Re: [PATCH 00/13] mm: PMD-level swap entries for anonymous THPs

[Zi Yan]

On 27 Apr 2026, at 16:12, Usama Arif wrote:

> On 27/04/2026 19:26, Zi Yan wrote:
>> +Ying, who did the original THP swap work[1].
>>
>> [1] https://lkml.org/lkml/2016/8/9/588
>>
>
> Thanks Zi!
>
> Sorry Ying for not CCing you! checkpatch on the whole series produced
> a really long list and I wasnt sure if people would start thinking of
> it as spam. I added reviewers and maintainers of swap and THP + a few
> folks that commented on previous related work from which this kicked off.
> I should have just CC'ed everyone.
>
>> On 27 Apr 2026, at 6:01, Usama Arif wrote:
>>
>>> When reclaim swaps out a PMD-mapped anonymous THP today, the PMD is
>>> split into 512 PTE-level swap entries via TTU_SPLIT_HUGE_PMD before
>>> unmap.
>>>
>>> This series introduces a PMD-level swap entry. The huge mapping is
>>> preserved across the swap round-trip, and do_huge_pmd_swap_page()
>>> resolves the entire 2 MB region in a single fault on swap-in,
>>> no khugepaged involvement is needed. swap_map metadata is identical
>>> either way (512 single-slot counts), so the PTE split buys nothing
>>> on the swap side, it is purely a page-table representation change.
>>>
>>> This work was brought about after Hugh reported that one of the
>>> major blockers for having lazy page table deposit is the lack of
>>> PMD swap entries [1]. However, this series has benefits of its
>>> own:
>>> - The huge mapping is restored on swap-in.  Today even when the
>>>   folio is still in swap cache as a single 2 MB folio, the swap-in
>>>   path installs 512 PTE mappings -- the PMD mapping is gone, the
>>>   freshly-materialised PTE table sticks around, and only
>>>   khugepaged can later collapse the range back into a THP.
>>>   do_huge_pmd_swap_page() reinstalls the PMD mapping directly in
>>>   one fault, no khugepaged involvement.
>>> - Memory saved per swapped-out THP *once lazy page table deposit is
>>>   merged* [2]. With lazy page table deposit [2], splitting a PMD into
>>>   512 PTE swap entries forces allocation of a 4 KB PTE table page.
>>>   The new path leaves the pgtable hierarchy at PMD level and avoids
>>>   that allocation entirely.
>>>   This will save memory when swapping, which is likely when there is
>>>   memory pressure and exactly when allocations are most likely to
>>>   fail.
>>> - Walkers (zap, mprotect, smaps, pagemap, soft-dirty, uffd-wp)
>>>   visit one PMD entry instead of 512 PTEs, reducing traversal
>>>   time and lock-hold windows.
>>>
>>> The swap entry value is identical to 512 PTE swap entries (same
>>> type, same starting offset), so swap_map refcounting is unchanged.
>>> Only the page-table representation differs; the swap slot allocator,
>>> swap I/O, and swap cache are untouched.  The new path falls back to
>>> the existing PTE-split path whenever a PMD-order resource is
>>> unavailable: zswap enabled, non-contiguous swap allocation
>>> (THP_SWPOUT_FALLBACK), PMD-order folio allocation failure on swap-in
>>> or fork, racing folio split, or rmap-driven split on a swapcache
>>> folio.  Walkers that previously assumed every non-present PMD encodes
>>> a PFN (migration / device_private) are taught to recognise PMD swap
>>> entries.
>>>
>>> Patch breakdown:
>>>
>>> The series is ordered to preserve git bisectability: every consumer
>>> of a PMD swap entry (split, fork, swapoff, walkers, UFFDIO_MOVE,
>>> swap-in fault) lands before the producer.  The swap-out path that
>>> actually installs PMD swap entries is the very last functional patch
>>> (12), so no intermediate commit can leave the kernel handling a
>>> PMD swap entry it does not yet understand.
>>>
>>> The first 4 patches are preparatory patches. Some of them (like
>>> softleaf_to_pmd() change in patch 1) are not exactly needed but its
>>> done to hopefully improve code quality and so that the PMD swap
>>> entry changes look well integrated with the rest of mm.
>>>
>>> Prep patches:
>>>   1. mm: add softleaf_to_pmd() and convert existing callers
>>>      PMD counterpart to softleaf_to_pte(); needed to construct a
>>>      PMD from a swap entry in later patches.
>>>   2. mm: extract ensure_on_mmlist() helper
>>>      Hoists the "register mm with swapoff" double-checked-locking
>>>      pattern out of try_to_unmap_one() / copy_nonpresent_pte() so
>>>      the PMD swap-out and PMD fork paths can reuse it without a
>>>      third open-coded copy.
>>>   3. fs/proc: use softleaf_has_pfn() in pagemap PMD walker
>>>      pagemap_pmd_range_thp() today calls softleaf_to_page()
>>>      unconditionally; a PMD swap entry has no PFN and would crash
>>>      it.
>>>   4. mm/huge_memory: move softleaf_to_folio() inside migration branch
>>>      change_non_present_huge_pmd() today calls softleaf_to_folio()
>>>      before branching on entry type, so a PMD swap entry would
>>>      produce a bogus folio pointer that the migration-only code
>>>      below would then dereference.
>>>
>>> Core patches:
>>>   5. PMD swap entry detection (pmd_is_swap_entry,
>>>      softleaf_is_valid_pmd_entry) and per-arch pmd_swp_*exclusive
>>>      helpers (x86/arm64/s390/riscv/loongarch).
>>>   6. __split_huge_pmd_locked() learns to split a PMD swap entry
>>>      into 512 PTE swap entries, used as the fallback when a
>>>      PMD-order resource is unavailable.
>>
>> I was wondering how to handle insufficient memory during swap-in.
>> Here it is. I have not read the code, but the split should be
>> straightforward, since we already have a contiguous swap space at
> m> swap-out time and the split is just to enable PTE-level swap in, right?
>>
>
> Yes that is correct. Actually patch 6 was one of the easier patches.
> If the kernel can't allocate 2M, memcg charge fails and a few other reasons,
> we split THP.

