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* [PATCH v9 0/6] userfaultfd: add minor fault handling
@ 2021-03-01 22:27 Axel Rasmussen
  2021-03-01 22:27 ` [PATCH v9 1/6] userfaultfd: add minor fault registration mode Axel Rasmussen
                   ` (5 more replies)
  0 siblings, 6 replies; 7+ messages in thread
From: Axel Rasmussen @ 2021-03-01 22:27 UTC (permalink / raw)
  To: Alexander Viro, Alexey Dobriyan, Andrea Arcangeli, Andrew Morton,
	Anshuman Khandual, Catalin Marinas, Chinwen Chang, Huang Ying,
	Ingo Molnar, Jann Horn, Jerome Glisse, Lokesh Gidra,
	Matthew Wilcox (Oracle), Michael Ellerman, Michal Koutný,
	Michel Lespinasse, Mike Kravetz, Mike Rapoport, Nicholas Piggin,
	Peter Xu, Shaohua Li, Shawn Anastasio, Steven Rostedt,
	Steven Price, Vlastimil Babka
  Cc: linux-kernel, linux-fsdevel, linux-mm, Adam Ruprecht,
	Axel Rasmussen, Cannon Matthews, Dr . David Alan Gilbert,
	David Rientjes, Mina Almasry, Oliver Upton

Base
====

This series is based on v5.12-rc1. Additionally, this series depends on
Peter Xu's series to allow disabling huge pmd sharing.

[1] https://lore.kernel.org/patchwork/cover/1382204/

Changelog
=========

v8->v9:
- Removed an unneeded double !! from a VM_BUG_ON check in handle_userfault.
- Introduced a handle_userfault helper in hugetlb.c, to reduce repetition.
- Rebased to v5.12-rc1, which has Mike's hugetlb changes which originally
  motivated rebasing onto akpm's tree (so, it also applies cleanly to akpm's
  tree).

v7->v8:
- Check CONFIG_HAVE_ARCH_USERFAULTFD_MINOR instead of commenting in
  userfaultfd_register.
- Remove redundant "ret = -EINVAL;" in userfaultfd_register.
- Revert removing trailing \ in include/trace/events/mmflags.h.
- Don't set "*pagep = NULL" in the is_continue case in
  hugetlb_mcopy_atomic_pte.

v6->v7:
- Based upon discussion, switched back to the VM_* flags approach which was used
  in v5, instead of implementing this as an API feature. Switched to using a
  high bit (instead of brokenly conflicting with VM_LOCKED), which implies
  introducing CONFIG_HAVE_ARCH_USERFAULTFD_MINOR and selecting it only on 64-bit
  architectures (x86_64 and arm64 for now).

v5->v6:
- Fixed the condition guarding a second case where we unlock_page() in
  hugetlb_mcopy_atomic_pte().
- Significantly refactored how minor registration works. Because there are no
  VM_* flags available to use, it has to be a userfaultfd API feature, rather
  than a registration mode. This has a few knock on consequences worth calling
  out:
    - userfaultfd_minor() can no longer be inline, because we have to inspect
      the userfaultfd_ctx, which is only defined in fs/userfaultfd.c. This means
      slightly more overhead (1 function call) on all hugetlbfs minor faults.
    - vma_can_userfault() no longer changes. It seems valid to me to create an
      FD with the minor fault feature enabled, and then register e.g. some
      non-hugetlbfs region in MISSING mode, fully expecting to not get any minor
      faults for it, alongside some other region which you *do* want minor
      faults for. So, at registration time, either should be accepted.
    - Since I'm no longer adding a new registration mode, I'm no longer
      introducing __VM_UFFD_FLAGS or UFFD_API_REGISTER_MODES, and all the
      related cleanups have been reverted.

v4->v5:
- Typo fix in the documentation update.
- Removed comment in vma_can_userfault. The same information is better covered
  in the documentation update, so the comment is unnecessary (and slightly
  confusing as written).
- Reworded comment for MCOPY_ATOMIC_CONTINUE mode.
- For non-shared CONTINUE, only make the PTE(s) non-writable, don't change flags
  on the VMA.
- In hugetlb_mcopy_atomic_pte, always unlock the page in MCOPY_ATOMIC_CONTINUE,
  even if we don't have VM_SHARED.
- In hugetlb_mcopy_atomic_pte, introduce "bool is_continue" to make that kind of
  mode check more terse.
- Merged two nested if()s into a single expression in __mcopy_atomic_hugetlb.
- Moved "return -EINVAL if MCOPY_CONTINUE isn't supported for this vma type" up
  one level, into __mcopy_atomic.
- Rebased onto linux-next/akpm, instead of the latest 5.11 RC. Resolved
  conflicts with Mike's recent hugetlb changes.

v3->v4:
- Relaxed restriction for minor registration to allow any hugetlb VMAs, not
  just those with VM_SHARED. Fixed setting VM_WRITE flag in a CONTINUE ioctl
  for non-VM_SHARED VMAs.
- Reordered if() branches in hugetlb_mcopy_atomic_pte, so the conditions are
  simpler and easier to read.
- Reverted most of the mfill_atomic_pte change (the anon / shmem path). Just
  return -EINVAL for CONTINUE, and set zeropage = (mode ==
  MCOPY_ATOMIC_ZEROPAGE), so we can keep the delta small.
- Split out adding #ifdef CONFIG_USERFAULTFD to a separate patch (instead of
  lumping it together with adding UFFDIO_CONTINUE).
- Fixed signature of hugetlb_mcopy_atomic_pte for !CONFIG_HUGETLB_PAGE
  (signature must be the same in either case).
- Rebased onto a newer version of Peter's patches to disable huge PMD sharing.

v2->v3:
- Added #ifdef CONFIG_USERFAULTFD around hugetlb helper functions, to fix build
  errors when building without CONFIG_USERFAULTFD set.

v1->v2:
- Fixed a bug in the hugetlb_mcopy_atomic_pte retry case. We now plumb in the
  enum mcopy_atomic_mode, so we can differentiate between the three cases this
  function needs to handle:
  1) We're doing a COPY op, and need to allocate a page, add to cache, etc.
  2) We're doing a COPY op, but allocation in this function failed previously;
     we're in the retry path. The page was allocated, but not e.g. added to page
     cache, so that still needs to be done.
  3) We're doing a CONTINUE op, we need to look up an existing page instead of
     allocating a new one.
- Rebased onto a newer version of Peter's patches to disable huge PMD sharing,
  which fixes syzbot complaints on some non-x86 architectures.
- Moved __VM_UFFD_FLAGS into userfaultfd_k.h, so inline helpers can use it.
- Renamed UFFD_FEATURE_MINOR_FAULT_HUGETLBFS to UFFD_FEATURE_MINOR_HUGETLBFS,
  for consistency with other existing feature flags.
- Moved the userfaultfd_minor hook in hugetlb.c into the else block, so we don't
  have to explicitly check for !new_page.

RFC->v1:
- Rebased onto Peter Xu's patches for disabling huge PMD sharing for certain
  userfaultfd-registered areas.
- Added commits which update documentation, and add a self test which exercises
  the new feature.
- Fixed reporting CONTINUE as a supported ioctl even for non-MINOR ranges.

Overview
========

This series adds a new userfaultfd feature, UFFD_FEATURE_MINOR_HUGETLBFS. When
enabled (via the UFFDIO_API ioctl), this feature means that any hugetlbfs VMAs
registered with UFFDIO_REGISTER_MODE_MISSING will *also* get events for "minor"
faults. By "minor" fault, I mean the following situation:

Let there exist two mappings (i.e., VMAs) to the same page(s) (shared memory).
One of the mappings is registered with userfaultfd (in minor mode), and the
other is not. Via the non-UFFD mapping, the underlying pages have already been
allocated & filled with some contents. The UFFD mapping has not yet been
faulted in; when it is touched for the first time, this results in what I'm
calling a "minor" fault. As a concrete example, when working with hugetlbfs, we
have huge_pte_none(), but find_lock_page() finds an existing page.

We also add a new ioctl to resolve such faults: UFFDIO_CONTINUE. The idea is,
userspace resolves the fault by either a) doing nothing if the contents are
already correct, or b) updating the underlying contents using the second,
non-UFFD mapping (via memcpy/memset or similar, or something fancier like RDMA,
or etc...). In either case, userspace issues UFFDIO_CONTINUE to tell the kernel
"I have ensured the page contents are correct, carry on setting up the mapping".

