UFFDIO_MOVE
              operation.

Link: https://lkml.kernel.org/r/20231206103702.3873743-3-surenb@google.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Nicolas Geoffray <ngeoffray@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: ZhangPeng <zhangpeng362@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/userfaultfd.rst

@@ -113,6 +113,9 @@ events, except page fault notifications, may be generated:
   areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating
   support for shmem virtual memory areas.
 
+- ``UFFD_FEATURE_MOVE`` indicates that the kernel supports moving an
+  existing page contents from userspace.
+
 The userland application should set the feature flags it intends to use
 when invoking the ``UFFDIO_API`` ioctl, to request that those features be
 enabled if supported.

fs/userfaultfd.c

@@ -2005,6 +2005,75 @@ static inline unsigned int uffd_ctx_features(__u64 user_features)
 	return (unsigned int)user_features | UFFD_FEATURE_INITIALIZED;
 }
 
+static int userfaultfd_move(struct userfaultfd_ctx *ctx,
+			    unsigned long arg)
+{
+	__s64 ret;
+	struct uffdio_move uffdio_move;
+	struct uffdio_move __user *user_uffdio_move;
+	struct userfaultfd_wake_range range;
+	struct mm_struct *mm = ctx->mm;
+
+	user_uffdio_move = (struct uffdio_move __user *) arg;
+
+	if (atomic_read(&ctx->mmap_changing))
+		return -EAGAIN;
+
+	if (copy_from_user(&uffdio_move, user_uffdio_move,
+			   /* don't copy "move" last field */
+			   sizeof(uffdio_move)-sizeof(__s64)))
+		return -EFAULT;
+
+	/* Do not allow cross-mm moves. */
+	if (mm != current->mm)
+		return -EINVAL;
+
+	ret = validate_range(mm, uffdio_move.dst, uffdio_move.len);
+	if (ret)
+		return ret;
+
+	ret = validate_range(mm, uffdio_move.src, uffdio_move.len);
+	if (ret)
+		return ret;
+
+	if (uffdio_move.mode & ~(UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES|
+				  UFFDIO_MOVE_MODE_DONTWAKE))
+		return -EINVAL;
+
+	if (mmget_not_zero(mm)) {
+		mmap_read_lock(mm);
+
+		/* Re-check after taking mmap_lock */
+		if (likely(!atomic_read(&ctx->mmap_changing)))
+			ret = move_pages(ctx, mm, uffdio_move.dst, uffdio_move.src,
+					 uffdio_move.len, uffdio_move.mode);
+		else
+			ret = -EINVAL;
+
+		mmap_read_unlock(mm);
+		mmput(mm);
+	} else {
+		return -ESRCH;
+	}
+
+	if (unlikely(put_user(ret, &user_uffdio_move->move)))
+		return -EFAULT;
+	if (ret < 0)
+		goto out;
+
+	/* len == 0 would wake all */
+	VM_WARN_ON(!ret);
+	range.len = ret;
+	if (!(uffdio_move.mode & UFFDIO_MOVE_MODE_DONTWAKE)) {
+		range.start = uffdio_move.dst;
+		wake_userfault(ctx, &range);
+	}
+	ret = range.len == uffdio_move.len ? 0 : -EAGAIN;
+
+out:
+	return ret;
+}
+
 /*
  * userland asks for a certain API version and we return which bits
  * and ioctl commands are implemented in this kernel for such API
@@ -2097,6 +2166,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
 	case UFFDIO_ZEROPAGE:
 		ret = userfaultfd_zeropage(ctx, arg);
 		break;
+	case UFFDIO_MOVE:
+		ret = userfaultfd_move(ctx, arg);
+		break;
 	case UFFDIO_WRITEPROTECT:
 		ret = userfaultfd_writeprotect(ctx, arg);
 		break;

include/linux/rmap.h

@@ -121,6 +121,11 @@ static inline void anon_vma_lock_write(struct anon_vma *anon_vma)
 	down_write(&anon_vma->root->rwsem);
 }
 
+static inline int anon_vma_trylock_write(struct anon_vma *anon_vma)
+{
+	return down_write_trylock(&anon_vma->root->rwsem);
+}
+
 static inline void anon_vma_unlock_write(struct anon_vma *anon_vma)
 {
 	up_write(&anon_vma->root->rwsem);

include/linux/userfaultfd_k.h

@@ -93,6 +93,17 @@ extern int mwriteprotect_range(struct mm_struct *dst_mm,
 extern long uffd_wp_range(struct vm_area_struct *vma,
 			  unsigned long start, unsigned long len, bool enable_wp);
 
