From: "Mika Penttilä" <mpenttil@redhat.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, dri-devel@lists.freedesktop.org,
intel-xe@lists.freedesktop.org, linux-kernel@vger.kernel.org,
David Hildenbrand <david@kernel.org>,
Jason Gunthorpe <jgg@nvidia.com>,
Leon Romanovsky <leonro@nvidia.com>,
Alistair Popple <apopple@nvidia.com>,
Balbir Singh <balbirs@nvidia.com>, Zi Yan <ziy@nvidia.com>,
Matthew Brost <matthew.brost@intel.com>,
Lorenzo Stoakes <ljs@kernel.org>,
"Liam R. Howlett" <Liam.Howlett@oracle.com>,
Vlastimil Babka <vbabka@suse.cz>, Mike Rapoport <rppt@kernel.org>,
Suren Baghdasaryan <surenb@google.com>,
Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH 00/12] [PATCH v14 00/12] migrate on fault for device pages
Date: Thu, 24 Sep 2026 03:10:43 +0300 [thread overview]
Message-ID: <63211ea4-9eae-4241-8926-9e81334c5f7a@redhat.com> (raw)
In-Reply-To: <20260923141936.d1f9e5b85355f0bdc6a6ecd2@linux-foundation.org>
On 9/24/26 00:19, Andrew Morton wrote:
> On Wed, 23 Sep 2026 08:29:34 +0300 Mika Penttilä <mpenttil@redhat.com> 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?
Yes I think Alistair mentioned those walks showed clearly in perf traces
in his experiments, and that served as a motivation for this series
>
>> 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.
>
> Under what circumstances to these crashes occur? Can/should we we fix
> those in a minimal -stable backportable fashion before moving on to speedups?
It is about races between concurrent MADV_DONTNEED and collecting, where the
collecting arrays overflow if page table is cleared, recycled and reinstantiated
as large page.
I am not sure if there is minimal fix to all the corner cases,
or would it be more like the approach in this series.
--Mika
prev parent reply other threads:[~2026-09-24 0:10 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-22 5:34 mpenttil
2026-09-22 5:34 ` [PATCH v14 01/12] mm/Kconfig: changes for " mpenttil
2026-09-22 22:27 ` Balbir Singh
2026-09-23 5:42 ` Mika Penttilä
2026-09-22 5:34 ` [PATCH v14 02/12] mm: add helper to convert HMM pfn to migrate pfn mpenttil
2026-09-22 5:34 ` [PATCH v14 03/12] mm/hmm: preparations for HMM to participate in migration mpenttil
2026-09-22 5:34 ` [PATCH v14 04/12] mm/hmm: do the plumbing " mpenttil
2026-09-22 5:34 ` [PATCH v14 05/12] mm/hmm: implement folio split for migrate needs in HMM pagewalk mpenttil
2026-09-22 5:34 ` [PATCH v14 06/12] mm/hmm: migrate collection in HMM pagewalk - pte level mpenttil
2026-09-22 5:34 ` [PATCH v14 07/12] mm/hmm: migrate collection in HMM pagewalk - pmd level mpenttil
2026-09-22 5:34 ` [PATCH v14 08/12] mm/hmm: add lazy MMU mode support for migration in HMM pagewalk mpenttil
2026-09-22 5:34 ` [PATCH v14 09/12] mm/hmm: implement rollback for device page " mpenttil
2026-09-22 5:34 ` [PATCH v14 10/12] mm: enable device page migration from " mpenttil
2026-09-22 5:34 ` [PATCH v14 11/12] lib/test_hmm: add a new testcase for the migrate on fault mpenttil
2026-09-22 5:34 ` [PATCH v14 12/12] Documentation/mm/hmm: document migration through hmm_range_fault() mpenttil
2026-09-23 2:27 ` [PATCH 00/12] [PATCH v14 00/12] migrate on fault for device pages Andrew Morton
2026-09-23 5:29 ` Mika Penttilä
2026-09-23 21:19 ` Andrew Morton
2026-09-23 23:24 ` Jason Gunthorpe
2026-09-24 0:14 ` Mika Penttilä
2026-09-24 0:10 ` Mika Penttilä [this message]
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