[PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI

Suren Baghdasaryan
2023-09-23T01:32:10Z

From: Andrea Arcangeli <aarcange@redhat.com>

This implements the uABI of UFFDIO_REMAP.

Notably one mode bitflag is also forwarded (and in turn known) by the
lowlevel remap_pages method.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
Changes since v1:
- add mmget_not_zero in userfaultfd_remap, per Jann Horn
- removed extern from function definitions, per Matthew Wilcox
- converted to folios in remap_pages_huge_pmd, per Matthew Wilcox
- use PageAnonExclusive in remap_pages_huge_pmd, per David Hildenbrand
- handle pgtable transfers between MMs, per Jann Horn
- ignore concurrent A/D pte bit changes, per Jann Horn
- split functions into smaller units, per David Hildenbrand
- test for folio_test_large in remap_anon_pte, per Matthew Wilcox
- use pte_swp_exclusive for swapcount check, per David Hildenbrand
- eliminated use of mmu_notifier_invalidate_range_start_nonblock,
per Jann Horn
- simplified THP alignment checks, per Jann Horn
- refactored the loop inside remap_pages, per Jann Horn
- additional clarifying comments, per Jann Horn

fs/userfaultfd.c | 63 ++++
include/linux/rmap.h | 5 +
include/linux/userfaultfd_k.h | 12 +
include/uapi/linux/userfaultfd.h | 22 ++
mm/huge_memory.c | 130 +++++++
mm/khugepaged.c | 3 +
mm/userfaultfd.c | 590 +++++++++++++++++++++++++++++++
7 files changed, 825 insertions(+)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 56eaae9dac1a..5b6bb20f4518 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -2027,6 +2027,66 @@ static inline unsigned int uffd_ctx_features(__u64 user_features)
return (unsigned int)user_features | UFFD_FEATURE_INITIALIZED;
}

+static int userfaultfd_remap(struct userfaultfd_ctx *ctx,
+ unsigned long arg)
+{
+ __s64 ret;
+ struct uffdio_remap uffdio_remap;
+ struct uffdio_remap __user *user_uffdio_remap;
+ struct userfaultfd_wake_range range;
+
+ user_uffdio_remap = (struct uffdio_remap __user *) arg;
+
+ ret = -EAGAIN;
+ if (atomic_read(&ctx->mmap_changing))
+ goto out;
+
+ ret = -EFAULT;
+ if (copy_from_user(&uffdio_remap, user_uffdio_remap,
+ /* don't copy "remap" last field */
+ sizeof(uffdio_remap)-sizeof(__s64)))
+ goto out;
+
+ ret = validate_range(ctx->mm, uffdio_remap.dst, uffdio_remap.len);
+ if (ret)
+ goto out;
+
+ ret = validate_range(current->mm, uffdio_remap.src, uffdio_remap.len);
+ if (ret)
+ goto out;
+
+ ret = -EINVAL;
+ if (uffdio_remap.mode & ~(UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES|
+ UFFDIO_REMAP_MODE_DONTWAKE))
+ goto out;
+
+ if (mmget_not_zero(ctx->mm)) {
+ ret = remap_pages(ctx->mm, current->mm,
+ uffdio_remap.dst, uffdio_remap.src,
+ uffdio_remap.len, uffdio_remap.mode);
+ mmput(ctx->mm);
+ } else {
+ return -ESRCH;
+ }
+
+ if (unlikely(put_user(ret, &user_uffdio_remap->remap)))
+ return -EFAULT;
+ if (ret < 0)
+ goto out;
+
+ /* len == 0 would wake all */
+ BUG_ON(!ret);
+ range.len = ret;
+ if (!(uffdio_remap.mode & UFFDIO_REMAP_MODE_DONTWAKE)) {
+ range.start = uffdio_remap.dst;
+ wake_userfault(ctx, &range);
+ }
+ ret = range.len == uffdio_remap.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
@@ -2113,6 +2173,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
case UFFDIO_ZEROPAGE:
