[PATCH v7 4/6] userfaultfd: add UFFDIO_CONTINUE ioctl

From: Axel Rasmussen
Date: Thu Feb 18 2021 - 19:50:31 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, and avoid adding the
existing page to the page cache or calling set_page_huge_active() on
it.)

Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
---
fs/userfaultfd.c | 67 ++++++++++++++++++++++++++++++++
include/linux/hugetlb.h | 3 ++
include/linux/userfaultfd_k.h | 18 +++++++++
include/uapi/linux/userfaultfd.h | 21 +++++++++-
mm/hugetlb.c | 41 ++++++++++++-------
mm/userfaultfd.c | 37 +++++++++++-------
6 files changed, 157 insertions(+), 30 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 041449e47870..3b42c09eb043 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1488,6 +1488,10 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_WP))
ioctls_out &= ~((__u64)1 << _UFFDIO_WRITEPROTECT);

+ /* CONTINUE ioctl is only supported for MINOR ranges. */
+ if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR))
+ ioctls_out &= ~((__u64)1 << _UFFDIO_CONTINUE);
+
/*
* Now that we scanned all vmas we can already tell
* userland which ioctls methods are guaranteed to
@@ -1841,6 +1845,66 @@ 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)
{
/*
@@ -1928,6 +1992,9 @@ 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/hugetlb.h b/include/linux/hugetlb.h
index 7e6d2f126df3..8f2a4fc11b1f 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -11,6 +11,7 @@
#include <linux/kref.h>
#include <linux/pgtable.h>
#include <linux/gfp.h>
+#include <linux/userfaultfd_k.h>

struct ctl_table;
struct user_struct;
@@ -139,6 +140,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm, pte_t *dst_pte,
struct vm_area_struct *dst_vma,
unsigned long dst_addr,
unsigned long src_addr,
+ enum mcopy_atomic_mode mode,
struct page **pagep);
#endif /* CONFIG_USERFAULTFD */
bool hugetlb_reserve_pages(struct inode *inode, long from, long to,
@@ -318,6 +320,7 @@ static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
struct vm_area_struct *dst_vma,
unsigned long dst_addr,
unsigned long src_addr,
+ enum mcopy_atomic_mode mode,
struct page **pagep)
{
BUG();
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index e060d5f77cc5..794d1538b8ba 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -37,6 +37,22 @@ extern int sysctl_unprivileged_userfaultfd;

extern vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason);

+/*
+ * The mode of operation for __mcopy_atomic and its helpers.
+ *
+ * This is almost an implementation detail (mcopy_atomic below doesn't take this
+ * as a parameter), but it's exposed here because memory-kind-specific
+ * implementations (e.g. hugetlbfs) need to know the mode of operation.
+ */
+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 install pte(s) with the existing page(s) in the page cache. */
+ MCOPY_ATOMIC_CONTINUE,
+};
+
extern 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);
@@ -44,6 +60,8 @@ 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 f24dd4fcbad9..bafbeb1a2624 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 301b6b64c04e..2553b0d525aa 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -39,7 +39,6 @@
#include <linux/hugetlb.h>
#include <linux/hugetlb_cgroup.h>
#include <linux/node.h>
-#include <linux/userfaultfd_k.h>
#include <linux/page_owner.h>
#include "internal.h"

@@ -4634,8 +4633,10 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
struct vm_area_struct *dst_vma,
unsigned long dst_addr,
unsigned long src_addr,
+ enum mcopy_atomic_mode mode,
struct page **pagep)
{
+ bool is_continue = (mode == MCOPY_ATOMIC_CONTINUE);
struct address_space *mapping;
pgoff_t idx;
unsigned long size;
@@ -4645,8 +4646,18 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
spinlock_t *ptl;
int ret;
struct page *page;
+ int writable;

- if (!*pagep) {
+ mapping = dst_vma->vm_file->f_mapping;
+ idx = vma_hugecache_offset(h, dst_vma, dst_addr);
+
+ if (is_continue) {
+ ret = -EFAULT;
+ page = find_lock_page(mapping, idx);
+ *pagep = NULL;
+ if (!page)
+ goto out;
+ } else if (!*pagep) {
ret = -ENOMEM;
page = alloc_huge_page(dst_vma, dst_addr, 0);
if (IS_ERR(page))
@@ -4675,13 +4686,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
*/
__SetPageUptodate(page);

- 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 && !is_continue) {
size = i_size_read(mapping->host) >> huge_page_shift(h);
ret = -EFAULT;
if (idx >= size)
@@ -4726,8 +4732,14 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
hugepage_add_new_anon_rmap(page, dst_vma, dst_addr);
}

- _dst_pte = make_huge_pte(dst_vma, page, dst_vma->vm_flags & VM_WRITE);
- if (dst_vma->vm_flags & VM_WRITE)
+ /* For CONTINUE on a non-shared VMA, don't set VM_WRITE for CoW. */
+ if (is_continue && !vm_shared)
+ writable = 0;
+ else
+ writable = dst_vma->vm_flags & VM_WRITE;
+
+ _dst_pte = make_huge_pte(dst_vma, page, writable);
+ if (writable)
_dst_pte = huge_pte_mkdirty(_dst_pte);
_dst_pte = pte_mkyoung(_dst_pte);

@@ -4741,15 +4753,16 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
update_mmu_cache(dst_vma, dst_addr, dst_pte);

spin_unlock(ptl);
- SetHPageMigratable(page);
- if (vm_shared)
+ if (!is_continue)
+ SetHPageMigratable(page);
+ if (vm_shared || is_continue)
unlock_page(page);
ret = 0;
out:
return ret;
out_release_unlock:
spin_unlock(ptl);
- if (vm_shared)
+ if (vm_shared || is_continue)
unlock_page(page);
out_release_nounlock:
put_page(page);
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index b2ce61c1b50d..ce6cb4760d2c 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -207,7 +207,7 @@ 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 +227,7 @@ 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 +273,6 @@ 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);

/*
@@ -297,16 +295,16 @@ 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)) {
+ if (mode != MCOPY_ATOMIC_CONTINUE &&
+ !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,
- dst_addr, src_addr, &page);
+ dst_addr, src_addr, mode, &page);

mutex_unlock(&hugetlb_fault_mutex_table[hash]);
i_mmap_unlock_read(mapping);
@@ -408,7 +406,7 @@ 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,
@@ -458,7 +456,7 @@ 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)
{
@@ -469,6 +467,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
long copied;
struct page *page;
bool wp_copy;
+ bool zeropage = (mcopy_mode == MCOPY_ATOMIC_ZEROPAGE);

/*
* Sanitize the command parameters:
@@ -527,10 +526,12 @@ 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;
+ if (mcopy_mode == MCOPY_ATOMIC_CONTINUE)
+ goto out_unlock;

/*
* Ensure the dst_vma has a anon_vma or this page
@@ -626,14 +627,22 @@ 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,
--
2.30.0.617.g56c4b15f3c-goog