Use Case
========

Consider the use case of VM live migration (e.g. under QEMU/KVM):

1. While a VM is still running, we copy the contents of its memory to a
   target machine. The pages are populated on the target by writing to the
   non-UFFD mapping, using the setup described above. The VM is still running
   (and therefore its memory is likely changing), so this may be repeated
   several times, until we decide the target is "up to date enough".

2. We pause the VM on the source, and start executing on the target machine.
   During this gap, the VM's user(s) will *see* a pause, so it is desirable to
   minimize this window.

3. Between the last time any page was copied from the source to the target, and
   when the VM was paused, the contents of that page may have changed - and
   therefore the copy we have on the target machine is out of date. Although we
   can keep track of which pages are out of date, for VMs with large amounts of
   memory, it is "slow" to transfer this information to the target machine. We
   want to resume execution before such a transfer would complete.

4. So, the guest begins executing on the target machine. The first time it
   touches its memory (via the UFFD-registered mapping), userspace wants to
   intercept this fault. Userspace checks whether or not the page is up to date,
   and if not, copies the updated page from the source machine, via the non-UFFD
   mapping. Finally, whether a copy was performed or not, userspace issues a
   UFFDIO_CONTINUE ioctl to tell the kernel "I have ensured the page contents
   are correct, carry on setting up the mapping".

We don't have to do all of the final updates on-demand. The userfaultfd manager
can, in the background, also copy over updated pages once it receives the map of
which pages are up-to-date or not.

Interaction with Existing APIs
==============================

Because this is a feature, a registered VMA could potentially receive both
missing and minor faults. I spent some time thinking through how the existing
API interacts with the new feature:

UFFDIO_CONTINUE cannot be used to resolve non-minor faults, as it does not
allocate a new page. If UFFDIO_CONTINUE is used on a non-minor fault:

- For non-shared memory or shmem, -EINVAL is returned.
- For hugetlb, -EFAULT is returned.

UFFDIO_COPY and UFFDIO_ZEROPAGE cannot be used to resolve minor faults. Without
modifications, the existing codepath assumes a new page needs to be allocated.
This is okay, since userspace must have a second non-UFFD-registered mapping
anyway, thus there isn't much reason to want to use these in any case (just
memcpy or memset or similar).

- If UFFDIO_COPY is used on a minor fault, -EEXIST is returned.
- If UFFDIO_ZEROPAGE is used on a minor fault, -EEXIST is returned (or -EINVAL
  in the case of hugetlb, as UFFDIO_ZEROPAGE is unsupported in any case).
- UFFDIO_WRITEPROTECT simply doesn't work with shared memory, and returns
  -ENOENT in that case (regardless of the kind of fault).

Future Work
===========

This series only supports hugetlbfs. I have a second series in flight to support
shmem as well, extending the functionality. This series is more mature than the
shmem support at this point, and the functionality works fully on hugetlbfs, so
this series can be merged first and then shmem support will follow.

Axel Rasmussen (6):
  userfaultfd: add minor fault registration mode
  userfaultfd: disable huge PMD sharing for MINOR registered VMAs
  userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled
  userfaultfd: add UFFDIO_CONTINUE ioctl
  userfaultfd: update documentation to describe minor fault handling
  userfaultfd/selftests: add test exercising minor fault handling

 Documentation/admin-guide/mm/userfaultfd.rst | 107 +++++++-----
 arch/arm64/Kconfig                           |   1 +
 arch/x86/Kconfig                             |   1 +
 fs/proc/task_mmu.c                           |   3 +
 fs/userfaultfd.c                             | 145 ++++++++++++----
 include/linux/hugetlb.h                      |   7 +
 include/linux/mm.h                           |   7 +
 include/linux/userfaultfd_k.h                |  46 +++++-
 include/trace/events/mmflags.h               |   7 +
 include/uapi/linux/userfaultfd.h             |  36 +++-
 init/Kconfig                                 |   5 +
 mm/hugetlb.c                                 | 121 +++++++++-----
 mm/userfaultfd.c                             |  37 +++--
 tools/testing/selftests/vm/userfaultfd.c     | 164 ++++++++++++++++++-
 14 files changed, 545 insertions(+), 142 deletions(-)

--
2.30.1.766.gb4fecdf3b7-goog


^ permalink raw reply	[flat|nested] 7+ messages in thread

* [PATCH v9 1/6] userfaultfd: add minor fault registration mode
  2021-03-01 22:27 [PATCH v9 0/6] userfaultfd: add minor fault handling Axel Rasmussen
@ 2021-03-01 22:27 ` Axel Rasmussen
  2021-03-01 22:27 ` [PATCH v9 2/6] userfaultfd: disable huge PMD sharing for MINOR registered VMAs Axel Rasmussen
                   ` (4 subsequent siblings)
  5 siblings, 0 replies; 7+ messages in thread
From: Axel Rasmussen @ 2021-03-01 22:27 UTC (permalink / raw)
  To: Alexander Viro, Alexey Dobriyan, Andrea Arcangeli, Andrew Morton,
	Anshuman Khandual, Catalin Marinas, Chinwen Chang, Huang Ying,
	Ingo Molnar, Jann Horn, Jerome Glisse, Lokesh Gidra,
	Matthew Wilcox (Oracle), Michael Ellerman, Michal Koutný,
	Michel Lespinasse, Mike Kravetz, Mike Rapoport, Nicholas Piggin,
	Peter Xu, Shaohua Li, Shawn Anastasio, Steven Rostedt,
	Steven Price, Vlastimil Babka
  Cc: linux-kernel, linux-fsdevel, linux-mm, Adam Ruprecht,
	Axel Rasmussen, Cannon Matthews, Dr . David Alan Gilbert,
	David Rientjes, Mina Almasry, Oliver Upton

This feature allows userspace to intercept "minor" faults. By "minor"
faults, I mean the following situation:

Let there exist two mappings (i.e., VMAs) to the same page(s). One of
the mappings is registered with userfaultfd (in minor mode), and the
other is not. Via the non-UFFD mapping, the underlying pages have
already been allocated & filled with some contents. The UFFD mapping
has not yet been faulted in; when it is touched for the first time,
this results in what I'm calling a "minor" fault. As a concrete
example, when working with hugetlbfs, we have huge_pte_none(), but
find_lock_page() finds an existing page.

This commit adds the new registration mode, and sets the relevant flag
on the VMAs being registered. In the hugetlb fault path, if we find
that we have huge_pte_none(), but find_lock_page() does indeed find an
existing page, then we have a "minor" fault, and if the VMA has the
userfaultfd registration flag, we call into userfaultfd to handle it.

This is implemented as a new registration mode, instead of an API
feature. This is because the alternative implementation has significant
drawbacks [1].

However, doing it this was requires we allocate a VM_* flag for the new
registration mode. On 32-bit systems, there are no unused bits, so this
feature is only supported on architectures with
CONFIG_ARCH_USES_HIGH_VMA_FLAGS. When attempting to register a VMA in
MINOR mode on 32-bit architectures, we return -EINVAL.

[1] https://lore.kernel.org/patchwork/patch/1380226/

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 arch/arm64/Kconfig               |  1 +
 arch/x86/Kconfig                 |  1 +
 fs/proc/task_mmu.c               |  3 ++
 fs/userfaultfd.c                 | 78 ++++++++++++++++++-------------
 include/linux/mm.h               |  7 +++
 include/linux/userfaultfd_k.h    | 15 +++++-
 include/trace/events/mmflags.h   |  7 +++
 include/uapi/linux/userfaultfd.h | 15 +++++-
 init/Kconfig                     |  5 ++
 mm/hugetlb.c                     | 79 +++++++++++++++++++++-----------
 10 files changed, 149 insertions(+), 62 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1f212b47a48a..ce6044273ef1 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -208,6 +208,7 @@ config ARM64
 	select SWIOTLB
 	select SYSCTL_EXCEPTION_TRACE
 	select THREAD_INFO_IN_TASK
+	select HAVE_ARCH_USERFAULTFD_MINOR if USERFAULTFD
 	help
 	  ARM 64-bit (AArch64) Linux support.
 
