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

[Lorenzo Stoakes]

On Wed, Apr 29, 2026 at 10:39:23AM +0100, Usama Arif wrote:
>
>
> On 28/04/2026 20:54, David Hildenbrand (Arm) wrote:
> > 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 :(
> >
>
> Thanks! Appreciate it!
>

My email is a disaster right now, various other stuff + lately working hard on
the thing-I'm-going-to-talk-about-at-LSF and the-slides-for-that has left me
with only backlog but... :) will want to have a look post-LSF also. But May
likely to be slow for me alos.

Cheers, Lorenzo