Re: [PATCH 00/12] [PATCH v14 00/12] migrate on fault for device pages

[Jason Gunthorpe]

On Wed, Sep 23, 2026 at 02:19:36PM -0700, Andrew Morton wrote:
> On Wed, 23 Sep 2026 08:29:34 +0300 Mika Penttilä <mpenttil@xxxxxxxxxx> wrote:
> 
> > 
> > >> migration). It is also tricky to code correctly. One page table walk
> > >> could costs over 1000 cpu cycles on X86-64, which is a significant hit.
> > >>
> > >> We should be able to walk the page table once, faulting
> > >> pages in as required and replacing them with migration entries if
> > >> requested.
> > > Sounds sensible.
> > >
> > >> Tested in X86-64 VM with HMM test device, passing the selftests.
> > >> For performance, the migrate throughput tests from the selftests
> > >> show similar numbers (within error margin) as unmodified kernel.
> > > But no performance benefits are demonstrated?
> > 
> > There are no performance regressions for current tests.
> > Real benefits come if want to do migrate on fault.
> > For migrate on fault today missing pages are collected as not-present and
> > the caller has to fault them and re-run migrate_vma_setup(); folding 
> > HMM_PFN_REQ_FAULT into the collecting walk removes that extra
> > fault+retry round-trip, dropping two page table walks. Page table walks
> > are not cheap. Not to mention simplified implementation for driver.
> > Also, the vma looked up as part of the walk is readily available for
> > migration, eliminating the need for explicit vma lookup - one more
> > performance benefit. Net effect two saved page table walks and
> > one vma lookup.
> 
> It certainly sounds that this series will result in performance
> improvements, but have those improvements been quantified?
> 
> > This series also addresses the vanished/reborn page table while
> > collecting problem which can crash current implementation.
> 
> Oh.  I didn't get that message from the v14 changelogs, and crashes in
> the current implementation are not what we want.  In fact, addressing
> those is more important than speeding things up.

Yeah, what is this about?

The series still has lots of sashiko remarks, are they legit?

Jason