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

Usama Arif
2026-04-27T20:12:33Z

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.

>> 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@google.com/
>> [2] https://lore.kernel.org/all/20260327021403.214713-1-usama.arif@linux.dev/
>>
>> 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