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 2792879d398e..7f71b71ed372 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -164,6 +164,7 @@ config X86
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
 	select HAVE_ARCH_USERFAULTFD_WP         if X86_64 && USERFAULTFD
+	select HAVE_ARCH_USERFAULTFD_MINOR	if X86_64 && USERFAULTFD
 	select HAVE_ARCH_VMAP_STACK		if X86_64
 	select HAVE_ARCH_WITHIN_STACK_FRAMES
 	select HAVE_ASM_MODVERSIONS
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 3cec6fbef725..e1c9095ebe70 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -661,6 +661,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 		[ilog2(VM_PKEY_BIT4)]	= "",
 #endif
 #endif /* CONFIG_ARCH_HAS_PKEYS */
+#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
+		[ilog2(VM_UFFD_MINOR)]	= "ui",
+#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
 	};
 	size_t i;
 
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index e5ce3b4e6c3d..ba35cafa8b0d 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -197,24 +197,21 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
 	msg_init(&msg);
 	msg.event = UFFD_EVENT_PAGEFAULT;
 	msg.arg.pagefault.address = address;
+	/*
+	 * These flags indicate why the userfault occurred:
+	 * - UFFD_PAGEFAULT_FLAG_WP indicates a write protect fault.
+	 * - UFFD_PAGEFAULT_FLAG_MINOR indicates a minor fault.
+	 * - Neither of these flags being set indicates a MISSING fault.
+	 *
+	 * Separately, UFFD_PAGEFAULT_FLAG_WRITE indicates it was a write
+	 * fault. Otherwise, it was a read fault.
+	 */
 	if (flags & FAULT_FLAG_WRITE)
-		/*
-		 * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
-		 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WRITE
-		 * was not set in a UFFD_EVENT_PAGEFAULT, it means it
-		 * was a read fault, otherwise if set it means it's
-		 * a write fault.
-		 */
 		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
 	if (reason & VM_UFFD_WP)
-		/*
-		 * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
-		 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WP was
-		 * not set in a UFFD_EVENT_PAGEFAULT, it means it was
-		 * a missing fault, otherwise if set it means it's a
-		 * write protect fault.
-		 */
 		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
+	if (reason & VM_UFFD_MINOR)
+		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_MINOR;
 	if (features & UFFD_FEATURE_THREAD_ID)
 		msg.arg.pagefault.feat.ptid = task_pid_vnr(current);
 	return msg;
@@ -401,8 +398,10 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 
 	BUG_ON(ctx->mm != mm);
 
-	VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
-	VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
+	/* Any unrecognized flag is a bug. */
+	VM_BUG_ON(reason & ~__VM_UFFD_FLAGS);
+	/* 0 or > 1 flags set is a bug; we expect exactly 1. */
+	VM_BUG_ON(!reason || (reason & (reason - 1)));
 
 	if (ctx->features & UFFD_FEATURE_SIGBUS)
 		goto out;
@@ -612,7 +611,7 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
 		for (vma = mm->mmap; vma; vma = vma->vm_next)
 			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
 				vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
-				vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
+				vma->vm_flags &= ~__VM_UFFD_FLAGS;
 			}
 		mmap_write_unlock(mm);
 
@@ -644,7 +643,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
 	octx = vma->vm_userfaultfd_ctx.ctx;
 	if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
-		vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
+		vma->vm_flags &= ~__VM_UFFD_FLAGS;
 		return 0;
 	}
 
@@ -726,7 +725,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
 	} else {
 		/* Drop uffd context if remap feature not enabled */
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
-		vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
+		vma->vm_flags &= ~__VM_UFFD_FLAGS;
 	}
 }
 
@@ -867,12 +866,12 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
 		cond_resched();
 		BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
-		       !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
+		       !!(vma->vm_flags & __VM_UFFD_FLAGS));
 		if (vma->vm_userfaultfd_ctx.ctx != ctx) {
 			prev = vma;
 			continue;
 		}
-		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
+		new_flags = vma->vm_flags & ~__VM_UFFD_FLAGS;
 		prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
 				 new_flags, vma->anon_vma,
 				 vma->vm_file, vma->vm_pgoff,
@@ -1262,9 +1261,19 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
 				     unsigned long vm_flags)
 {
 	/* FIXME: add WP support to hugetlbfs and shmem */
-	return vma_is_anonymous(vma) ||
-		((is_vm_hugetlb_page(vma) || vma_is_shmem(vma)) &&
-		 !(vm_flags & VM_UFFD_WP));
+	if (vm_flags & VM_UFFD_WP) {
+		if (is_vm_hugetlb_page(vma) || vma_is_shmem(vma))
+			return false;
+	}
+
+	if (vm_flags & VM_UFFD_MINOR) {
+		/* FIXME: Add minor fault interception for shmem. */
+		if (!is_vm_hugetlb_page(vma))
+			return false;
+	}
+
+	return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
+	       vma_is_shmem(vma);
 }
 
 static int userfaultfd_register(struct userfaultfd_ctx *ctx,
@@ -1290,14 +1299,19 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 	ret = -EINVAL;
 	if (!uffdio_register.mode)
 		goto out;
-	if (uffdio_register.mode & ~(UFFDIO_REGISTER_MODE_MISSING|
-				     UFFDIO_REGISTER_MODE_WP))
+	if (uffdio_register.mode & ~UFFD_API_REGISTER_MODES)
 		goto out;
 	vm_flags = 0;
 	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
 		vm_flags |= VM_UFFD_MISSING;
 	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP)
 		vm_flags |= VM_UFFD_WP;
+	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR) {
+#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
+		goto out;
+#endif
+		vm_flags |= VM_UFFD_MINOR;
+	}
 
 	ret = validate_range(mm, &uffdio_register.range.start,
 			     uffdio_register.range.len);
@@ -1341,7 +1355,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 		cond_resched();
 
 		BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
-		       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
+		       !!(cur->vm_flags & __VM_UFFD_FLAGS));
 
 		/* check not compatible vmas */
 		ret = -EINVAL;
@@ -1421,8 +1435,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 			start = vma->vm_start;
 		vma_end = min(end, vma->vm_end);
 
-		new_flags = (vma->vm_flags &
-			     ~(VM_UFFD_MISSING|VM_UFFD_WP)) | vm_flags;
+		new_flags = (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags;
 		prev = vma_merge(mm, prev, start, vma_end, new_flags,
 				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
 				 vma_policy(vma),
@@ -1544,7 +1557,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 		cond_resched();
 
 		BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
-		       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
+		       !!(cur->vm_flags & __VM_UFFD_FLAGS));
 
 		/*
 		 * Check not compatible vmas, not strictly required
@@ -1595,7 +1608,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 			wake_userfault(vma->vm_userfaultfd_ctx.ctx, &range);
 		}
 
-		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
+		new_flags = vma->vm_flags & ~__VM_UFFD_FLAGS;
 		prev = vma_merge(mm, prev, start, vma_end, new_flags,
 				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
 				 vma_policy(vma),
@@ -1863,6 +1876,9 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
 		goto err_out;
 	/* report all available features and ioctls to userland */
 	uffdio_api.features = UFFD_API_FEATURES;
+#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
+	uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS;
+#endif
 	uffdio_api.ioctls = UFFD_API_IOCTLS;
 	ret = -EFAULT;
 	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 77e64e3eac80..5ed1316d6ed6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -362,6 +362,13 @@ extern unsigned int kobjsize(const void *objp);
 # define VM_GROWSUP	VM_NONE
 #endif
 
+#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
+# define VM_UFFD_MINOR_BIT	37
+# define VM_UFFD_MINOR		BIT(VM_UFFD_MINOR_BIT)	/* UFFD minor faults */
+#else /* !CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
+# define VM_UFFD_MINOR		VM_NONE
+#endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
+
 /* Bits set in the VMA until the stack is in its final location */
 #define VM_STACK_INCOMPLETE_SETUP	(VM_RAND_READ | VM_SEQ_READ)
 
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index c63ccdae3eab..0390e5ac63b3 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -17,6 +17,9 @@
 #include <linux/mm.h>
 #include <asm-generic/pgtable_uffd.h>
 
+/* The set of all possible UFFD-related VM flags. */
+#define __VM_UFFD_FLAGS (VM_UFFD_MISSING | VM_UFFD_WP | VM_UFFD_MINOR)
+
 /*
  * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining
  * new flags, since they might collide with O_* ones. We want
@@ -71,6 +74,11 @@ static inline bool userfaultfd_wp(struct vm_area_struct *vma)
 	return vma->vm_flags & VM_UFFD_WP;
 }
 
+static inline bool userfaultfd_minor(struct vm_area_struct *vma)
+{
+	return vma->vm_flags & VM_UFFD_MINOR;
+}
+
 static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
 				      pte_t pte)
 {
@@ -85,7 +93,7 @@ static inline bool userfaultfd_huge_pmd_wp(struct vm_area_struct *vma,
 
 static inline bool userfaultfd_armed(struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP);
+	return vma->vm_flags & __VM_UFFD_FLAGS;
 }
 
 extern int dup_userfaultfd(struct vm_area_struct *, struct list_head *);
@@ -132,6 +140,11 @@ static inline bool userfaultfd_wp(struct vm_area_struct *vma)
 	return false;
 }
 
