Re: [PATCH] mm/madvise: reclaim isolated folios if PTE restart fails

[Gregory Price <gourry@gourry.net>]

On Wed, Sep 16, 2026 at 02:03:29PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Tue, Sep 15, 2026 at 01:12:03PM -0400, Gregory Price wrote:
> > On Tue, Sep 15, 2026 at 04:55:47PM +0100, Lorenzo Stoakes (ARM) wrote:
> > > On Sat, Sep 12, 2026 at 07:08:32AM -0400, Gregory Price wrote:
> > > > MADV_PAGEOUT collects isolated folios on a local list before reclaiming
> > > > them after the PTE walk.  The reschedule path drops the PTE lock and then
> > > > restarts the mapping with pte_offset_map_lock().
> > >
> > > Is this reschedule path even necessary? It's pretty bloody sketchy.
> > >
> > > I thought the modern approach (TM) was to not do cond_resched() and friends so
> > > can we actually look at removing this?
> > >
> >
> > Sorry - meant to reply to this.
> >
> > Honestly there's so much jammed into this function i can't give you a
> > straight answer. I can take a short detour to figure out if we can
> > rework this as a whole.
> 
> s/short detour/long journey through hell/ perhaps? :P ;)
> 

Not as much as you'd think.

I did it yesterday and left an agent to validate it overnight with
existing in-tree tests, LTP tests, and added some new tests.

The naming could use some work (cold_or_pageout -> lru_op, bleh),
but new call graph / pseudo-code:

• walk_page_range_vma()
  └── madvise_lru_op_pmd()              [PMD callback]
      │
      ├── PMD is a huge mapping
      │   └── madvise_lru_op_huge_pmd()
      │       ├── acquire PMD lock
      │       ├── madvise_lru_op_huge_pmd_locked()
      │       │   ├── process whole folio, or
      │       │   └── return locked+referenced split candidate
      │       ├── release PMD lock
      │       └── split folio / reclaim isolated folios
      │
      └── PMD points to a PTE table
          └── madvise_lru_op_ptes()
              ├── map PTE table and acquire PTL
              ├── madvise_lru_op_pte_range_locked()
              │   └── madvise_lru_op_pte_batch_locked()
              │       ├── process one folio/PTE batch, or
              │       └── return locked+referenced split candidate
              ├── leave lazy-MMU mode
              ├── unmap PTE table and release PTL
              ├── split candidate if necessary
              ├── reclaim isolated folios
              └── reschedule and restart from current address if necessary

Looks like a normal table walk now.  Significantly easier to review and
doesn't make my eyes vomit. (there's still an ifdef wart, but it's not
mid-function anymore... so that's nice).

Going to spend a little more time testing and tweaking before I post it.

I've been working on a making more extensive unit tests for certain
parts of mm/ and this might be a good time to look at whether I can
introduce a piece of it.

~Gregory