[PATCH v2 00/46] hugetlb: introduce HugeTLB high-granularity mapping

From: James Houghton
Date: Fri Feb 17 2023 - 19:28:52 EST

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This series introduces the concept of HugeTLB high-granularity mapping
(HGM). This series teaches HugeTLB how to map HugeTLB pages at
high-granularity, similar to how THPs can be PTE-mapped.

Support for HGM in this series is for MAP_SHARED VMAs on x86_64 only. Other
architectures and (some) support for MAP_PRIVATE will come later.

This series is based on latest mm-unstable (ccd6a73daba9).

Notable changes with this series
================================

- hugetlb_add_file_rmap / hugetlb_remove_rmap are added to handle
mapcounting for non-anon hugetlb.
- The mapcounting scheme uses subpages' mapcounts for high-granularity
mappings, but it does not use subpages_mapcount(). This scheme
prevents the HugeTLB VMEMMAP optimization from being used, so it
will be improved in a later series.
- page_add_file_rmap and page_remove_rmap are updated so they can be
used by hugetlb_add_file_rmap / hugetlb_remove_rmap.
- MADV_SPLIT has been added to enable the userspace API changes that
HGM allows for: high-granularity UFFDIO_CONTINUE (and maybe other
changes in the future). MADV_SPLIT does NOT force all the mappings to
be PAGE_SIZE.
- MADV_COLLAPSE is expanded to include HugeTLB mappings.

Old versions:
v1: https://lore.kernel.org/linux-mm/20230105101844.1893104-1-jthoughton@xxxxxxxxxx/
RFC v2: https://lore.kernel.org/linux-mm/20221021163703.3218176-1-jthoughton@xxxxxxxxxx/
RFC v1: https://lore.kernel.org/linux-mm/20220624173656.2033256-1-jthoughton@xxxxxxxxxx/

Changelog:
v1 -> v2 (thanks Peter for all your suggestions!):
- Changed mapcount to be more THP-like, and make HGM incompatible with
HVO.
- HGM is now disabled by default to leave HVO enabled by default.
- Added refcount overflow check.
- Removed cond_resched() in hugetlb_collapse().
- Take mmap_lock for writing in hugetlb_collapse().
- Fixed high-granularity UFFDIO_CONTINUE on a UFFDIO_WRITEPROTECTed page (+tests)
- Fixed vaddr math in follow_hugetlb_page.
- Fixed Kconfig to limit HGM to x86_64.
- Fixed some compile errors.
RFC v2 -> v1:
- Userspace API to enable HGM changed from
UFFD_FEATURE_MINOR_HUGETLBFS_HGM to MADV_SPLIT.
- Picked up Acked-bys and Reviewed-bys. Thanks Mike, Peter, and Mina!
- Rebased onto latest mm-unstable, notably picking up Peter's
HugeTLB walk synchronization fix [1].
- Changed MADV_COLLAPSE to take i_mmap_rwsem for writing to make its
synchronization the same as huge_pmd_unshare, so anywhere where
hugetlb_pte_walk() is safe, HGM walks are also safe.
- hugetlb_hgm_walk API has changed -- should reduce complexity where
callers wish to do HGM walks.
- Always round addresses properly before populating hugetlb_ptes (always
pick up first PTE in a contiguous bunch).
- Added a VMA flag for HGM: VM_HUGETLB_HGM; the hugetlb_shared_vma_data
struct has been removed.
- Make hugetlb_pte.ptl always hold the PTL to use.
- Added a requirement that overlapping contiguous and non-contiguous
PTEs must use the same PTL.
- Some things have been slightly renamed for clarity, and I've added
lots of comments that I said I would.
- Added a test for fork() + uffd-wp to cover
copy_hugetlb_page_range().