+static inline bool userfaultfd_minor(struct vm_area_struct *vma)
+{
+	return false;
+}
+
 static inline bool userfaultfd_pte_wp(struct vm_area_struct *vma,
 				      pte_t pte)
 {
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index 67018d367b9f..629c7a0eaff2 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -137,6 +137,12 @@ IF_HAVE_PG_ARCH_2(PG_arch_2,		"arch_2"	)
 #define IF_HAVE_VM_SOFTDIRTY(flag,name)
 #endif
 
+#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
+# define IF_HAVE_UFFD_MINOR(flag, name) {flag, name},
+#else
+# define IF_HAVE_UFFD_MINOR(flag, name)
+#endif
+
 #define __def_vmaflag_names						\
 	{VM_READ,			"read"		},		\
 	{VM_WRITE,			"write"		},		\
@@ -148,6 +154,7 @@ IF_HAVE_PG_ARCH_2(PG_arch_2,		"arch_2"	)
 	{VM_MAYSHARE,			"mayshare"	},		\
 	{VM_GROWSDOWN,			"growsdown"	},		\
 	{VM_UFFD_MISSING,		"uffd_missing"	},		\
+IF_HAVE_UFFD_MINOR(VM_UFFD_MINOR,	"uffd_minor"	)		\
 	{VM_PFNMAP,			"pfnmap"	},		\
 	{VM_DENYWRITE,			"denywrite"	},		\
 	{VM_UFFD_WP,			"uffd_wp"	},		\
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 5f2d88212f7c..f24dd4fcbad9 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -19,15 +19,19 @@
  * means the userland is reading).
  */
 #define UFFD_API ((__u64)0xAA)
+#define UFFD_API_REGISTER_MODES (UFFDIO_REGISTER_MODE_MISSING |	\
+				 UFFDIO_REGISTER_MODE_WP |	\
+				 UFFDIO_REGISTER_MODE_MINOR)
 #define UFFD_API_FEATURES (UFFD_FEATURE_PAGEFAULT_FLAG_WP |	\
 			   UFFD_FEATURE_EVENT_FORK |		\
 			   UFFD_FEATURE_EVENT_REMAP |		\
-			   UFFD_FEATURE_EVENT_REMOVE |	\
+			   UFFD_FEATURE_EVENT_REMOVE |		\
 			   UFFD_FEATURE_EVENT_UNMAP |		\
 			   UFFD_FEATURE_MISSING_HUGETLBFS |	\
 			   UFFD_FEATURE_MISSING_SHMEM |		\
 			   UFFD_FEATURE_SIGBUS |		\
-			   UFFD_FEATURE_THREAD_ID)
+			   UFFD_FEATURE_THREAD_ID |		\
+			   UFFD_FEATURE_MINOR_HUGETLBFS)
 #define UFFD_API_IOCTLS				\
 	((__u64)1 << _UFFDIO_REGISTER |		\
 	 (__u64)1 << _UFFDIO_UNREGISTER |	\
@@ -127,6 +131,7 @@ struct uffd_msg {
 /* flags for UFFD_EVENT_PAGEFAULT */
 #define UFFD_PAGEFAULT_FLAG_WRITE	(1<<0)	/* If this was a write fault */
 #define UFFD_PAGEFAULT_FLAG_WP		(1<<1)	/* If reason is VM_UFFD_WP */
+#define UFFD_PAGEFAULT_FLAG_MINOR	(1<<2)	/* If reason is VM_UFFD_MINOR */
 
 struct uffdio_api {
 	/* userland asks for an API number and the features to enable */
@@ -171,6 +176,10 @@ struct uffdio_api {
 	 *
 	 * UFFD_FEATURE_THREAD_ID pid of the page faulted task_struct will
 	 * be returned, if feature is not requested 0 will be returned.
+	 *
+	 * UFFD_FEATURE_MINOR_HUGETLBFS indicates that minor faults
+	 * can be intercepted (via REGISTER_MODE_MINOR) for
+	 * hugetlbfs-backed pages.
 	 */
 #define UFFD_FEATURE_PAGEFAULT_FLAG_WP		(1<<0)
 #define UFFD_FEATURE_EVENT_FORK			(1<<1)
@@ -181,6 +190,7 @@ struct uffdio_api {
 #define UFFD_FEATURE_EVENT_UNMAP		(1<<6)
 #define UFFD_FEATURE_SIGBUS			(1<<7)
 #define UFFD_FEATURE_THREAD_ID			(1<<8)
+#define UFFD_FEATURE_MINOR_HUGETLBFS		(1<<9)
 	__u64 features;
 
 	__u64 ioctls;
@@ -195,6 +205,7 @@ struct uffdio_register {
 	struct uffdio_range range;
 #define UFFDIO_REGISTER_MODE_MISSING	((__u64)1<<0)
 #define UFFDIO_REGISTER_MODE_WP		((__u64)1<<1)
+#define UFFDIO_REGISTER_MODE_MINOR	((__u64)1<<2)
 	__u64 mode;
 
 	/*
diff --git a/init/Kconfig b/init/Kconfig
index 22946fe5ded9..d13d942309ce 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1628,6 +1628,11 @@ config HAVE_ARCH_USERFAULTFD_WP
 	help
 	  Arch has userfaultfd write protection support
 
+config HAVE_ARCH_USERFAULTFD_MINOR
+	bool
+	help
+	  Arch has userfaultfd minor fault support
+
 config MEMBARRIER
 	bool "Enable membarrier() system call" if EXPERT
 	default y
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index cad303306abc..61fd15185f0a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4230,6 +4230,44 @@ int huge_add_to_page_cache(struct page *page, struct address_space *mapping,
 	return 0;
 }
 
+static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
+						  struct address_space *mapping,
+						  pgoff_t idx,
+						  unsigned int flags,
+						  unsigned long haddr,
+						  unsigned long reason)
+{
+	vm_fault_t ret;
+	u32 hash;
+	struct vm_fault vmf = {
+		.vma = vma,
+		.address = haddr,
+		.flags = flags,
+
+		/*
+		 * Hard to debug if it ends up being
+		 * used by a callee that assumes
+		 * something about the other
+		 * uninitialized fields... same as in
+		 * memory.c
+		 */
+	};
+
+	/*
+	 * hugetlb_fault_mutex and i_mmap_rwsem must be
+	 * dropped before handling userfault.  Reacquire
+	 * after handling fault to make calling code simpler.
+	 */
+	hash = hugetlb_fault_mutex_hash(mapping, idx);
+	mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+	i_mmap_unlock_read(mapping);
+	ret = handle_userfault(&vmf, reason);
+	i_mmap_lock_read(mapping);
+	mutex_lock(&hugetlb_fault_mutex_table[hash]);
+
+	return ret;
+}
+
 static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 			struct vm_area_struct *vma,
 			struct address_space *mapping, pgoff_t idx,
@@ -4268,35 +4306,11 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 retry:
 	page = find_lock_page(mapping, idx);
 	if (!page) {
-		/*
-		 * Check for page in userfault range
-		 */
+		/* Check for page in userfault range */
 		if (userfaultfd_missing(vma)) {
-			u32 hash;
-			struct vm_fault vmf = {
-				.vma = vma,
-				.address = haddr,
-				.flags = flags,
-				/*
-				 * Hard to debug if it ends up being
-				 * used by a callee that assumes
-				 * something about the other
-				 * uninitialized fields... same as in
-				 * memory.c
-				 */
-			};
-
-			/*
-			 * hugetlb_fault_mutex and i_mmap_rwsem must be
-			 * dropped before handling userfault.  Reacquire
-			 * after handling fault to make calling code simpler.
-			 */
-			hash = hugetlb_fault_mutex_hash(mapping, idx);
-			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
-			i_mmap_unlock_read(mapping);
-			ret = handle_userfault(&vmf, VM_UFFD_MISSING);
-			i_mmap_lock_read(mapping);
-			mutex_lock(&hugetlb_fault_mutex_table[hash]);
+			ret = hugetlb_handle_userfault(vma, mapping, idx,
+						       flags, haddr,
+						       VM_UFFD_MISSING);
 			goto out;
 		}
 
@@ -4355,6 +4369,15 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 				VM_FAULT_SET_HINDEX(hstate_index(h));
 			goto backout_unlocked;
 		}
+
+		/* Check for page in userfault range. */
+		if (userfaultfd_minor(vma)) {
+			unlock_page(page);
+			ret = hugetlb_handle_userfault(vma, mapping, idx,
+						       flags, haddr,
+						       VM_UFFD_MINOR);
+			goto out;
+		}
 	}
 