ret = userfaultfd_zeropage(ctx, arg);
break;
+ case UFFDIO_REMAP:
+ ret = userfaultfd_remap(ctx, arg);
+ break;
case UFFDIO_WRITEPROTECT:
ret = userfaultfd_writeprotect(ctx, arg);
break;
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 51cc21ebb568..614c4b439907 100644
--- a/include/linux/rmap.h
+++ b/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);
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index ac8c6854097c..9ea2c43ad4b7 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -93,6 +93,18 @@ 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);

+/* remap_pages */
+void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2);
+void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2);
+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);
+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/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 62151706c5a3..22d1c43e39f9 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -49,6 +49,7 @@
((__u64)1 << _UFFDIO_WAKE | \
(__u64)1 << _UFFDIO_COPY | \
(__u64)1 << _UFFDIO_ZEROPAGE | \
+ (__u64)1 << _UFFDIO_REMAP | \
(__u64)1 << _UFFDIO_WRITEPROTECT | \
(__u64)1 << _UFFDIO_CONTINUE | \
(__u64)1 << _UFFDIO_POISON)
@@ -72,6 +73,7 @@
#define _UFFDIO_WAKE (0x02)
#define _UFFDIO_COPY (0x03)
#define _UFFDIO_ZEROPAGE (0x04)
+#define _UFFDIO_REMAP (0x05)
#define _UFFDIO_WRITEPROTECT (0x06)
#define _UFFDIO_CONTINUE (0x07)
#define _UFFDIO_POISON (0x08)
@@ -91,6 +93,8 @@
struct uffdio_copy)
#define UFFDIO_ZEROPAGE _IOWR(UFFDIO, _UFFDIO_ZEROPAGE, \
struct uffdio_zeropage)
+#define UFFDIO_REMAP _IOWR(UFFDIO, _UFFDIO_REMAP, \
+ struct uffdio_remap)
#define UFFDIO_WRITEPROTECT _IOWR(UFFDIO, _UFFDIO_WRITEPROTECT, \
struct uffdio_writeprotect)
#define UFFDIO_CONTINUE _IOWR(UFFDIO, _UFFDIO_CONTINUE, \
@@ -340,6 +344,24 @@ struct uffdio_poison {
__s64 updated;
};

+struct uffdio_remap {
+ __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_REMAP_MODE_DONTWAKE ((__u64)1<<0)
+#define UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES ((__u64)1<<1)
+ __u64 mode;
+ /*
+ * "remap" is written by the ioctl and must be at the end: the
+ * copy_from_user will not read the last 8 bytes.
+ */
+ __s64 remap;
+};
+
/*
* Flags for the userfaultfd(2) system call itself.
*/
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 064fbd90822b..a8c898df36db 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1932,6 +1932,136 @@ 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. 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 folio *src_folio;
+ struct anon_vma *src_anon_vma, *dst_anon_vma;
+ spinlock_t *src_ptl, *dst_ptl;
+ pgtable_t src_pgtable, dst_pgtable;
+ struct mmu_notifier_range range;
+ int err = 0;
+
+ src_pmdval = *src_pmd;
+ src_ptl = pmd_lockptr(src_mm, src_pmd);
+
+ BUG_ON(!spin_is_locked(src_ptl));
+ mmap_assert_locked(src_mm);
+ mmap_assert_locked(dst_mm);
+
+ BUG_ON(!pmd_trans_huge(src_pmdval));
+ BUG_ON(!pmd_none(dst_pmdval));
+ BUG_ON(src_addr & ~HPAGE_PMD_MASK);
+ BUG_ON(dst_addr & ~HPAGE_PMD_MASK);
+
+ 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);
+
+ /* preallocate dst_pgtable if needed */
+ if (dst_mm != src_mm) {
+ dst_pgtable = pte_alloc_one(dst_mm);
+ if (unlikely(!dst_pgtable)) {
+ err = -ENOMEM;
