linux-kernel - Re: [v3 03/11] mm/migrate_device: THP migration of zone device pages

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Message-ID: <aLDi16JRf2VRWVI+@lstrano-desk.jf.intel.com>
Date: Thu, 28 Aug 2025 16:14:31 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: Balbir Singh <balbirs@...dia.com>
CC: Mika Penttilä <mpenttil@...hat.com>,
	<dri-devel@...ts.freedesktop.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>, Joshua Hahn
	<joshua.hahnjy@...il.com>, Rakie Kim <rakie.kim@...com>, Byungchul Park
	<byungchul@...com>, Gregory Price <gourry@...rry.net>, Ying Huang
	<ying.huang@...ux.alibaba.com>, Alistair Popple <apopple@...dia.com>, "Oscar
 Salvador" <osalvador@...e.de>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>, "Liam R. Howlett"
	<Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>, Ryan Roberts
	<ryan.roberts@....com>, Dev Jain <dev.jain@....com>, Barry Song
	<baohua@...nel.org>, Lyude Paul <lyude@...hat.com>, Danilo Krummrich
	<dakr@...nel.org>, David Airlie <airlied@...il.com>, Simona Vetter
	<simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>, Francois Dugast
	<francois.dugast@...el.com>
Subject: Re: [v3 03/11] mm/migrate_device: THP migration of zone device pages

On Thu, Aug 21, 2025 at 08:24:00PM +1000, Balbir Singh wrote:
> On 8/15/25 10:04, Matthew Brost wrote:
> > On Fri, Aug 15, 2025 at 08:51:21AM +1000, Balbir Singh wrote:
> >> On 8/13/25 10:07, Mika Penttilä wrote:
> >>>
> >>> On 8/13/25 02:36, Balbir Singh wrote:
> >>>
> >>>> On 8/12/25 15:35, Mika Penttilä wrote:
> >>>>> Hi,
> >>>>>
> >>>>> On 8/12/25 05:40, Balbir Singh wrote:
> ...
> 
> >> I've not run into this with my testing, let me try with more mTHP sizes enabled. I'll wait on Matthew
> >> to post his test case or any results, issues seen
> >>
> > 
> > I’ve hit this. In the code I shared privately, I split THPs in the
> > page-collection path. You omitted that in v2 and v3; I believe you’ll
> > need those changes. The code I'm referring to had the below comment.
> > 
> >  416         /*
> >  417          * XXX: No clean way to support higher-order folios that don't match PMD
> >  418          * boundaries for now — split them instead. Once mTHP support lands, add
> >  419          * proper support for this case.
> >  420          *
> >  421          * The test, which exposed this as problematic, remapped (memremap) a
> >  422          * large folio to an unaligned address, resulting in the folio being
> >  423          * found in the middle of the PTEs. The requested number of pages was
> >  424          * less than the folio size. Likely to be handled gracefully by upper
> >  425          * layers eventually, but not yet.
> >  426          */
> > 
> > I triggered it by doing some odd mremap operations, which caused the CPU
> > page-fault handler to spin indefinitely iirc. In that case, a large device
> > folio had been moved into the middle of a PMD.
> > 
> > Upstream could see the same problem if the device fault handler enforces
> > a must-migrate-to-device policy and mremap moves a large CPU folio into
> > the middle of a PMD.
> > 
> > I’m in the middle of other work; when I circle back, I’ll try to create
> > a selftest to reproduce this. My current test is a fairly convoluted IGT
> > with a bunch of threads doing remap nonsense, but I’ll try to distill it
> > into a concise selftest.
> > 
> 
> I ran into this while doing some testing as well, I fixed it in a manner similar
> to split_folio() for partial unmaps. I will consolidate the folio splits into
> a single helper and post it with v4.
> 

I created a selftest for this one. I'm going to send these over along +
the fixes I've applied in v3. Please include my selftests in the v4.

Matt 

> 
> Balbir Singh