Patch breakdown:
Patches 1-4: Cleanup.
Patch 5: rmap preliminary changes.
Patches 6-9: Add HGM config option, VM flag, MADV_SPLIT.
Patches 10-15: Create hugetlb_pte and implement HGM basics.
Patches 16-30: Make existing routines compatible with HGM.
Patches 31-34: Extend userfaultfd to support high-granularity
CONTINUEs.
Patch 35: Add HugeTLB HGM support to MADV_COLLAPSE.
Patch 36: Add refcount overflow check.
Patches 37-40: Cleanup, add HGM stats, and enable HGM for x86_64.
Patches 41-47: Documentation and selftests.

Motivation
==========

Being able to map HugeTLB pages at PAGE_SIZE has important use cases in
post-copy live migration and memory poisoning.

- Live Migration (userfaultfd)
For post-copy live migration, using userfaultfd, currently we have to
install an entire hugepage before we can allow a guest to access that
page. This is because, right now, either the WHOLE hugepage is mapped or
NONE of it is. So either the guest can access the WHOLE hugepage or NONE
of it. This makes post-copy live migration for 1G HugeTLB-backed VMs
completely infeasible.

With high-granularity mapping, we can map PAGE_SIZE pieces of a
hugepage, thereby allowing the guest to access only PAGE_SIZE chunks,
and getting page faults on the rest (and triggering another
demand-fetch). This gives userspace the flexibility to install PAGE_SIZE
chunks of memory into a hugepage, making migration of 1G-backed VMs
perfectly feasible, and it vastly reduces the vCPU stall time during
post-copy for 2M-backed VMs.

At Google, for a 48 vCPU VM in post-copy, we can expect these approximate
per-page median fetch latencies:
4K: <100us
2M: >10ms
Being able to unpause a vCPU 100x quicker is helpful for guest stability,
and being able to use 1G pages at all can significant improve
steady-state guest performance.

After fully copying a hugepage over the network, we will want to
collapse the mapping down to what it would normally be (e.g., one PUD
for a 1G page). Rather than having the kernel do this automatically,
we leave it up to userspace to tell us to collapse a range (via
MADV_COLLAPSE).

- Memory Failure
When a memory error is found within a HugeTLB page, it would be ideal
if we could unmap only the PAGE_SIZE section that contained the error.
This is what THPs are able to do. Using high-granularity mapping, we
could do this, but this isn't tackled in this patch series.

Userspace API
=============

This series introduces the first application of high-granularity
mapping: high-granularity userfaultfd post-copy for HugeTLB.

The userspace API for this consists of:
- MADV_SPLIT: to enable the following userfaultfd API changes.
1. read(uffd): addresses are rounded to PAGE_SIZE instead of the
hugepage size.
2. UFFDIO_CONTINUE for HugeTLB VMAs is now allowed in
PAGE_SIZE-aligned chunks.
- MADV_COLLAPSE is now available for MAP_SHARED HugeTLB VMAs. It is used
to collapse the page table mappings, but it does not undo the API
changes that MADV_SPLIT provides.

HugeTLB changes
===============

- hugetlb_pte
`hugetlb_pte` is used to keep track of "HugeTLB" PTEs, which are PTEs at
any level and of any size. page_vma_mapped_walk and pagewalk have both
been changed to provide `hugetlb_pte`s to callers so that they can get
size+level information that, before, came from the hstate.

- Mapcount
Previously, file-backed HugeTLB pages had their mapcount incremented by
page_dup_file_rmap. This is replaced with page_add_file_rmap, which is
wrapped by hugetlb_add_file_rmap to implement new mapcount behavior.

HugeTLB pages mapped at hugepage-granularity still have their
compound_mapcount incremented by 1, but when a page is mapped at
high granularity, we increase the subpages' mapcounts for all the
subpages that get mapped. For example, for a 1G page, if a 2M piece of
it is mapped with a PMD, the mapcount for all the 4K pages within the 2M
piece have their mapcount's incremented.