+ goto put_folio;
+ }
+ } else {
+ dst_pgtable = NULL;
+ }
+
+ mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, src_mm, src_addr,
+ src_addr + HPAGE_PMD_SIZE);
+ mmu_notifier_invalidate_range_start(&range);
+
+ /* block all concurrent rmap walks */
+ 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(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) ||
+ folio_mapcount(src_folio) != 1)) {
+ double_pt_unlock(src_ptl, dst_ptl);
+ err = -EAGAIN;
+ goto put_anon_vma;
+ }
+
+ BUG_ON(!folio_test_head(src_folio));
+ BUG_ON(!folio_test_anon(src_folio));
+
+ dst_anon_vma = (void *)dst_vma->anon_vma + PAGE_MAPPING_ANON;
+ WRITE_ONCE(src_folio->mapping, (struct address_space *) dst_anon_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);
+ _dst_pmd = mk_huge_pmd(&src_folio->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);
+
+ src_pgtable = pgtable_trans_huge_withdraw(src_mm, src_pmd);
+ if (dst_pgtable) {
+ pgtable_trans_huge_deposit(dst_mm, dst_pmd, dst_pgtable);
+ pte_free(src_mm, src_pgtable);
+ dst_pgtable = NULL;
+
+ mm_inc_nr_ptes(dst_mm);
+ mm_dec_nr_ptes(src_mm);
+ add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
+ add_mm_counter(src_mm, MM_ANONPAGES, -HPAGE_PMD_NR);
+ } else {
+ pgtable_trans_huge_deposit(dst_mm, dst_pmd, src_pgtable);
+ }
+ double_pt_unlock(src_ptl, dst_ptl);
+
+put_anon_vma:
+ 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);
+ if (dst_pgtable)
+ pte_free(dst_mm, dst_pgtable);
+put_folio:
+ folio_put(src_folio);
+
+ return err;
+}
+#endif /* CONFIG_USERFAULTFD */
+
/*
* Returns page table lock pointer if a given pmd maps a thp, NULL otherwise.
*
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 88433cc25d8a..af23248b3551 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1135,6 +1135,9 @@ 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_REMAP is prevented to race as well thanks to the
+ * mmap_lock.
*/
mmap_write_lock(mm);
result = hugepage_vma_revalidate(mm, address, true, &vma, cc);
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 96d9eae5c7cc..5ce5e364373c 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -842,3 +842,593 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
mmap_read_unlock(dst_mm);
return err;
}
+
+
+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);
+}
+
+
+static int remap_anon_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
+ struct vm_area_struct *dst_vma,
+ struct vm_area_struct *src_vma,
+ unsigned long dst_addr, unsigned long src_addr,
+ pte_t *dst_pte, pte_t *src_pte,
+ pte_t orig_dst_pte, pte_t orig_src_pte,
+ spinlock_t *dst_ptl, spinlock_t *src_ptl,
+ struct folio *src_folio)
+{
+ struct anon_vma *dst_anon_vma;
+
+ double_pt_lock(dst_ptl, src_ptl);
+
+ if (!pte_same(*src_pte, orig_src_pte) ||
+ !pte_same(*dst_pte, orig_dst_pte) ||
+ folio_test_large(src_folio) ||
+ folio_estimated_sharers(src_folio) != 1) {
+ double_pt_unlock(dst_ptl, src_ptl);
+ return -EAGAIN;
+ }
+
+ BUG_ON(!folio_test_anon(src_folio));
+
+ dst_anon_vma = (void *)dst_vma->anon_vma + PAGE_MAPPING_ANON;
+ WRITE_ONCE(src_folio->mapping,
+ (struct address_space *) dst_anon_vma);
+ WRITE_ONCE(src_folio->index, linear_page_index(dst_vma,
+ dst_addr));
+
+ orig_src_pte = ptep_clear_flush(src_vma, src_addr, src_pte);
