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

[David Hildenbrand (Arm)]

On 4/27/26 12:01, 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.

Ack, that's nice.

> - 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.

Also ack.

> - Walkers (zap, mprotect, smaps, pagemap, soft-dirty, uffd-wp)
>   visit one PMD entry instead of 512 PTEs, reducing traversal
>   time and lock-hold windows.

Right.

> 
> 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.

All sounds nice. I'll get to review this soon. LSF/MM and travel will slow me a
bit down in May :(

-- 
Cheers,

David