 	/*
-- 
2.30.1.766.gb4fecdf3b7-goog


^ permalink raw reply	[flat|nested] 7+ messages in thread

* [PATCH v9 2/6] userfaultfd: disable huge PMD sharing for MINOR registered VMAs
  2021-03-01 22:27 [PATCH v9 0/6] userfaultfd: add minor fault handling Axel Rasmussen
  2021-03-01 22:27 ` [PATCH v9 1/6] userfaultfd: add minor fault registration mode Axel Rasmussen
@ 2021-03-01 22:27 ` Axel Rasmussen
  2021-03-01 22:27 ` [PATCH v9 3/6] userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled Axel Rasmussen
                   ` (3 subsequent siblings)
  5 siblings, 0 replies; 7+ messages in thread
From: Axel Rasmussen @ 2021-03-01 22:27 UTC (permalink / raw)
  To: Alexander Viro, Alexey Dobriyan, Andrea Arcangeli, Andrew Morton,
	Anshuman Khandual, Catalin Marinas, Chinwen Chang, Huang Ying,
	Ingo Molnar, Jann Horn, Jerome Glisse, Lokesh Gidra,
	Matthew Wilcox (Oracle), Michael Ellerman, Michal Koutný,
	Michel Lespinasse, Mike Kravetz, Mike Rapoport, Nicholas Piggin,
	Peter Xu, Shaohua Li, Shawn Anastasio, Steven Rostedt,
	Steven Price, Vlastimil Babka
  Cc: linux-kernel, linux-fsdevel, linux-mm, Adam Ruprecht,
	Axel Rasmussen, Cannon Matthews, Dr . David Alan Gilbert,
	David Rientjes, Mina Almasry, Oliver Upton

As the comment says: for the MINOR fault use case, although the page
might be present and populated in the other (non-UFFD-registered) half
of the mapping, it may be out of date, and we explicitly want userspace
to get a minor fault so it can check and potentially update the page's
contents.

Huge PMD sharing would prevent these faults from occurring for
suitably aligned areas, so disable it upon UFFD registration.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 include/linux/userfaultfd_k.h | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 0390e5ac63b3..e060d5f77cc5 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -56,12 +56,19 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
 }
 
 /*
- * Never enable huge pmd sharing on uffd-wp registered vmas, because uffd-wp
- * protect information is per pgtable entry.
+ * Never enable huge pmd sharing on some uffd registered vmas:
+ *
+ * - VM_UFFD_WP VMAs, because write protect information is per pgtable entry.
+ *
+ * - VM_UFFD_MINOR VMAs, because otherwise we would never get minor faults for
+ *   VMAs which share huge pmds. (If you have two mappings to the same
+ *   underlying pages, and fault in the non-UFFD-registered one with a write,
+ *   with huge pmd sharing this would *also* setup the second UFFD-registered
+ *   mapping, and we'd not get minor faults.)
  */
 static inline bool uffd_disable_huge_pmd_share(struct vm_area_struct *vma)
 {
-	return vma->vm_flags & VM_UFFD_WP;
+	return vma->vm_flags & (VM_UFFD_WP | VM_UFFD_MINOR);
 }
 
 static inline bool userfaultfd_missing(struct vm_area_struct *vma)
-- 
2.30.1.766.gb4fecdf3b7-goog


^ permalink raw reply	[flat|nested] 7+ messages in thread

* [PATCH v9 3/6] userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled
  2021-03-01 22:27 [PATCH v9 0/6] userfaultfd: add minor fault handling Axel Rasmussen
  2021-03-01 22:27 ` [PATCH v9 1/6] userfaultfd: add minor fault registration mode Axel Rasmussen
  2021-03-01 22:27 ` [PATCH v9 2/6] userfaultfd: disable huge PMD sharing for MINOR registered VMAs Axel Rasmussen
@ 2021-03-01 22:27 ` Axel Rasmussen
  2021-03-01 22:27 ` [PATCH v9 4/6] userfaultfd: add UFFDIO_CONTINUE ioctl Axel Rasmussen
                   ` (2 subsequent siblings)
  5 siblings, 0 replies; 7+ messages in thread
From: Axel Rasmussen @ 2021-03-01 22:27 UTC (permalink / raw)
  To: Alexander Viro, Alexey Dobriyan, Andrea Arcangeli, Andrew Morton,
	Anshuman Khandual, Catalin Marinas, Chinwen Chang, Huang Ying,
	Ingo Molnar, Jann Horn, Jerome Glisse, Lokesh Gidra,
	Matthew Wilcox (Oracle), Michael Ellerman, Michal Koutný,
	Michel Lespinasse, Mike Kravetz, Mike Rapoport, Nicholas Piggin,
	Peter Xu, Shaohua Li, Shawn Anastasio, Steven Rostedt,
	Steven Price, Vlastimil Babka
  Cc: linux-kernel, linux-fsdevel, linux-mm, Adam Ruprecht,
	Axel Rasmussen, Cannon Matthews, Dr . David Alan Gilbert,
	David Rientjes, Mina Almasry, Oliver Upton

For background, mm/userfaultfd.c provides a general mcopy_atomic
implementation. But some types of memory (i.e., hugetlb and shmem) need
a slightly different implementation, so they provide their own helpers
for this. In other words, userfaultfd is the only caller of these
functions.

This patch achieves two things:

1. Don't spend time compiling code which will end up never being
referenced anyway (a small build time optimization).

2. In patches later in this series, we extend the signature of these
helpers with UFFD-specific state (a mode enumeration). Once this
happens, we *have to* either not compile the helpers, or unconditionally
define the UFFD-only state (which seems messier to me). This includes
the declarations in the headers, as otherwise they'd yield warnings
about implicitly defining the type of those arguments.

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 include/linux/hugetlb.h | 4 ++++
 mm/hugetlb.c            | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index c0b10f0c7f23..7b86bf809d7a 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -134,11 +134,13 @@ void hugetlb_show_meminfo(void);
 unsigned long hugetlb_total_pages(void);
 vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 			unsigned long address, unsigned int flags);
+#ifdef CONFIG_USERFAULTFD
 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,
 				struct page **pagep);
+#endif /* CONFIG_USERFAULTFD */
 bool hugetlb_reserve_pages(struct inode *inode, long from, long to,
 						struct vm_area_struct *vma,
 						vm_flags_t vm_flags);
@@ -310,6 +312,7 @@ static inline void hugetlb_free_pgd_range(struct mmu_gather *tlb,
 	BUG();
 }
 
+#ifdef CONFIG_USERFAULTFD
 static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 						pte_t *dst_pte,
 						struct vm_area_struct *dst_vma,
@@ -320,6 +323,7 @@ static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	BUG();
 	return 0;
 }
+#endif /* CONFIG_USERFAULTFD */
 
 static inline pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr,
 					unsigned long sz)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 61fd15185f0a..4422dab8db9a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4618,6 +4618,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	return ret;
 }
 
+#ifdef CONFIG_USERFAULTFD
 /*
  * Used by userfaultfd UFFDIO_COPY.  Based on mcopy_atomic_pte with
  * modifications for huge pages.
@@ -4748,6 +4749,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	put_page(page);
 	goto out;
 }
+#endif /* CONFIG_USERFAULTFD */
 
 static void record_subpages_vmas(struct page *page, struct vm_area_struct *vma,
 				 int refs, struct page **pages,
-- 
2.30.1.766.gb4fecdf3b7-goog


^ permalink raw reply	[flat|nested] 7+ messages in thread

* [PATCH v9 4/6] userfaultfd: add UFFDIO_CONTINUE ioctl
  2021-03-01 22:27 [PATCH v9 0/6] userfaultfd: add minor fault handling Axel Rasmussen
                   ` (2 preceding siblings ...)
  2021-03-01 22:27 ` [PATCH v9 3/6] userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled Axel Rasmussen
@ 2021-03-01 22:27 ` Axel Rasmussen
  2021-03-01 22:27 ` [PATCH v9 5/6] userfaultfd: update documentation to describe minor fault handling Axel Rasmussen
  2021-03-01 22:27 ` [PATCH v9 6/6] userfaultfd/selftests: add test exercising " Axel Rasmussen
  5 siblings, 0 replies; 7+ messages in thread
From: Axel Rasmussen @ 2021-03-01 22:27 UTC (permalink / raw)
  To: Alexander Viro, Alexey Dobriyan, Andrea Arcangeli, Andrew Morton,
	Anshuman Khandual, Catalin Marinas, Chinwen Chang, Huang Ying,
	Ingo Molnar, Jann Horn, Jerome Glisse, Lokesh Gidra,
	Matthew Wilcox (Oracle), Michael Ellerman, Michal Koutný,
	Michel Lespinasse, Mike Kravetz, Mike Rapoport, Nicholas Piggin,
	Peter Xu, Shaohua Li, Shawn Anastasio, Steven Rostedt,
	Steven Price, Vlastimil Babka
  Cc: linux-kernel, linux-fsdevel, linux-mm, Adam Ruprecht,
	Axel Rasmussen, Cannon Matthews, Dr . David Alan Gilbert,
	David Rientjes, Mina Almasry, Oliver Upton

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.)