+ orig_dst_pte = mk_pte(&src_folio->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);
+
+ return 0;
+}
+
+static int remap_swap_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
+ unsigned long dst_addr, unsigned long src_addr,
+ pte_t *dst_pte, pte_t *src_pte,
+ pte_t orig_dst_pte, pte_t orig_src_pte,
+ spinlock_t *dst_ptl, spinlock_t *src_ptl)
+{
+ if (!pte_swp_exclusive(orig_src_pte))
+ return -EBUSY;
+
+ double_pt_lock(dst_ptl, src_ptl);
+
+ if (!pte_same(*src_pte, orig_src_pte) ||
+ !pte_same(*dst_pte, orig_dst_pte)) {
+ double_pt_unlock(dst_ptl, src_ptl);
+ return -EAGAIN;
+ }
+
+ orig_src_pte = ptep_get_and_clear(src_mm, src_addr, src_pte);
+ 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;
+}
+
+/*
+ * The mmap_lock 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,
+ pmd_t *dst_pmd,
+ pmd_t *src_pmd,
+ struct vm_area_struct *dst_vma,
+ struct vm_area_struct *src_vma,
+ unsigned long dst_addr,
+ unsigned long src_addr,
+ __u64 mode)
+{
+ swp_entry_t entry;
+ pte_t orig_src_pte, orig_dst_pte;
+ spinlock_t *src_ptl, *dst_ptl;
+ pte_t *src_pte = NULL;
+ pte_t *dst_pte = NULL;
+
+ struct folio *src_folio = NULL;
+ struct anon_vma *src_anon_vma = NULL;
+ struct mmu_notifier_range range;
+ int err = 0;
+
+ mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, src_mm,
+ src_addr, src_addr + PAGE_SIZE);
+ mmu_notifier_invalidate_range_start(&range);
+retry:
+ dst_pte = pte_offset_map_nolock(dst_mm, dst_pmd, dst_addr, &dst_ptl);
+
+ /* If an huge pmd materialized from under us fail */
+ if (unlikely(!dst_pte)) {
+ err = -EFAULT;
+ goto out;
+ }
+
+ src_pte = pte_offset_map_nolock(src_mm, src_pmd, src_addr, &src_ptl);
+
+ /*
+ * We held the mmap_lock 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(!src_pte)) {
+ err = -EFAULT;
+ goto out;
+ }
+
+ 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));
+
+ spin_lock(dst_ptl);
+ orig_dst_pte = *dst_pte;
+ spin_unlock(dst_ptl);
+ if (!pte_none(orig_dst_pte)) {
+ err = -EEXIST;
+ goto out;
+ }
+
+ 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))
+ err = -ENOENT;
+ else /* nothing to do to remap a hole */
+ err = 0;
+ goto out;
+ }
+
+ if (pte_present(orig_src_pte)) {
+ /*
+ * Pin and lock both source folio and anon_vma. Since we are in
+ * RCU read section, we can't block, so on contention have to
+ * unmap the ptes, obtain the lock and retry.
+ */
+ if (!src_folio) {
+ struct folio *folio;
+
+ /*
+ * 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);
+ err = -EAGAIN;
+ goto out;
+ }
+
+ folio = vm_normal_folio(src_vma, src_addr, orig_src_pte);
+ if (!folio || !folio_test_anon(folio) ||
+ folio_test_large(folio) ||
+ folio_estimated_sharers(folio) != 1) {
+ spin_unlock(src_ptl);
+ err = -EBUSY;
+ goto out;
+ }
+
+ folio_get(folio);
+ src_folio = folio;
+ spin_unlock(src_ptl);
+
+ /* block all concurrent rmap walks */
+ if (!folio_trylock(src_folio)) {
+ pte_unmap(&orig_src_pte);
+ pte_unmap(&orig_dst_pte);
+ src_pte = dst_pte = NULL;
+ /* now we can block and wait */
+ folio_lock(src_folio);
+ goto retry;
+ }
+ }
+
+ if (!src_anon_vma) {
+ /*
+ * folio_referenced walks the anon_vma chain
+ * without the folio lock. Serialize against it with
+ * the anon_vma lock, the folio lock is not enough.