This behavior means that HGM is incompatible with the HugeTLB Vmemmap
Optimization (HVO). HGM is disabled by default, and if it gets enabled,
HVO will be disabled. Also, collapsing to the hugepage size requires us
to decrement the subpage mapcounts for all of the subpages we had
mapped. For a 1G page, this can get really slow. This thread[3] has some
discussion.

- Synchronization
Collapsing high-granularity HugeTLB mappings requires taking the
mmap_lock for writing.

Supporting arm64 & contiguous PTEs
==================================

As implemented, HGM does not yet fully support contiguous PTEs. To do
this, the HugeTLB API that architectures implement will need to change.
For example, set_huge_pte_at merely takes a `pte_t *`; there is no
information about the "size" of that PTE (like, if we need to overwrite
multiple contiguous PTEs).

To handle this, in a follow-up series, set_huge_pte_at and many other
similar functions will be replaced with variants that take
`hugetlb_pte`s. See [2] for how this may be implemented, plus a full HGM
implementation for arm64.

Supporting architectures beyond arm64
=====================================

Each architecture must audit their HugeTLB implementations to make sure
that they support HGM. For example, architectures that implement
arch_make_huge_pte need to ensure that a `shift` of `PAGE_SHIFT` is
acceptable.

Architectures must also audit code that might depend on HugeTLB always
having large mappings (i.e., check huge_page_size(), huge_page_shift(),
vma_kernel_pagesize(), and vma_mmu_pagesize() callers). For example, the
arm64 KVM MMU implementation thinks that all hugepages are mapped at
huge_page_size(), and thus builds the second-stage page table
accordingly. In an HGM world, this isn't true; it is corrected in [2].

[1]: https://lore.kernel.org/linux-mm/20221216155100.2043537-1-peterx@xxxxxxxxxx/
[2]: https://github.com/48ca/linux/tree/hgmv1-dec19-2
[3]: https://lore.kernel.org/linux-mm/CADrL8HUSx6=K0QXQtTmv9ZJQmvhe6KEb+FiAviRfO3HjmRUeTw@xxxxxxxxxxxxxx/

James Houghton (46):
hugetlb: don't set PageUptodate for UFFDIO_CONTINUE
hugetlb: remove mk_huge_pte; it is unused
hugetlb: remove redundant pte_mkhuge in migration path
hugetlb: only adjust address ranges when VMAs want PMD sharing
rmap: hugetlb: switch from page_dup_file_rmap to page_add_file_rmap
hugetlb: add CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
mm: add VM_HUGETLB_HGM VMA flag
hugetlb: add HugeTLB HGM enablement helpers
mm: add MADV_SPLIT to enable HugeTLB HGM
hugetlb: make huge_pte_lockptr take an explicit shift argument
hugetlb: add hugetlb_pte to track HugeTLB page table entries
hugetlb: add hugetlb_alloc_pmd and hugetlb_alloc_pte
hugetlb: add hugetlb_hgm_walk and hugetlb_walk_step
hugetlb: split PTE markers when doing HGM walks
hugetlb: add make_huge_pte_with_shift
hugetlb: make default arch_make_huge_pte understand small mappings
hugetlbfs: do a full walk to check if vma maps a page
hugetlb: add HGM support to __unmap_hugepage_range
hugetlb: add HGM support to hugetlb_change_protection
hugetlb: add HGM support to follow_hugetlb_page
hugetlb: add HGM support to hugetlb_follow_page_mask
hugetlb: add HGM support to copy_hugetlb_page_range
hugetlb: add HGM support to move_hugetlb_page_tables
hugetlb: add HGM support to hugetlb_fault and hugetlb_no_page
hugetlb: use struct hugetlb_pte for walk_hugetlb_range
mm: rmap: provide pte_order in page_vma_mapped_walk
mm: rmap: update try_to_{migrate,unmap} to handle mapcount for HGM
mm: rmap: in try_to_{migrate,unmap}, check head page for hugetlb page
flags
hugetlb: update page_vma_mapped to do high-granularity walks
hugetlb: add high-granularity migration support
hugetlb: sort hstates in hugetlb_init_hstates
hugetlb: add for_each_hgm_shift
hugetlb: userfaultfd: add support for high-granularity UFFDIO_CONTINUE
hugetlb: add MADV_COLLAPSE for hugetlb
hugetlb: add check to prevent refcount overflow via HGM
hugetlb: remove huge_pte_lock and huge_pte_lockptr
hugetlb: replace make_huge_pte with make_huge_pte_with_shift
mm: smaps: add stats for HugeTLB mapping size
hugetlb: x86: enable high-granularity mapping for x86_64
docs: hugetlb: update hugetlb and userfaultfd admin-guides with HGM
info
docs: proc: include information about HugeTLB HGM
selftests/mm: add HugeTLB HGM to userfaultfd selftest
KVM: selftests: add HugeTLB HGM to KVM demand paging selftest
selftests/mm: add anon and shared hugetlb to migration test
selftests/mm: add hugetlb HGM test to migration selftest
selftests/mm: add HGM UFFDIO_CONTINUE and hwpoison tests

