On 9/24/26 02:24, Jason Gunthorpe wrote:
> On Wed, Sep 23, 2026 at 02:19:36PM -0700, Andrew Morton wrote: >> On Wed, 23 Sep 2026 08:29:34 +0300 Mika Penttilä <[email protected]> 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? Some of them are legit some not. This was actually the first time sashiko managed to apply the series. I will address all the remarks in v15 soon. > > Jason --Mika >