+ */
+ src_anon_vma = folio_get_anon_vma(src_folio);
+ if (!src_anon_vma) {
+ /* page was unmapped from under us */
+ err = -EAGAIN;
+ goto out;
+ }
+ if (!anon_vma_trylock_write(src_anon_vma)) {
+ pte_unmap(&orig_src_pte);
+ pte_unmap(&orig_dst_pte);
+ src_pte = dst_pte = NULL;
+ /* now we can block and wait */
+ anon_vma_lock_write(src_anon_vma);
+ goto retry;
+ }
+ }
+
+ err = remap_anon_pte(dst_mm, src_mm, dst_vma, src_vma,
+ dst_addr, src_addr, dst_pte, src_pte,
+ orig_dst_pte, orig_src_pte,
+ dst_ptl, src_ptl, src_folio);
+ } else {
+ entry = pte_to_swp_entry(orig_src_pte);
+ if (non_swap_entry(entry)) {
+ if (is_migration_entry(entry)) {
+ pte_unmap(&orig_src_pte);
+ pte_unmap(&orig_dst_pte);
+ src_pte = dst_pte = NULL;
+ migration_entry_wait(src_mm, src_pmd,
+ src_addr);
+ err = -EAGAIN;
+ } else
+ err = -EFAULT;
+ goto out;
+ }
+
+ err = remap_swap_pte(dst_mm, src_mm, dst_addr, src_addr,
+ dst_pte, src_pte,
+ orig_dst_pte, orig_src_pte,
+ dst_ptl, src_ptl);
+ }
+
+out:
+ if (src_anon_vma) {
+ anon_vma_unlock_write(src_anon_vma);
+ put_anon_vma(src_anon_vma);
+ }
+ if (src_folio) {
+ folio_unlock(src_folio);
+ folio_put(src_folio);
+ }
+ if (dst_pte)
+ pte_unmap(dst_pte);
+ if (src_pte)
+ pte_unmap(src_pte);
+ mmu_notifier_invalidate_range_end(&range);
+
+ return err;
+}
+
+static int validate_remap_areas(struct vm_area_struct *src_vma,
+ struct vm_area_struct *dst_vma)
+{
+ /* Only allow remapping if both have the same access and protection */
+ if ((src_vma->vm_flags & VM_ACCESS_FLAGS) != (dst_vma->vm_flags & VM_ACCESS_FLAGS) ||
+ pgprot_val(src_vma->vm_page_prot) != pgprot_val(dst_vma->vm_page_prot))
+ return -EINVAL;
+
+ /* Only allow remapping if both are mlocked or both aren't */
+ if ((src_vma->vm_flags & VM_LOCKED) != (dst_vma->vm_flags & VM_LOCKED))
+ return -EINVAL;
+
+ /*
+ * 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)
+ return -EINVAL;
+
+ /*
+ * FIXME: only allow remapping across anonymous vmas,
+ * tmpfs should be added.
+ */
+ if (!vma_is_anonymous(src_vma) || !vma_is_anonymous(dst_vma))
+ return -EINVAL;
+
+ /*
+ * Ensure the dst_vma has a anon_vma or this page
+ * would get a NULL anon_vma when moved in the
+ * dst_vma.
+ */
+ if (unlikely(anon_vma_prepare(dst_vma)))
+ return -ENOMEM;
+
+ 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 provides a zero copy mechanism to handle userspace page faults.
+ * The source vma pages should have mapcount == 1, which can be
+ * enforced by using madvise(MADV_DONTFORK) on src vma.
+ *
+ * The thread receiving the page during the userland page fault
+ * 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 successful. 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 folio lock (for example split_huge_page and
+ * folio_referenced), they will have to verify if the folio->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;
+ unsigned long src_addr, dst_addr;
+ pmd_t *src_pmd, *dst_pmd;
+ long err = -EINVAL;
+ 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.