Documentation/admin-guide/mm/hugetlbpage.rst | 4 +
Documentation/admin-guide/mm/userfaultfd.rst | 8 +-
Documentation/filesystems/proc.rst | 56 +-
arch/alpha/include/uapi/asm/mman.h | 2 +
arch/mips/include/uapi/asm/mman.h | 2 +
arch/parisc/include/uapi/asm/mman.h | 2 +
arch/powerpc/mm/pgtable.c | 6 +-
arch/s390/include/asm/hugetlb.h | 5 -
arch/s390/mm/gmap.c | 12 +-
arch/x86/Kconfig | 1 +
arch/xtensa/include/uapi/asm/mman.h | 2 +
fs/Kconfig | 13 +
fs/hugetlbfs/inode.c | 17 +-
fs/proc/task_mmu.c | 190 ++-
fs/userfaultfd.c | 14 +-
include/asm-generic/hugetlb.h | 5 -
include/asm-generic/tlb.h | 6 +-
include/linux/huge_mm.h | 12 +-
include/linux/hugetlb.h | 170 +-
include/linux/mm.h | 7 +
include/linux/pagewalk.h | 10 +-
include/linux/rmap.h | 1 +
include/linux/swapops.h | 8 +-
include/trace/events/mmflags.h | 7 +
include/uapi/asm-generic/mman-common.h | 2 +
mm/damon/vaddr.c | 41 +-
mm/debug_vm_pgtable.c | 2 +-
mm/hmm.c | 20 +-
mm/hugetlb.c | 1390 ++++++++++++++---
mm/khugepaged.c | 4 +-
mm/madvise.c | 56 +-
mm/memory-failure.c | 17 +-
mm/mempolicy.c | 28 +-
mm/migrate.c | 21 +-
mm/mincore.c | 17 +-
mm/mprotect.c | 18 +-
mm/page_vma_mapped.c | 60 +-
mm/pagewalk.c | 20 +-
mm/rmap.c | 85 +-
mm/userfaultfd.c | 40 +-
.../selftests/kvm/demand_paging_test.c | 2 +-
.../testing/selftests/kvm/include/test_util.h | 2 +
.../selftests/kvm/include/userfaultfd_util.h | 6 +-
tools/testing/selftests/kvm/lib/kvm_util.c | 2 +-
tools/testing/selftests/kvm/lib/test_util.c | 14 +
.../selftests/kvm/lib/userfaultfd_util.c | 14 +-
tools/testing/selftests/mm/Makefile | 1 +
tools/testing/selftests/mm/hugetlb-hgm.c | 608 +++++++
tools/testing/selftests/mm/migration.c | 229 ++-
tools/testing/selftests/mm/userfaultfd.c | 84 +-
50 files changed, 2841 insertions(+), 502 deletions(-)
create mode 100644 tools/testing/selftests/mm/hugetlb-hgm.c

--
2.39.2.637.g21b0678d19-goog

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