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

[Mika Penttilä]

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 <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?

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


>