+ */
+ mmap_read_lock(dst_mm);
+ if (dst_mm != src_mm)
+ mmap_read_lock(src_mm);
+
+ /*
+ * 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;
+
+ err = validate_remap_areas(src_vma, dst_vma);
+ if (err)
+ goto out;
+
+ for (src_addr = src_start, dst_addr = dst_start;
+ src_addr < src_start + len;) {
+ spinlock_t *ptl;
+ pmd_t dst_pmdval;
+ unsigned long step_size;
+
+ BUG_ON(dst_addr >= dst_start + len);
+ /*
+ * Below works because anonymous area would not have a
+ * transparent huge PUD. If file-backed support is added,
+ * that case would need to be handled here.
+ */
+ src_pmd = mm_find_pmd(src_mm, src_addr);
+ if (unlikely(!src_pmd)) {
+ if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES)) {
+ err = -ENOENT;
+ break;
+ }
+ 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 = pmdp_get_lockless(dst_pmd);
+ /*
+ * If the dst_pmd is mapped as THP don't override it and just
+ * be strict. If dst_pmd changes into TPH after this check, the
+ * remap_pages_huge_pmd() will detect the change and retry
+ * while remap_pages_pte() will detect the change and fail.
+ */
+ if (unlikely(pmd_trans_huge(dst_pmdval))) {
+ err = -EEXIST;
+ break;
+ }
+
+ ptl = pmd_trans_huge_lock(src_pmd, src_vma);
+ if (ptl && !pmd_trans_huge(*src_pmd)) {
+ spin_unlock(ptl);
+ ptl = NULL;
+ }
+
+ if (ptl) {
+ /*
+ * 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 ((src_addr & ~HPAGE_PMD_MASK) || (dst_addr & ~HPAGE_PMD_MASK) ||
+ src_start + len - src_addr < HPAGE_PMD_SIZE || !pmd_none(dst_pmdval)) {
+ spin_unlock(ptl);
+ split_huge_pmd(src_vma, src_pmd, src_addr);
+ continue;
+ }
+
+ err = remap_pages_huge_pmd(dst_mm, src_mm,
+ dst_pmd, src_pmd,
+ dst_pmdval,
+ dst_vma, src_vma,
+ dst_addr, src_addr);
+ step_size = HPAGE_PMD_SIZE;
+ } else {
+ if (pmd_none(*src_pmd)) {
+ if (!(mode & UFFDIO_REMAP_MODE_ALLOW_SRC_HOLES)) {
+ err = -ENOENT;
+ break;
+ }
+ if (unlikely(__pte_alloc(src_mm, src_pmd))) {
+ err = -ENOMEM;
+ break;
+ }
+ }
+
+ if (unlikely(pte_alloc(dst_mm, dst_pmd))) {
+ err = -ENOMEM;
+ break;
+ }
+
+ err = remap_pages_pte(dst_mm, src_mm,
+ dst_pmd, src_pmd,
+ dst_vma, src_vma,
+ dst_addr, src_addr,
+ mode);
+ step_size = PAGE_SIZE;
+ }
+
+ cond_resched();
+
+ if (!err) {
+ dst_addr += step_size;
+ src_addr += step_size;
+ moved += step_size;
+ }
+
+ if ((!err || err == -EAGAIN) &&
+ fatal_signal_pending(current))
+ err = -EINTR;
+
+ if (err && err != -EAGAIN)
+ break;
+ }
+
+out:
+ mmap_read_unlock(dst_mm);
+ if (dst_mm != src_mm)
+ mmap_read_unlock(src_mm);
+ BUG_ON(moved < 0);
+ BUG_ON(err > 0);
+ BUG_ON(!moved && !err);
+ return moved ? moved : err;
+}
--
2.42.0.515.g380fc7ccd1-goog