Re: [v3 00/24] mm: thp: lazy PTE page table allocation at PMD split time

[Usama Arif <usama.arif@linux.dev>]

On 27/03/2026 11:51, David Hildenbrand (Arm) wrote:
> On 3/27/26 03:08, Usama Arif wrote:
>> When the kernel creates a PMD-level THP mapping for anonymous pages, it
>> pre-allocates a PTE page table via pgtable_trans_huge_deposit(). This
>> page table sits unused in a deposit list for the lifetime of the THP
>> mapping, only to be withdrawn when the PMD is split or zapped. Every
>> anonymous THP therefore wastes 4KB of memory unconditionally. On large
>> servers where hundreds of gigabytes of memory are mapped as THPs, this
>> adds up: roughly 200MB wasted per 100GB of THP memory. This memory
>> could otherwise satisfy other allocations, including the very PTE page
>> table allocations needed when splits eventually occur.
>>
>> This series removes the pre-deposit and allocates the PTE page table
>> lazily — only when a PMD split actually happens. Since a large number
>> of THPs are never split (they are zapped wholesale when processes exit or
>> munmap the full range), the allocation is avoided entirely in the common
>> case.
>>
>> The pre-deposit pattern exists because split_huge_pmd was designed as an
>> operation that must never fail: if the kernel decides to split, it needs
>> a PTE page table, so one is deposited in advance. But "must never fail"
>> is an unnecessarily strong requirement. A PMD split is typically triggered
>> by a partial operation on a sub-PMD range — partial munmap, partial
>> mprotect, COW on a pinned folio, GUP with FOLL_SPLIT_PMD, and similar.
>> All of these operations already have well-defined error handling for
>> allocation failures (e.g., -ENOMEM, VM_FAULT_OOM). Allowing split to
>> fail and propagating the error through these existing paths is the natural
>> thing to do. Furthermore, if the system cannot satisfy a single order-0
>> allocation for a page table, it is under extreme memory pressure and
>> failing the operation is the correct response.
>>
>> Designing functions like split_huge_pmd as operations that cannot fail
>> has a subtle but real cost to code quality. It forces a pre-allocation
>> pattern - every THP creation path must deposit a page table, and every
>> split or zap path must withdraw one, creating a hidden coupling between
>> widely separated code paths.
>>
>> This also serves as a code cleanup. On every architecture except powerpc
>> with hash MMU, the deposit/withdraw machinery becomes dead code. The
>> series removes the generic implementations in pgtable-generic.c and the
>> s390/sparc overrides, replacing them with no-op stubs guarded by
>> arch_needs_pgtable_deposit(), which evaluates to false at compile time
>> on all non-powerpc architectures.
>>
>> The series is structured as follows:
>>
>> Patches 1-2:    Infrastructure — make split functions return int and
>>                 propagate errors from vma_adjust_trans_huge() through
>>                 __split_vma, vma_shrink, and commit_merge.
>>
>> Patches 3-15:   Handle split failure at every call site — copy_huge_pmd,
>>                 do_huge_pmd_wp_page, zap_pmd_range, wp_huge_pmd,
>>                 change_pmd_range (mprotect), follow_pmd_mask (GUP),
>>                 walk_pmd_range (pagewalk), move_page_tables (mremap),
>>                 move_pages (userfaultfd), device migration,
>>                 pagemap_scan_thp_entry (proc), powerpc subpage_prot,
>>                 and dax_iomap_pmd_fault (DAX). The code will become
>>                 effective in Patch 17 when split functions start
>>                 returning -ENOMEM.
>>
>> Patch 16:       Add __must_check to __split_huge_pmd(), split_huge_pmd()
>>                 and split_huge_pmd_address() so the compiler warns on
>>                 unchecked return values.
>>
>> Patch 17:       The actual change — allocate PTE page tables lazily at
>>                 split time instead of pre-depositing at THP creation.
>>                 This is when split functions will actually start returning
>>                 -ENOMEM.
>>
>> Patch 18:       Remove the now-dead deposit/withdraw code on
>>                 non-powerpc architectures.
>>
>> Patch 19:       Add THP_SPLIT_PMD_FAILED vmstat counter for monitoring
>>                 split failures.
>>
>> Patches 20-24:  Selftests covering partial munmap, mprotect, mlock,
>>                 mremap, and MADV_DONTNEED on THPs to exercise the
>>                 split paths.
>>
>> The error handling patches are placed before the lazy allocation patch so
>> that every call site is already prepared to handle split failures before
>> the failure mode is introduced. This makes each patch independently safe
>> to apply and bisect through.
>>
>> The patches were tested with CONFIG_DEBUG_ATOMIC_SLEEP and CONFIG_DEBUG_VM
>> enabled. The test results are below:
>>
>> TAP version 13
>> 1..5
>> # Starting 5 tests from 1 test cases.
>> #  RUN           thp_pmd_split.partial_munmap ...
>> # thp_pmd_split_test.c:60:partial_munmap:thp_split_pmd: 0 -> 1
>> # thp_pmd_split_test.c:62:partial_munmap:thp_split_pmd_failed: 0 -> 0
>> #            OK  thp_pmd_split.partial_munmap
>> ok 1 thp_pmd_split.partial_munmap
>> #  RUN           thp_pmd_split.partial_mprotect ...
>> # thp_pmd_split_test.c:60:partial_mprotect:thp_split_pmd: 1 -> 2
>> # thp_pmd_split_test.c:62:partial_mprotect:thp_split_pmd_failed: 0 -> 0
>> #            OK  thp_pmd_split.partial_mprotect
>> ok 2 thp_pmd_split.partial_mprotect
>> #  RUN           thp_pmd_split.partial_mlock ...
>> # thp_pmd_split_test.c:60:partial_mlock:thp_split_pmd: 2 -> 3
>> # thp_pmd_split_test.c:62:partial_mlock:thp_split_pmd_failed: 0 -> 0
>> #            OK  thp_pmd_split.partial_mlock
>> ok 3 thp_pmd_split.partial_mlock
>> #  RUN           thp_pmd_split.partial_mremap ...
>> # thp_pmd_split_test.c:60:partial_mremap:thp_split_pmd: 3 -> 4
>> # thp_pmd_split_test.c:62:partial_mremap:thp_split_pmd_failed: 0 -> 0
>> #            OK  thp_pmd_split.partial_mremap
>> ok 4 thp_pmd_split.partial_mremap
>> #  RUN           thp_pmd_split.partial_madv_dontneed ...
>> # thp_pmd_split_test.c:60:partial_madv_dontneed:thp_split_pmd: 4 -> 5
>> # thp_pmd_split_test.c:62:partial_madv_dontneed:thp_split_pmd_failed: 0 -> 0
>> #            OK  thp_pmd_split.partial_madv_dontneed
>> ok 5 thp_pmd_split.partial_madv_dontneed
>> # PASSED: 5 / 5 tests passed.
>> # Totals: pass:5 fail:0 xfail:0 xpass:0 skip:0 error:0
>>
>> The patches are based off of mm-unstable as of 25 Mar
>> git hash: d6f51e38433489eb22cb65d1bf72ac7993c5bdec
>>
>> RFC v2 -> v3: https://lore.kernel.org/all/de0dc7ec-7a8d-4b1a-a419-1d97d2e4d510@linux.dev/
> 
> Note that we usually go from RFC to v1.

ack.

> 
> I'll put this series on my review backlog, but it will take some time
> until I get to it (it won't make the next release either way :) ).
> 

No worries and Thanks!