+/* move_pages */
+void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2);
+void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2);
+ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
+		   unsigned long dst_start, unsigned long src_start,
+		   unsigned long len, __u64 flags);
+int move_pages_huge_pmd(struct mm_struct *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)

include/uapi/linux/userfaultfd.h

@@ -41,7 +41,8 @@
 			   UFFD_FEATURE_WP_HUGETLBFS_SHMEM |	\
 			   UFFD_FEATURE_WP_UNPOPULATED |	\
 			   UFFD_FEATURE_POISON |		\
-			   UFFD_FEATURE_WP_ASYNC)
+			   UFFD_FEATURE_WP_ASYNC |		\
+			   UFFD_FEATURE_MOVE)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
@@ -50,6 +51,7 @@
 	((__u64)1 << _UFFDIO_WAKE |		\
 	 (__u64)1 << _UFFDIO_COPY |		\
 	 (__u64)1 << _UFFDIO_ZEROPAGE |		\
+	 (__u64)1 << _UFFDIO_MOVE |		\
 	 (__u64)1 << _UFFDIO_WRITEPROTECT |	\
 	 (__u64)1 << _UFFDIO_CONTINUE |		\
 	 (__u64)1 << _UFFDIO_POISON)
@@ -73,6 +75,7 @@
 #define _UFFDIO_WAKE			(0x02)
 #define _UFFDIO_COPY			(0x03)
 #define _UFFDIO_ZEROPAGE		(0x04)
+#define _UFFDIO_MOVE			(0x05)
 #define _UFFDIO_WRITEPROTECT		(0x06)
 #define _UFFDIO_CONTINUE		(0x07)
 #define _UFFDIO_POISON			(0x08)
@@ -92,6 +95,8 @@
 				      struct uffdio_copy)
 #define UFFDIO_ZEROPAGE		_IOWR(UFFDIO, _UFFDIO_ZEROPAGE,	\
 				      struct uffdio_zeropage)
+#define UFFDIO_MOVE		_IOWR(UFFDIO, _UFFDIO_MOVE,	\
+				      struct uffdio_move)
 #define UFFDIO_WRITEPROTECT	_IOWR(UFFDIO, _UFFDIO_WRITEPROTECT, \
 				      struct uffdio_writeprotect)
 #define UFFDIO_CONTINUE		_IOWR(UFFDIO, _UFFDIO_CONTINUE,	\
@@ -222,6 +227,9 @@ struct uffdio_api {
 	 * asynchronous mode is supported in which the write fault is
 	 * automatically resolved and write-protection is un-set.
 	 * It implies UFFD_FEATURE_WP_UNPOPULATED.
+	 *
+	 * UFFD_FEATURE_MOVE indicates that the kernel supports moving an
+	 * existing page contents from userspace.
 	 */
 #define UFFD_FEATURE_PAGEFAULT_FLAG_WP		(1<<0)
 #define UFFD_FEATURE_EVENT_FORK			(1<<1)
@@ -239,6 +247,7 @@ struct uffdio_api {
 #define UFFD_FEATURE_WP_UNPOPULATED		(1<<13)
 #define UFFD_FEATURE_POISON			(1<<14)
 #define UFFD_FEATURE_WP_ASYNC			(1<<15)
+#define UFFD_FEATURE_MOVE			(1<<16)
 	__u64 features;
 
 	__u64 ioctls;
@@ -347,6 +356,24 @@ struct uffdio_poison {
 	__s64 updated;
 };
 
+struct uffdio_move {
+	__u64 dst;
+	__u64 src;
+	__u64 len;
+	/*
+	 * Especially if used to atomically remove memory from the
+	 * address space the wake on the dst range is not needed.
+	 */
+#define UFFDIO_MOVE_MODE_DONTWAKE		((__u64)1<<0)
+#define UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES	((__u64)1<<1)
+	__u64 mode;
+	/*
+	 * "move" is written by the ioctl and must be at the end: the
+	 * copy_from_user will not read the last 8 bytes.
+	 */
+	__s64 move;
+};
+
 /*
  * Flags for the userfaultfd(2) system call itself.
  */

mm/huge_memory.c

@@ -2141,6 +2141,128 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	return ret;
 }
 
