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Subject[RFC PATCH 00/26] hugetlb: Introduce HugeTLB high-granularity mapping
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This RFC introduces the concept of HugeTLB high-granularity mapping
(HGM)[1]. In broad terms, this series teaches HugeTLB how to map HugeTLB
pages at different granularities, and, more importantly, to partially map
a HugeTLB page. This cover letter will go over
- the motivation for these changes
- userspace API
- some of the changes to HugeTLB to make this work
- limitations & future enhancements

High-granularity mapping does *not* involve dissolving the hugepages
themselves; it only affects how they are mapped.

---- Motivation ----

Being able to map HugeTLB memory with PAGE_SIZE PTEs has important use
cases in post-copy live migration and memory failure handling.

- 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, co-opting
the API that is being introduced for THPs[2]).

- 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 patch series introduces a single way to take advantage of
high-granularity mapping: via UFFDIO_CONTINUE. UFFDIO_CONTINUE allows
userspace to resolve MINOR page faults on shared VMAs.

To collapse a HugeTLB address range that has been mapped with several
UFFDIO_CONTINUE operations, userspace can issue MADV_COLLAPSE. We expect
userspace to know when all pages (that they care about) have been fetched.

---- HugeTLB Changes ----

- Mapcount
The way mapcount is handled is different from the way that it was handled
before. If the PUD for a hugepage is not none, a hugepage's mapcount will
be increased. This scheme means that, for hugepages that aren't mapped at
high granularity, their mapcounts will remain the same as what they would
have been pre-HGM.

- Page table walking and manipulation
A new function, hugetlb_walk_to, handles walking HugeTLB page tables for
high-granularity mappings. Eventually, it's possible to merge
hugetlb_walk_to with huge_pte_offset and huge_pte_alloc.

We keep track of HugeTLB page table entries with a new struct, hugetlb_pte.
This is because we generally need to know the "size" of a PTE (previously
always just huge_page_size(hstate)).

For every page table manipulation function that has a huge version (e.g.
huge_ptep_get and ptep_get), there is a wrapper for it (e.g.
hugetlb_ptep_get). The correct version is used depending on if a HugeTLB
PTE really is "huge".

- Synchronization
For existing bits of HugeTLB, synchronization is unchanged. For splitting
and collapsing HugeTLB PTEs, we require that the i_mmap_rw_sem is held for
writing, and for doing high-granularity page table walks, we require it to
be held for reading.

---- Limitations & Future Changes ----

This patch series only implements high-granularity mapping for VM_SHARED
VMAs. I intend to implement enough HGM to support 4K unmapping for memory
failure recovery for both shared and private mappings.

The memory failure use case poses its own challenges that can be
addressed, but I will do so in a separate RFC.

Performance has not been heavily scrutinized with this patch series. There
are places where lock contention can significantly reduce performance. This
will be addressed later.

The patch series, as it stands right now, is compatible with the VMEMMAP
page struct optimization[3], as we do not need to modify data contained
in the subpage page structs.

Other omissions:
- Compatibility with userfaultfd write-protect (will be included in v1).
- Support for mremap() (will be included in v1). This looks a lot like
the support we have for fork().
- Documentation changes (will be included in v1).
- Completely ignores PMD sharing and hugepage migration (will be included
in v1).
- Implementations for architectures that don't use GENERAL_HUGETLB other
than arm64.

---- Patch Breakdown ----

Patch 1 - Preliminary changes
Patch 2-10 - HugeTLB HGM core changes
Patch 11-13 - HugeTLB HGM page table walking functionality
Patch 14-19 - HugeTLB HGM compatibility with other bits
Patch 20-23 - Userfaultfd and collapse changes
Patch 24-26 - arm64 support and selftests

[1] This used to be called HugeTLB double mapping, a bad and confusing
name. "High-granularity mapping" is not a great name either. I am open
to better names.
[2] https://lore.kernel.org/linux-mm/20220604004004.954674-10-zokeefe@google.com/
[3] commit f41f2ed43ca5 ("mm: hugetlb: free the vmemmap pages associated with each HugeTLB page")

James Houghton (26):
hugetlb: make hstate accessor functions const
hugetlb: sort hstates in hugetlb_init_hstates
hugetlb: add make_huge_pte_with_shift
hugetlb: make huge_pte_lockptr take an explicit shift argument.
hugetlb: add CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
mm: make free_p?d_range functions public
hugetlb: add hugetlb_pte to track HugeTLB page table entries
hugetlb: add hugetlb_free_range to free PT structures
hugetlb: add hugetlb_hgm_enabled
hugetlb: add for_each_hgm_shift
hugetlb: add hugetlb_walk_to to do PT walks
hugetlb: add HugeTLB splitting functionality
hugetlb: add huge_pte_alloc_high_granularity
hugetlb: add HGM support for hugetlb_fault and hugetlb_no_page
hugetlb: make unmapping compatible with high-granularity mappings
hugetlb: make hugetlb_change_protection compatible with HGM
hugetlb: update follow_hugetlb_page to support HGM
hugetlb: use struct hugetlb_pte for walk_hugetlb_range
hugetlb: add HGM support for copy_hugetlb_page_range
hugetlb: add support for high-granularity UFFDIO_CONTINUE
hugetlb: add hugetlb_collapse
madvise: add uapi for HugeTLB HGM collapse: MADV_COLLAPSE
userfaultfd: add UFFD_FEATURE_MINOR_HUGETLBFS_HGM
arm64/hugetlb: add support for high-granularity mappings
selftests: add HugeTLB HGM to userfaultfd selftest
selftests: add HugeTLB HGM to KVM demand paging selftest

arch/arm64/Kconfig | 1 +
arch/arm64/mm/hugetlbpage.c | 63 ++
arch/powerpc/mm/pgtable.c | 3 +-
arch/s390/mm/gmap.c | 8 +-
fs/Kconfig | 7 +
fs/proc/task_mmu.c | 35 +-
fs/userfaultfd.c | 10 +-
include/asm-generic/tlb.h | 6 +-
include/linux/hugetlb.h | 177 +++-
include/linux/mm.h | 7 +
include/linux/pagewalk.h | 3 +-
include/uapi/asm-generic/mman-common.h | 2 +
include/uapi/linux/userfaultfd.h | 2 +
mm/damon/vaddr.c | 34 +-
mm/hmm.c | 7 +-
mm/hugetlb.c | 987 +++++++++++++++---
mm/madvise.c | 23 +
mm/memory.c | 8 +-
mm/mempolicy.c | 11 +-
mm/migrate.c | 3 +-
mm/mincore.c | 4 +-
mm/mprotect.c | 6 +-
mm/page_vma_mapped.c | 3 +-
mm/pagewalk.c | 18 +-
mm/userfaultfd.c | 57 +-
.../testing/selftests/kvm/include/test_util.h | 2 +
tools/testing/selftests/kvm/lib/kvm_util.c | 2 +-
tools/testing/selftests/kvm/lib/test_util.c | 14 +
tools/testing/selftests/vm/userfaultfd.c | 61 +-
29 files changed, 1314 insertions(+), 250 deletions(-)

--
2.37.0.rc0.161.g10f37bed90-goog

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