[RFC PATCH 2/2] userfaultfd: add UFFDIO_CONTINUE ioctl

From: Axel Rasmussen
Date: Thu Jan 07 2021 - 14:06:34 EST

This ioctl is how userspace ought to resolve "minor" userfaults. The
idea is, userspace is notified that a minor fault has occurred. It might
change the contents of the page using its second non-UFFD mapping, or
not. Then, it calls UFFDIO_CONTINUE to tell the kernel "I have ensured
the page contents are correct, carry on setting up the mapping".

Note that it doesn't make much sense to use UFFDIO_{COPY,ZEROPAGE} for
MINOR registered VMAs. ZEROPAGE maps the VMA to the zero page; but in
the minor fault case, we already have some pre-existing underlying page.
Likewise, UFFDIO_COPY isn't useful if we have a second non-UFFD mapping.
We'd just use memcpy() or similar instead.

It turns out hugetlb_mcopy_atomic_pte() already does very close to what
we want, if an existing page is provided via `struct page **pagep`. We
already special-case the behavior a bit for the UFFDIO_ZEROPAGE case, so
just extend that design: add an enum for the three modes of operation,
and make the small adjustments needed for the MCOPY_ATOMIC_CONTINUE
case. (Basically, look up the existing page. This in turn causes
hugetlb_mcopy_atomic_pte() to not (double-)add the page to the page
cache, and it also avoid set_page_huge_active() (as this would have been
done when the page was allocated).

Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
---
fs/userfaultfd.c | 63 +++++++++++++++++++++++
include/linux/userfaultfd_k.h | 2 +
include/uapi/linux/userfaultfd.h | 21 +++++++-
mm/hugetlb.c | 11 ++--
mm/userfaultfd.c | 86 ++++++++++++++++++++++++--------
5 files changed, 154 insertions(+), 29 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 0a661422eb19..08ff561426c1 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1840,6 +1840,66 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
return ret;
}

+static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
+{
+ __s64 ret;
+ struct uffdio_continue uffdio_continue;
+ struct uffdio_continue __user *user_uffdio_continue;
+ struct userfaultfd_wake_range range;
+
+ user_uffdio_continue = (struct uffdio_continue __user *)arg;
+
+ ret = -EAGAIN;
+ if (READ_ONCE(ctx->mmap_changing))
+ goto out;
+
+ ret = -EFAULT;
+ if (copy_from_user(&uffdio_continue, user_uffdio_continue,
+ /* don't copy the output fields */
+ sizeof(uffdio_continue) - (sizeof(__s64))))
+ goto out;
+
+ ret = validate_range(ctx->mm, &uffdio_continue.range.start,
+ uffdio_continue.range.len);
+ if (ret)
+ goto out;
+
+ ret = -EINVAL;
+ /* double check for wraparound just in case. */
+ if (uffdio_continue.range.start + uffdio_continue.range.len <=
+ uffdio_continue.range.start) {
+ goto out;
+ }
+ if (uffdio_continue.mode & ~UFFDIO_CONTINUE_MODE_DONTWAKE)
+ goto out;
+
+ if (mmget_not_zero(ctx->mm)) {
+ ret = mcopy_continue(ctx->mm, uffdio_continue.range.start,
+ uffdio_continue.range.len,
+ &ctx->mmap_changing);
+ mmput(ctx->mm);
+ } else {
+ return -ESRCH;
+ }
+
+ if (unlikely(put_user(ret, &user_uffdio_continue->mapped)))
+ return -EFAULT;
+ if (ret < 0)
+ goto out;
+
+ /* len == 0 would wake all */
+ BUG_ON(!ret);
+ range.len = ret;
+ if (!(uffdio_continue.mode & UFFDIO_CONTINUE_MODE_DONTWAKE)) {
+ range.start = uffdio_continue.range.start;
+ wake_userfault(ctx, &range);
+ }
+ ret = range.len == uffdio_continue.range.len ? 0 : -EAGAIN;
+
+out:
+ return ret;
+}
+
static inline unsigned int uffd_ctx_features(__u64 user_features)
{
/*
@@ -1924,6 +1984,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
case UFFDIO_WRITEPROTECT:
ret = userfaultfd_writeprotect(ctx, arg);
break;
+ case UFFDIO_CONTINUE:
+ ret = userfaultfd_continue(ctx, arg);
+ break;
}
return ret;
}
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 0c8c5fa5efc8..b1f5aaec8dad 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -41,6 +41,8 @@ extern ssize_t mfill_zeropage(struct mm_struct *dst_mm,
unsigned long dst_start,
unsigned long len,
bool *mmap_changing);
+extern ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long dst_start,
+ unsigned long len, bool *mmap_changing);
extern int mwriteprotect_range(struct mm_struct *dst_mm,
unsigned long start, unsigned long len,
bool enable_wp, bool *mmap_changing);
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 1cc2cd8a5279..9a48305f4bdd 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -40,10 +40,12 @@
((__u64)1 << _UFFDIO_WAKE | \
(__u64)1 << _UFFDIO_COPY | \
(__u64)1 << _UFFDIO_ZEROPAGE | \
- (__u64)1 << _UFFDIO_WRITEPROTECT)
+ (__u64)1 << _UFFDIO_WRITEPROTECT | \
+ (__u64)1 << _UFFDIO_CONTINUE)
#define UFFD_API_RANGE_IOCTLS_BASIC \
((__u64)1 << _UFFDIO_WAKE | \
- (__u64)1 << _UFFDIO_COPY)
+ (__u64)1 << _UFFDIO_COPY | \
+ (__u64)1 << _UFFDIO_CONTINUE)

