From: Jason Gunthorpe <jgg@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Mika Penttilä" <mpenttil@redhat.com>,
linux-mm@kvack.org, dri-devel@lists.freedesktop.org,
intel-xe@lists.freedesktop.org, linux-kernel@vger.kernel.org,
"David Hildenbrand" <david@kernel.org>,
"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: Wed, 23 Sep 2026 20:24:02 -0300 [thread overview]
Message-ID: <20260923232402.GI2545495@nvidia.com> (raw)
In-Reply-To: <20260923141936.d1f9e5b85355f0bdc6a6ecd2@linux-foundation.org>
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@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?
>
> > 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
next prev parent reply other threads:[~2026-09-23 23:24 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 [this message]
2026-09-24 0:14 ` Mika Penttilä
2026-09-24 0:10 ` Mika Penttilä
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