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 fs/userfaultfd.c                 | 67 ++++++++++++++++++++++++++++++++
 include/linux/hugetlb.h          |  3 ++
 include/linux/userfaultfd_k.h    | 18 +++++++++
 include/uapi/linux/userfaultfd.h | 21 +++++++++-
 mm/hugetlb.c                     | 40 ++++++++++++-------
 mm/userfaultfd.c                 | 37 +++++++++++-------
 6 files changed, 156 insertions(+), 30 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index ba35cafa8b0d..14f92285d04f 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1487,6 +1487,10 @@ 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
@@ -1840,6 +1844,66 @@ 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)
 {
 	/*
@@ -1927,6 +1991,9 @@ 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 7b86bf809d7a..1d3246b31a41 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 4422dab8db9a..eecde8fb2c36 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"
 
@@ -4628,8 +4627,10 @@ 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;
@@ -4639,8 +4640,17 @@ 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);
+		if (!page)
+			goto out;
+	} else if (!*pagep) {
 		ret = -ENOMEM;
 		page = alloc_huge_page(dst_vma, dst_addr, 0);
 		if (IS_ERR(page))
@@ -4669,13 +4679,8 @@ 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)
@@ -4720,8 +4725,14 @@ 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);
 
@@ -4735,15 +4746,16 @@ 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,
-- 
2.30.1.766.gb4fecdf3b7-goog


^ permalink raw reply	[flat|nested] 7+ messages in thread

* [PATCH v9 5/6] userfaultfd: update documentation to describe minor fault handling
  2021-03-01 22:27 [PATCH v9 0/6] userfaultfd: add minor fault handling Axel Rasmussen
                   ` (3 preceding siblings ...)
  2021-03-01 22:27 ` [PATCH v9 4/6] userfaultfd: add UFFDIO_CONTINUE ioctl Axel Rasmussen
@ 2021-03-01 22:27 ` Axel Rasmussen
  2021-03-01 22:27 ` [PATCH v9 6/6] userfaultfd/selftests: add test exercising " Axel Rasmussen
  5 siblings, 0 replies; 7+ messages in thread
From: Axel Rasmussen @ 2021-03-01 22:27 UTC (permalink / raw)
  To: Alexander Viro, Alexey Dobriyan, Andrea Arcangeli, Andrew Morton,
	Anshuman Khandual, Catalin Marinas, Chinwen Chang, Huang Ying,
	Ingo Molnar, Jann Horn, Jerome Glisse, Lokesh Gidra,
	Matthew Wilcox (Oracle), Michael Ellerman, Michal Koutný,
	Michel Lespinasse, Mike Kravetz, Mike Rapoport, Nicholas Piggin,
	Peter Xu, Shaohua Li, Shawn Anastasio, Steven Rostedt,
	Steven Price, Vlastimil Babka
  Cc: linux-kernel, linux-fsdevel, linux-mm, Adam Ruprecht,
	Axel Rasmussen, Cannon Matthews, Dr . David Alan Gilbert,
	David Rientjes, Mina Almasry, Oliver Upton

Reword / reorganize things a little bit into "lists", so new features /
modes / ioctls can sort of just be appended.

Describe how UFFDIO_REGISTER_MODE_MINOR and UFFDIO_CONTINUE can be used
to intercept and resolve minor faults. Make it clear that COPY and
ZEROPAGE are used for MISSING faults, whereas CONTINUE is used for MINOR
faults.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 Documentation/admin-guide/mm/userfaultfd.rst | 107 ++++++++++++-------
 1 file changed, 66 insertions(+), 41 deletions(-)

diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index 65eefa66c0ba..3aa38e8b8361 100644
--- a/Documentation/admin-guide/mm/userfaultfd.rst
+++ b/Documentation/admin-guide/mm/userfaultfd.rst
@@ -63,36 +63,36 @@ the generic ioctl available.
 
 The ``uffdio_api.features`` bitmask returned by the ``UFFDIO_API`` ioctl
 defines what memory types are supported by the ``userfaultfd`` and what
-events, except page fault notifications, may be generated.
-
-If the kernel supports registering ``userfaultfd`` ranges on hugetlbfs
-virtual memory areas, ``UFFD_FEATURE_MISSING_HUGETLBFS`` will be set in
-``uffdio_api.features``. Similarly, ``UFFD_FEATURE_MISSING_SHMEM`` will be
-set if the kernel supports registering ``userfaultfd`` ranges on shared
-memory (covering all shmem APIs, i.e. tmpfs, ``IPCSHM``, ``/dev/zero``,
-``MAP_SHARED``, ``memfd_create``, etc).
-
-The userland application that wants to use ``userfaultfd`` with hugetlbfs
-or shared memory need to set the corresponding flag in
-``uffdio_api.features`` to enable those features.
-
-If the userland desires to receive notifications for events other than
-page faults, it has to verify that ``uffdio_api.features`` has appropriate
-``UFFD_FEATURE_EVENT_*`` bits set. These events are described in more
-detail below in `Non-cooperative userfaultfd`_ section.
-
-Once the ``userfaultfd`` has been enabled the ``UFFDIO_REGISTER`` ioctl should
-be invoked (if present in the returned ``uffdio_api.ioctls`` bitmask) to
-register a memory range in the ``userfaultfd`` by setting the
+events, except page fault notifications, may be generated:
+
+- The ``UFFD_FEATURE_EVENT_*`` flags indicate that various other events
+  other than page faults are supported. These events are described in more
+  detail below in the `Non-cooperative userfaultfd`_ section.
+
+- ``UFFD_FEATURE_MISSING_HUGETLBFS`` and ``UFFD_FEATURE_MISSING_SHMEM``
+  indicate that the kernel supports ``UFFDIO_REGISTER_MODE_MISSING``
+  registrations for hugetlbfs and shared memory (covering all shmem APIs,
+  i.e. tmpfs, ``IPCSHM``, ``/dev/zero``, ``MAP_SHARED``, ``memfd_create``,
+  etc) virtual memory areas, respectively.
+
+- ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports
+  ``UFFDIO_REGISTER_MODE_MINOR`` registration for hugetlbfs virtual memory
+  areas.
+
+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.
+
+Once the ``userfaultfd`` API has been enabled the ``UFFDIO_REGISTER``
+ioctl should be invoked (if present in the returned ``uffdio_api.ioctls``
+bitmask) to register a memory range in the ``userfaultfd`` by setting the
 uffdio_register structure accordingly. The ``uffdio_register.mode``
 bitmask will specify to the kernel which kind of faults to track for
-the range (``UFFDIO_REGISTER_MODE_MISSING`` would track missing
-pages). The ``UFFDIO_REGISTER`` ioctl will return the
+the range. The ``UFFDIO_REGISTER`` ioctl will return the
 ``uffdio_register.ioctls`` bitmask of ioctls that are suitable to resolve
 userfaults on the range registered. Not all ioctls will necessarily be
-supported for all memory types depending on the underlying virtual
-memory backend (anonymous memory vs tmpfs vs real filebacked
-mappings).
+supported for all memory types (e.g. anonymous memory vs. shmem vs.
+hugetlbfs), or all types of intercepted faults.
 
 Userland can use the ``uffdio_register.ioctls`` to manage the virtual
 address space in the background (to add or potentially also remove
@@ -100,21 +100,46 @@ memory from the ``userfaultfd`` registered range). This means a userfault
 could be triggering just before userland maps in the background the
 user-faulted page.
 