/*
* Valid ioctl command number range with this API is from 0x00 to
@@ -59,6 +61,7 @@
#define _UFFDIO_COPY (0x03)
#define _UFFDIO_ZEROPAGE (0x04)
#define _UFFDIO_WRITEPROTECT (0x06)
+#define _UFFDIO_CONTINUE (0x07)
#define _UFFDIO_API (0x3F)

/* userfaultfd ioctl ids */
@@ -77,6 +80,8 @@
struct uffdio_zeropage)
#define UFFDIO_WRITEPROTECT _IOWR(UFFDIO, _UFFDIO_WRITEPROTECT, \
struct uffdio_writeprotect)
+#define UFFDIO_CONTINUE _IOR(UFFDIO, _UFFDIO_CONTINUE, \
+ struct uffdio_continue)

/* read() structure */
struct uffd_msg {
@@ -268,6 +273,18 @@ struct uffdio_writeprotect {
__u64 mode;
};

+struct uffdio_continue {
+ struct uffdio_range range;
+#define UFFDIO_CONTINUE_MODE_DONTWAKE ((__u64)1<<0)
+ __u64 mode;
+
+ /*
+ * Fields below here are written by the ioctl and must be at the end:
+ * the copy_from_user will not read past here.
+ */
+ __s64 mapped;
+};
+
/*
* Flags for the userfaultfd(2) system call itself.
*/
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 0ba8f2f5a4ae..889b9cf7f107 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4666,12 +4666,14 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
spinlock_t *ptl;
int ret;
struct page *page;
+ bool new_page = false;

if (!*pagep) {
ret = -ENOMEM;
page = alloc_huge_page(dst_vma, dst_addr, 0);
if (IS_ERR(page))
goto out;
+ new_page = true;

ret = copy_huge_page_from_user(page,
(const void __user *) src_addr,
@@ -4699,10 +4701,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
mapping = dst_vma->vm_file->f_mapping;
idx = vma_hugecache_offset(h, dst_vma, dst_addr);

- /*
- * If shared, add to page cache
- */
- if (vm_shared) {
+ /* Add shared, newly allocated pages to the page cache. */
+ if (vm_shared && new_page) {
size = i_size_read(mapping->host) >> huge_page_shift(h);
ret = -EFAULT;
if (idx >= size)
@@ -4762,7 +4762,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
update_mmu_cache(dst_vma, dst_addr, dst_pte);

spin_unlock(ptl);
- set_page_huge_active(page);
+ if (new_page)
+ set_page_huge_active(page);
if (vm_shared)
unlock_page(page);
ret = 0;
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 9a3d451402d7..15ba09bb4ceb 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -197,6 +197,16 @@ static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
return pmd_alloc(mm, pud, address);
}

+/* The mode of operation for __mcopy_atomic and its helpers. */
+enum mcopy_atomic_mode {
+ /* A normal copy_from_user into the destination range. */
+ MCOPY_ATOMIC_NORMAL,
+ /* Don't copy; map the destination range to the zero page. */
+ MCOPY_ATOMIC_ZEROPAGE,
+ /* Just setup the dst_vma, without modifying the underlying page(s). */
+ MCOPY_ATOMIC_CONTINUE,
+};
+
#ifdef CONFIG_HUGETLB_PAGE
/*
* __mcopy_atomic processing for HUGETLB vmas. Note that this routine is
@@ -207,7 +217,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
unsigned long dst_start,
unsigned long src_start,
unsigned long len,
- bool zeropage)
+ enum mcopy_atomic_mode mode)
{
int vm_alloc_shared = dst_vma->vm_flags & VM_SHARED;
int vm_shared = dst_vma->vm_flags & VM_SHARED;
@@ -227,7 +237,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
* by THP. Since we can not reliably insert a zero page, this
* feature is not supported.
*/
- if (zeropage) {
+ if (mode == MCOPY_ATOMIC_ZEROPAGE) {
mmap_read_unlock(dst_mm);
return -EINVAL;
}
@@ -273,8 +283,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
}

while (src_addr < src_start + len) {
- pte_t dst_pteval;
-
BUG_ON(dst_addr >= dst_start + len);