Thank you for the confirmation. I will be mostly AFK in May and will
probably check the patches later.
>
>
>>>   7. Fork: copy_huge_non_present_pmd() duplicates the PMD swap entry
>>>      in one folio_dup_swap() call, with GFP_KERNEL retry mirroring
>>>      copy_pte_range().
>>>   8. Swapoff: unuse_pmd() reads the whole 2 MB folio and reinstalls
>>>      the PMD; falls back to PTE-split + unuse_pte_range() on error.
>>>   9. Walker updates: zap_huge_pmd, change_huge_pmd,
>>>      change_non_present_huge_pmd, move_soft_dirty_pmd,
>>>      clear_soft_dirty_pmd, make_uffd_wp_pmd, smaps_pmd_entry,
>>>      queue_folios_pmd (mempolicy), check_pmd_state (khugepaged),
>>>      and the madvise_cold_or_pageout_pte_range / madvise_free_huge_pmd
>>>      VM_BUG_ON extensions.
>>>  10. UFFDIO_MOVE: move_pages_huge_pmd() learns to move a PMD swap
>>>      entry whole via a new move_swap_pmd() helper modeled on
>>>      move_swap_pte().
>>>  11. Swap-in: do_huge_pmd_swap_page() resolves a PMD swap fault in
>>>      one shot.  Handles racing splits, SWP_STABLE_WRITES read-only
>>>      mapping, immediate COW for write faults; falls back to PTE-split
>>>      on any PMD-order resource shortfall.
>>>  12. Swap-out: shrink_folio_list() drops TTU_SPLIT_HUGE_PMD for
>>>      PMD-mappable swapcache folios (when zswap is disabled), and
>>>      try_to_unmap_one() installs one PMD swap entry via
>>>      set_pmd_swap_entry() instead of splitting.
>>>
>>> Testing:
>>>  13. selftests/mm: 12 tests covering swap-out/in, fork, fork+COW,
>>>      repeated cycles, write fault, munmap, mprotect, mremap, pagemap,
>>>      MADV_FREE, UFFDIO_MOVE, swapoff.
>>>
>>> Making PMD swap entries work with zswap is another project on its own and
>>> should be in a separate follow up series.
>>>
>>> The patches are on top of mm-unstable from 23 April
>>> (2bcc13c29c711381d815c1ba5d5b25737400c71a).
>>>
>>> [1] https://lore.kernel.org/all/6869b7f0-84e1-fb93-03f1-9442cdfe476b@xxxxxxxxxx/
>>> [2] https://lore.kernel.org/all/20260327021403.214713-1-usama.arif@xxxxxxxxx/
>>>
>>> Usama Arif (13):
>>>   mm: add softleaf_to_pmd() and convert existing callers
>>>   mm: extract ensure_on_mmlist() helper
>>>   fs/proc: use softleaf_has_pfn() in pagemap PMD walker
>>>   mm/huge_memory: move softleaf_to_folio() inside migration branch
>>>   mm: add PMD swap entry detection support
>>>   mm: add PMD swap entry splitting support
>>>   mm: handle PMD swap entries in fork path
>>>   mm: swap in PMD swap entries as whole THPs during swapoff
>>>   mm: handle PMD swap entries in non-present PMD walkers
>>>   mm: handle PMD swap entries in UFFDIO_MOVE
>>>   mm: handle PMD swap entry faults on swap-in
>>>   mm: install PMD swap entries on swap-out
>>>   selftests/mm: add PMD swap entry tests
>>>
>>>  arch/arm64/include/asm/pgtable.h      |   4 +
>>>  arch/loongarch/include/asm/pgtable.h  |  17 +
>>>  arch/riscv/include/asm/pgtable.h      |  15 +
>>>  arch/s390/include/asm/pgtable.h       |  15 +
>>>  arch/x86/include/asm/pgtable.h        |  15 +
>>>  fs/proc/task_mmu.c                    |  47 +-
>>>  include/linux/huge_mm.h               |  11 +
>>>  include/linux/leafops.h               |  44 +-
>>>  include/linux/swap.h                  |   4 +-
>>>  include/linux/vm_event_item.h         |   1 +
>>>  mm/hmm.c                              |   3 +-
>>>  mm/huge_memory.c                      | 540 +++++++++++++++++++++--
>>>  mm/internal.h                         |  49 +++
>>>  mm/khugepaged.c                       |   6 +
>>>  mm/madvise.c                          |   5 +-
>>>  mm/memory.c                           |  51 +--
>>>  mm/mempolicy.c                        |   2 +
>>>  mm/rmap.c                             |  27 +-
>>>  mm/swap.h                             |   7 +
>>>  mm/swap_state.c                       |  35 ++
>>>  mm/swapfile.c                         | 144 +++++-
>>>  mm/vmscan.c                           |  14 +-
>>>  mm/vmstat.c                           |   1 +
>>>  tools/testing/selftests/mm/Makefile   |   1 +
>>>  tools/testing/selftests/mm/pmd_swap.c | 607 ++++++++++++++++++++++++++
>>>  25 files changed, 1554 insertions(+), 111 deletions(-)
>>>  create mode 100644 tools/testing/selftests/mm/pmd_swap.c
>>>
>>> -- 
>>> 2.52.0
>>
>>
>> Best Regards,
>> Yan, Zi


Best Regards,
Yan, Zi