-The primary ioctl to resolve userfaults is ``UFFDIO_COPY``. That
-atomically copies a page into the userfault registered range and wakes
-up the blocked userfaults
-(unless ``uffdio_copy.mode & UFFDIO_COPY_MODE_DONTWAKE`` is set).
-Other ioctl works similarly to ``UFFDIO_COPY``. They're atomic as in
-guaranteeing that nothing can see an half copied page since it'll
-keep userfaulting until the copy has finished.
+Resolving Userfaults
+--------------------
+
+There are three basic ways to resolve userfaults:
+
+- ``UFFDIO_COPY`` atomically copies some existing page contents from
+  userspace.
+
+- ``UFFDIO_ZEROPAGE`` atomically zeros the new page.
+
+- ``UFFDIO_CONTINUE`` maps an existing, previously-populated page.
+
+These operations are atomic in the sense that they guarantee nothing can
+see a half-populated page, since readers will keep userfaulting until the
+operation has finished.
+
+By default, these wake up userfaults blocked on the range in question.
+They support a ``UFFDIO_*_MODE_DONTWAKE`` ``mode`` flag, which indicates
+that waking will be done separately at some later time.
+
+Which ioctl to choose depends on the kind of page fault, and what we'd
+like to do to resolve it:
+
+- For ``UFFDIO_REGISTER_MODE_MISSING`` faults, the fault needs to be
+  resolved by either providing a new page (``UFFDIO_COPY``), or mapping
+  the zero page (``UFFDIO_ZEROPAGE``). By default, the kernel would map
+  the zero page for a missing fault. With userfaultfd, userspace can
+  decide what content to provide before the faulting thread continues.
+
+- For ``UFFDIO_REGISTER_MODE_MINOR`` faults, there is an existing page (in
+  the page cache). Userspace has the option of modifying the page's
+  contents before resolving the fault. Once the contents are correct
+  (modified or not), userspace asks the kernel to map the page and let the
+  faulting thread continue with ``UFFDIO_CONTINUE``.
 
 Notes:
 
-- If you requested ``UFFDIO_REGISTER_MODE_MISSING`` when registering then
-  you must provide some kind of page in your thread after reading from
-  the uffd.  You must provide either ``UFFDIO_COPY`` or ``UFFDIO_ZEROPAGE``.
-  The normal behavior of the OS automatically providing a zero page on
-  an anonymous mmaping is not in place.
+- You can tell which kind of fault occurred by examining
+  ``pagefault.flags`` within the ``uffd_msg``, checking for the
+  ``UFFD_PAGEFAULT_FLAG_*`` flags.
 
 - None of the page-delivering ioctls default to the range that you
   registered with.  You must fill in all fields for the appropriate
@@ -122,9 +147,9 @@ Notes:
 
 - You get the address of the access that triggered the missing page
   event out of a struct uffd_msg that you read in the thread from the
-  uffd.  You can supply as many pages as you want with ``UFFDIO_COPY`` or
-  ``UFFDIO_ZEROPAGE``.  Keep in mind that unless you used DONTWAKE then
-  the first of any of those IOCTLs wakes up the faulting thread.
+  uffd.  You can supply as many pages as you want with these IOCTLs.
+  Keep in mind that unless you used DONTWAKE then the first of any of
+  those IOCTLs wakes up the faulting thread.
 
 - Be sure to test for all errors including
   (``pollfd[0].revents & POLLERR``).  This can happen, e.g. when ranges
-- 
2.30.1.766.gb4fecdf3b7-goog


^ permalink raw reply	[flat|nested] 7+ messages in thread

* [PATCH v9 6/6] userfaultfd/selftests: add test exercising minor fault handling
  2021-03-01 22:27 [PATCH v9 0/6] userfaultfd: add minor fault handling Axel Rasmussen
                   ` (4 preceding siblings ...)
  2021-03-01 22:27 ` [PATCH v9 5/6] userfaultfd: update documentation to describe minor fault handling Axel Rasmussen
@ 2021-03-01 22:27 ` Axel Rasmussen
  5 siblings, 0 replies; 7+ messages in thread
From: Axel Rasmussen @ 2021-03-01 22:27 UTC (permalink / raw)
  To: Alexander Viro, Alexey Dobriyan, Andrea Arcangeli, Andrew Morton,
	Anshuman Khandual, Catalin Marinas, Chinwen Chang, Huang Ying,
	Ingo Molnar, Jann Horn, Jerome Glisse, Lokesh Gidra,
	Matthew Wilcox (Oracle), Michael Ellerman, Michal Koutný,
	Michel Lespinasse, Mike Kravetz, Mike Rapoport, Nicholas Piggin,
	Peter Xu, Shaohua Li, Shawn Anastasio, Steven Rostedt,
	Steven Price, Vlastimil Babka
  Cc: linux-kernel, linux-fsdevel, linux-mm, Adam Ruprecht,
	Axel Rasmussen, Cannon Matthews, Dr . David Alan Gilbert,
	David Rientjes, Mina Almasry, Oliver Upton

Fix a dormant bug in userfaultfd_events_test(), where we did
`return faulting_process(0)` instead of `exit(faulting_process(0))`.
This caused the forked process to keep running, trying to execute any
further test cases after the events test in parallel with the "real"
process.

Add a simple test case which exercises minor faults. In short, it does
the following:

1. "Sets up" an area (area_dst) and a second shared mapping to the same
   underlying pages (area_dst_alias).

2. Register one of these areas with userfaultfd, in minor fault mode.

3. Start a second thread to handle any minor faults.

4. Populate the underlying pages with the non-UFFD-registered side of
   the mapping. Basically, memset() each page with some arbitrary
   contents.

5. Then, using the UFFD-registered mapping, read all of the page
   contents, asserting that the contents match expectations (we expect
   the minor fault handling thread can modify the page contents before
   resolving the fault).

The minor fault handling thread, upon receiving an event, flips all the
bits (~) in that page, just to prove that it can modify it in some
arbitrary way. Then it issues a UFFDIO_CONTINUE ioctl, to setup the
mapping and resolve the fault. The reading thread should wake up and see
this modification.

Currently the minor fault test is only enabled in hugetlb_shared mode,
as this is the only configuration the kernel feature supports.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 tools/testing/selftests/vm/userfaultfd.c | 164 ++++++++++++++++++++++-
 1 file changed, 158 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 92b8ec423201..f5ab5e0312e7 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -81,6 +81,8 @@ static volatile bool test_uffdio_copy_eexist = true;
 static volatile bool test_uffdio_zeropage_eexist = true;
 /* Whether to test uffd write-protection */
 static bool test_uffdio_wp = false;
+/* Whether to test uffd minor faults */
+static bool test_uffdio_minor = false;
 
 static bool map_shared;
 static int huge_fd;
@@ -96,6 +98,7 @@ struct uffd_stats {
 	int cpu;
 	unsigned long missing_faults;
 	unsigned long wp_faults;
+	unsigned long minor_faults;
 };
 
 /* pthread_mutex_t starts at page offset 0 */
@@ -153,17 +156,19 @@ static void uffd_stats_reset(struct uffd_stats *uffd_stats,
 		uffd_stats[i].cpu = i;
 		uffd_stats[i].missing_faults = 0;
 		uffd_stats[i].wp_faults = 0;
+		uffd_stats[i].minor_faults = 0;
 	}
 }
 
 static void uffd_stats_report(struct uffd_stats *stats, int n_cpus)
 {
 	int i;
-	unsigned long long miss_total = 0, wp_total = 0;
+	unsigned long long miss_total = 0, wp_total = 0, minor_total = 0;
 
 	for (i = 0; i < n_cpus; i++) {
 		miss_total += stats[i].missing_faults;
 		wp_total += stats[i].wp_faults;
+		minor_total += stats[i].minor_faults;
 	}
 
 	printf("userfaults: %llu missing (", miss_total);
@@ -172,6 +177,9 @@ static void uffd_stats_report(struct uffd_stats *stats, int n_cpus)
 	printf("\b), %llu wp (", wp_total);
 	for (i = 0; i < n_cpus; i++)
 		printf("%lu+", stats[i].wp_faults);
+	printf("\b), %llu minor (", minor_total);
+	for (i = 0; i < n_cpus; i++)
+		printf("%lu+", stats[i].minor_faults);
 	printf("\b)\n");
 }
 
@@ -328,7 +336,7 @@ static struct uffd_test_ops shmem_uffd_test_ops = {
 };
 
 static struct uffd_test_ops hugetlb_uffd_test_ops = {
-	.expected_ioctls = UFFD_API_RANGE_IOCTLS_BASIC,
+	.expected_ioctls = UFFD_API_RANGE_IOCTLS_BASIC & ~(1 << _UFFDIO_CONTINUE),
 	.allocate_area	= hugetlb_allocate_area,
 	.release_pages	= hugetlb_release_pages,
 	.alias_mapping = hugetlb_alias_mapping,
@@ -362,6 +370,22 @@ static void wp_range(int ufd, __u64 start, __u64 len, bool wp)
 	}
 }
 