+#ifdef CONFIG_USERFAULTFD
+/*
+ * The PT lock for src_pmd and the mmap_lock for reading are held by
+ * the caller, but it must return after releasing the page_table_lock.
+ * 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 move_pages_huge_pmd(struct mm_struct *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 folio *src_folio;
+	struct anon_vma *src_anon_vma;
+	spinlock_t *src_ptl, *dst_ptl;
+	pgtable_t src_pgtable;
+	struct mmu_notifier_range range;
+	int err = 0;
+
+	src_pmdval = *src_pmd;
+	src_ptl = pmd_lockptr(mm, src_pmd);
+
+	lockdep_assert_held(src_ptl);
+	mmap_assert_locked(mm);
+
+	/* Sanity checks before the operation */
+	if (WARN_ON_ONCE(!pmd_none(dst_pmdval)) || WARN_ON_ONCE(src_addr & ~HPAGE_PMD_MASK) ||
+	    WARN_ON_ONCE(dst_addr & ~HPAGE_PMD_MASK)) {
+		spin_unlock(src_ptl);
+		return -EINVAL;
+	}
+
+	if (!pmd_trans_huge(src_pmdval)) {
+		spin_unlock(src_ptl);
+		if (is_pmd_migration_entry(src_pmdval)) {
+			pmd_migration_entry_wait(mm, &src_pmdval);
+			return -EAGAIN;
+		}
+		return -ENOENT;
+	}
+
+	src_page = pmd_page(src_pmdval);
+	if (unlikely(!PageAnonExclusive(src_page))) {
+		spin_unlock(src_ptl);
+		return -EBUSY;
+	}
+
+	src_folio = page_folio(src_page);
+	folio_get(src_folio);
+	spin_unlock(src_ptl);
+
+	flush_cache_range(src_vma, src_addr, src_addr + HPAGE_PMD_SIZE);
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, src_addr,
+				src_addr + HPAGE_PMD_SIZE);
+	mmu_notifier_invalidate_range_start(&range);
+
+	folio_lock(src_folio);
+
+	/*
+	 * 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 = folio_get_anon_vma(src_folio);
+	if (!src_anon_vma) {
+		err = -EAGAIN;
+		goto unlock_folio;
+	}
+	anon_vma_lock_write(src_anon_vma);
+
+	dst_ptl = pmd_lockptr(mm, dst_pmd);
+	double_pt_lock(src_ptl, dst_ptl);
+	if (unlikely(!pmd_same(*src_pmd, src_pmdval) ||
+		     !pmd_same(*dst_pmd, dst_pmdval))) {
+		err = -EAGAIN;
+		goto unlock_ptls;
+	}
+	if (folio_maybe_dma_pinned(src_folio) ||
+	    !PageAnonExclusive(&src_folio->page)) {
+		err = -EBUSY;
+		goto unlock_ptls;
+	}
+
+	if (WARN_ON_ONCE(!folio_test_head(src_folio)) ||
+	    WARN_ON_ONCE(!folio_test_anon(src_folio))) {
+		err = -EBUSY;
+		goto unlock_ptls;
+	}
+
+	folio_move_anon_rmap(src_folio, dst_vma);
+	WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
+
+	src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
+	/* Folio got pinned from under us. Put it back and fail the move. */
+	if (folio_maybe_dma_pinned(src_folio)) {
+		set_pmd_at(mm, src_addr, src_pmd, src_pmdval);
+		err = -EBUSY;
+		goto unlock_ptls;
+	}
+
+	_dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot);
+	/* Follow mremap() behavior and treat the entry dirty after the move */
+	_dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
+	set_pmd_at(mm, dst_addr, dst_pmd, _dst_pmd);
+
+	src_pgtable = pgtable_trans_huge_withdraw(mm, src_pmd);
+	pgtable_trans_huge_deposit(mm, dst_pmd, src_pgtable);
+unlock_ptls:
+	double_pt_unlock(src_ptl, dst_ptl);
+	anon_vma_unlock_write(src_anon_vma);
+	put_anon_vma(src_anon_vma);
+unlock_folio:
+	/* unblock rmap walks */
+	folio_unlock(src_folio);
+	mmu_notifier_invalidate_range_end(&range);
+	folio_put(src_folio);
+	return err;
+}
+#endif /* CONFIG_USERFAULTFD */
+
 /*
  * Returns page table lock pointer if a given pmd maps a thp, NULL otherwise.
  *

mm/khugepaged.c

@@ -1140,6 +1140,9 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
 	 * Prevent all access to pagetables with the exception of
 	 * gup_fast later handled by the ptep_clear_flush and the VM
 	 * handled by the anon_vma lock + PG_lock.
+	 *
+	 * UFFDIO_MOVE is prevented to race as well thanks to the
+	 * mmap_lock.
 	 */
 	mmap_write_lock(mm);
 	result = hugepage_vma_revalidate(mm, address, true, &vma, cc);

mm/rmap.c

@@ -490,6 +490,12 @@ void __init anon_vma_init(void)
  * page_remove_rmap() that the anon_vma pointer from page->mapping is valid
  * if there is a mapcount, we can dereference the anon_vma after observing
  * those.
+ *
+ * NOTE: the caller should normally hold folio lock when calling this.  If
+ * not, the caller needs to double check the anon_vma didn't change after
+ * taking the anon_vma lock for either read or write (UFFDIO_MOVE can modify it
+ * concurrently without folio lock protection). See folio_lock_anon_vma_read()
+ * which has already covered that, and comment above remap_pages().
  */
 struct anon_vma *folio_get_anon_vma(struct folio *folio)
 {