/*
@@ -297,12 +305,22 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
goto out_unlock;
}

- err = -EEXIST;
- dst_pteval = huge_ptep_get(dst_pte);
- if (!huge_pte_none(dst_pteval)) {
- mutex_unlock(&hugetlb_fault_mutex_table[hash]);
- i_mmap_unlock_read(mapping);
- goto out_unlock;
+ if (mode == MCOPY_ATOMIC_CONTINUE) {
+ /* hugetlb_mcopy_atomic_pte unlocks the page. */
+ page = find_lock_page(mapping, idx);
+ if (!page) {
+ err = -EFAULT;
+ mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+ i_mmap_unlock_read(mapping);
+ goto out_unlock;
+ }
+ } else {
+ if (!huge_pte_none(huge_ptep_get(dst_pte))) {
+ err = -EEXIST;
+ mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+ i_mmap_unlock_read(mapping);
+ goto out_unlock;
+ }
}

err = hugetlb_mcopy_atomic_pte(dst_mm, dst_pte, dst_vma,
@@ -408,7 +426,7 @@ extern ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
unsigned long dst_start,
unsigned long src_start,
unsigned long len,
- bool zeropage);
+ enum mcopy_atomic_mode mode);
#endif /* CONFIG_HUGETLB_PAGE */

static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
@@ -417,7 +435,7 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
unsigned long dst_addr,
unsigned long src_addr,
struct page **page,
- bool zeropage,
+ enum mcopy_atomic_mode mode,
bool wp_copy)
{
ssize_t err;
@@ -433,22 +451,38 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
* and not in the radix tree.
*/
if (!(dst_vma->vm_flags & VM_SHARED)) {
- if (!zeropage)
+ switch (mode) {
+ case MCOPY_ATOMIC_NORMAL:
err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
dst_addr, src_addr, page,
wp_copy);
- else
+ break;
+ case MCOPY_ATOMIC_ZEROPAGE:
err = mfill_zeropage_pte(dst_mm, dst_pmd,
dst_vma, dst_addr);
+ break;
+ /* It only makes sense to CONTINUE for shared memory. */
+ case MCOPY_ATOMIC_CONTINUE:
+ err = -EINVAL;
+ break;
+ }
} else {
VM_WARN_ON_ONCE(wp_copy);
- if (!zeropage)
+ switch (mode) {
+ case MCOPY_ATOMIC_NORMAL:
err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd,
dst_vma, dst_addr,
src_addr, page);
- else
+ break;
+ case MCOPY_ATOMIC_ZEROPAGE:
err = shmem_mfill_zeropage_pte(dst_mm, dst_pmd,
dst_vma, dst_addr);
+ break;
+ case MCOPY_ATOMIC_CONTINUE:
+ /* FIXME: Add minor fault interception for shmem. */
+ err = -EINVAL;
+ break;
+ }
}

return err;
@@ -458,7 +492,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
unsigned long dst_start,
unsigned long src_start,
unsigned long len,
- bool zeropage,
+ enum mcopy_atomic_mode mcopy_mode,
bool *mmap_changing,
__u64 mode)
{
@@ -527,7 +561,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
*/
if (is_vm_hugetlb_page(dst_vma))
return __mcopy_atomic_hugetlb(dst_mm, dst_vma, dst_start,
- src_start, len, zeropage);
+ src_start, len, mcopy_mode);

if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
goto out_unlock;
@@ -577,7 +611,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
BUG_ON(pmd_trans_huge(*dst_pmd));

err = mfill_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
- src_addr, &page, zeropage, wp_copy);
+ src_addr, &page, mcopy_mode, wp_copy);
cond_resched();

if (unlikely(err == -ENOENT)) {
@@ -626,14 +660,22 @@ ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,
unsigned long src_start, unsigned long len,
bool *mmap_changing, __u64 mode)
{
- return __mcopy_atomic(dst_mm, dst_start, src_start, len, false,
- mmap_changing, mode);
+ return __mcopy_atomic(dst_mm, dst_start, src_start, len,
+ MCOPY_ATOMIC_NORMAL, mmap_changing, mode);
}

ssize_t mfill_zeropage(struct mm_struct *dst_mm, unsigned long start,
unsigned long len, bool *mmap_changing)
{
- return __mcopy_atomic(dst_mm, start, 0, len, true, mmap_changing, 0);
+ return __mcopy_atomic(dst_mm, start, 0, len, MCOPY_ATOMIC_ZEROPAGE,
+ mmap_changing, 0);
+}
+
+ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long start,
+ unsigned long len, bool *mmap_changing)
+{
+ return __mcopy_atomic(dst_mm, start, 0, len, MCOPY_ATOMIC_CONTINUE,
+ mmap_changing, 0);
}

int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
--
2.29.2.729.g45daf8777d-goog