+static void continue_range(int ufd, __u64 start, __u64 len)
+{
+	struct uffdio_continue req;
+
+	req.range.start = start;
+	req.range.len = len;
+	req.mode = 0;
+
+	if (ioctl(ufd, UFFDIO_CONTINUE, &req)) {
+		fprintf(stderr,
+			"UFFDIO_CONTINUE failed for address 0x%" PRIx64 "\n",
+			(uint64_t)start);
+		exit(1);
+	}
+}
+
 static void *locking_thread(void *arg)
 {
 	unsigned long cpu = (unsigned long) arg;
@@ -569,8 +593,32 @@ static void uffd_handle_page_fault(struct uffd_msg *msg,
 	}
 
 	if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP) {
+		/* Write protect page faults */
 		wp_range(uffd, msg->arg.pagefault.address, page_size, false);
 		stats->wp_faults++;
+	} else if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_MINOR) {
+		uint8_t *area;
+		int b;
+
+		/*
+		 * Minor page faults
+		 *
+		 * To prove we can modify the original range for testing
+		 * purposes, we're going to bit flip this range before
+		 * continuing.
+		 *
+		 * Note that this requires all minor page fault tests operate on
+		 * area_dst (non-UFFD-registered) and area_dst_alias
+		 * (UFFD-registered).
+		 */
+
+		area = (uint8_t *)(area_dst +
+				   ((char *)msg->arg.pagefault.address -
+				    area_dst_alias));
+		for (b = 0; b < page_size; ++b)
+			area[b] = ~area[b];
+		continue_range(uffd, msg->arg.pagefault.address, page_size);
+		stats->minor_faults++;
 	} else {
 		/* Missing page faults */
 		if (bounces & BOUNCE_VERIFY &&
@@ -779,7 +827,7 @@ static int stress(struct uffd_stats *uffd_stats)
 	return 0;
 }
 
-static int userfaultfd_open(int features)
+static int userfaultfd_open_ext(uint64_t *features)
 {
 	struct uffdio_api uffdio_api;
 
@@ -792,7 +840,7 @@ static int userfaultfd_open(int features)
 	uffd_flags = fcntl(uffd, F_GETFD, NULL);
 
 	uffdio_api.api = UFFD_API;
-	uffdio_api.features = features;
+	uffdio_api.features = *features;
 	if (ioctl(uffd, UFFDIO_API, &uffdio_api)) {
 		fprintf(stderr, "UFFDIO_API failed.\nPlease make sure to "
 			"run with either root or ptrace capability.\n");
@@ -804,9 +852,15 @@ static int userfaultfd_open(int features)
 		return 1;
 	}
 
+	*features = uffdio_api.features;
 	return 0;
 }
 
+static int userfaultfd_open(uint64_t features)
+{
+	return userfaultfd_open_ext(&features);
+}
+
 sigjmp_buf jbuf, *sigbuf;
 
 static void sighndl(int sig, siginfo_t *siginfo, void *ptr)
@@ -1112,7 +1166,7 @@ static int userfaultfd_events_test(void)
 	}
 
 	if (!pid)
-		return faulting_process(0);
+		exit(faulting_process(0));
 
 	waitpid(pid, &err, 0);
 	if (err) {
@@ -1215,6 +1269,102 @@ static int userfaultfd_sig_test(void)
 	return userfaults != 0;
 }
 
+static int userfaultfd_minor_test(void)
+{
+	struct uffdio_register uffdio_register;
+	unsigned long expected_ioctls;
+	unsigned long p;
+	pthread_t uffd_mon;
+	uint8_t expected_byte;
+	void *expected_page;
+	char c;
+	struct uffd_stats stats = { 0 };
+	uint64_t features = UFFD_FEATURE_MINOR_HUGETLBFS;
+
+	if (!test_uffdio_minor)
+		return 0;
+
+	printf("testing minor faults: ");
+	fflush(stdout);
+
+	if (uffd_test_ops->release_pages(area_dst))
+		return 1;
+
+	if (userfaultfd_open_ext(&features))
+		return 1;
+	/* If kernel reports the feature isn't supported, skip the test. */
+	if (!(features & UFFD_FEATURE_MINOR_HUGETLBFS)) {
+		printf("skipping test due to lack of feature support\n");
+		fflush(stdout);
+		return 0;
+	}
+
+	uffdio_register.range.start = (unsigned long)area_dst_alias;
+	uffdio_register.range.len = nr_pages * page_size;
+	uffdio_register.mode = UFFDIO_REGISTER_MODE_MINOR;
+	if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register)) {
+		fprintf(stderr, "register failure\n");
+		exit(1);
+	}
+
+	expected_ioctls = uffd_test_ops->expected_ioctls;
+	expected_ioctls |= 1 << _UFFDIO_CONTINUE;
+	if ((uffdio_register.ioctls & expected_ioctls) != expected_ioctls) {
+		fprintf(stderr, "unexpected missing ioctl(s)\n");
+		exit(1);
+	}
+
+	/*
+	 * After registering with UFFD, populate the non-UFFD-registered side of
+	 * the shared mapping. This should *not* trigger any UFFD minor faults.
+	 */
+	for (p = 0; p < nr_pages; ++p) {
+		memset(area_dst + (p * page_size), p % ((uint8_t)-1),
+		       page_size);
+	}
+
+	if (pthread_create(&uffd_mon, &attr, uffd_poll_thread, &stats)) {
+		perror("uffd_poll_thread create");
+		exit(1);
+	}
+
+	/*
+	 * Read each of the pages back using the UFFD-registered mapping. We
+	 * expect that the first time we touch a page, it will result in a minor
+	 * fault. uffd_poll_thread will resolve the fault by bit-flipping the
+	 * page's contents, and then issuing a CONTINUE ioctl.
+	 */
+
+	if (posix_memalign(&expected_page, page_size, page_size)) {
+		fprintf(stderr, "out of memory\n");
+		return 1;
+	}
+
+	for (p = 0; p < nr_pages; ++p) {
+		expected_byte = ~((uint8_t)(p % ((uint8_t)-1)));
+		memset(expected_page, expected_byte, page_size);
+		if (my_bcmp(expected_page, area_dst_alias + (p * page_size),
+			    page_size)) {
+			fprintf(stderr,
+				"unexpected page contents after minor fault\n");
+			exit(1);
+		}
+	}
+
+	if (write(pipefd[1], &c, sizeof(c)) != sizeof(c)) {
+		perror("pipe write");
+		exit(1);
+	}
+	if (pthread_join(uffd_mon, NULL))
+		return 1;
+
+	close(uffd);
+
+	uffd_stats_report(&stats, 1);
+
+	return stats.missing_faults != 0 || stats.minor_faults != nr_pages;
+}
+
 static int userfaultfd_stress(void)
 {
 	void *area;
@@ -1413,7 +1563,7 @@ static int userfaultfd_stress(void)
 
 	close(uffd);
 	return userfaultfd_zeropage_test() || userfaultfd_sig_test()
-		|| userfaultfd_events_test();
+		|| userfaultfd_events_test() || userfaultfd_minor_test();
 }
 
 /*
@@ -1454,6 +1604,8 @@ static void set_test_type(const char *type)
 		map_shared = true;
 		test_type = TEST_HUGETLB;
 		uffd_test_ops = &hugetlb_uffd_test_ops;
+		/* Minor faults require shared hugetlb; only enable here. */
+		test_uffdio_minor = true;
 	} else if (!strcmp(type, "shmem")) {
 		map_shared = true;
 		test_type = TEST_SHMEM;
-- 
2.30.1.766.gb4fecdf3b7-goog


^ permalink raw reply	[flat|nested] 7+ messages in thread

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Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-03-01 22:27 [PATCH v9 0/6] userfaultfd: add minor fault handling Axel Rasmussen
2021-03-01 22:27 ` [PATCH v9 1/6] userfaultfd: add minor fault registration mode Axel Rasmussen
2021-03-01 22:27 ` [PATCH v9 2/6] userfaultfd: disable huge PMD sharing for MINOR registered VMAs Axel Rasmussen
2021-03-01 22:27 ` [PATCH v9 3/6] userfaultfd: hugetlbfs: only compile UFFD helpers if config enabled Axel Rasmussen
2021-03-01 22:27 ` [PATCH v9 4/6] userfaultfd: add UFFDIO_CONTINUE ioctl Axel Rasmussen
2021-03-01 22:27 ` [PATCH v9 5/6] userfaultfd: update documentation to describe minor fault handling Axel Rasmussen
2021-03-01 22:27 ` [PATCH v9 6/6] userfaultfd/selftests: add test exercising " Axel Rasmussen

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