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Subject[PATCH v6 0/7] userfaultfd: add minor fault handling
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Base
====

This series is based on linux-next/akpm. Additionally, this series depends on
Peter Xu's commits to disable huge pmd sharing in certain situations [1] -
those are not included directly here as they're being shared by several ongoing
projects.

[1] https://github.com/xzpeter/linux/tree/uffd-wp-shmem-hugetlbfs :
hugetlb: Pass vma into huge_pte_alloc() and huge_pmd_share()
hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled
mm/hugetlb: Move flush_hugetlb_tlb_range() into hugetlb.h
hugetlb/userfaultfd: Unshare all pmds for hugetlbfs when register wp

Changelog
=========

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
===========

Currently the patchset only supports hugetlbfs. There is no reason it can't work
with shmem, but I expect hugetlbfs to be much more commonly used since we're
talking about backing guest memory for VMs. I plan to implement shmem support in
a follow-up patch series.

Axel Rasmussen (7):
userfaultfd: introduce a new reason enum instead of using VM_* flags
userfaultfd: add minor fault registration mode
userfaultfd: disable huge PMD sharing for minor fault 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 | 109 ++++++++------
fs/userfaultfd.c | 136 +++++++++++++----
include/linux/hugetlb.h | 7 +
include/linux/mm.h | 2 +-
include/linux/userfaultfd_k.h | 65 ++++++--
include/uapi/linux/userfaultfd.h | 36 ++++-
mm/huge_memory.c | 4 +-
mm/hugetlb.c | 77 ++++++++--
mm/memory.c | 8 +-
mm/shmem.c | 2 +-
mm/userfaultfd.c | 37 +++--
tools/testing/selftests/vm/userfaultfd.c | 147 ++++++++++++++++++-
12 files changed, 501 insertions(+), 129 deletions(-)

--
2.30.0.478.g8a0d178c01-goog

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