linux-kernel - Re: [v2 01/11] mm/zone_device: support large zone device private folios

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Message-ID: <aJJ0hTS52PVAq1sp@lstrano-desk.jf.intel.com>
Date: Tue, 5 Aug 2025 14:15:49 -0700
From: Matthew Brost <matthew.brost@...el.com>
To: David Hildenbrand <david@...hat.com>
CC: Balbir Singh <balbirs@...dia.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, Karol Herbst <kherbst@...hat.com>, Lyude Paul
	<lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>, David Airlie
	<airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Jérôme Glisse <jglisse@...hat.com>, Shuah Khan
	<shuah@...nel.org>, Barry Song <baohua@...nel.org>, Baolin Wang
	<baolin.wang@...ux.alibaba.com>, Ryan Roberts <ryan.roberts@....com>,
	"Matthew Wilcox" <willy@...radead.org>, Peter Xu <peterx@...hat.com>, Zi Yan
	<ziy@...dia.com>, Kefeng Wang <wangkefeng.wang@...wei.com>, Jane Chu
	<jane.chu@...cle.com>, Alistair Popple <apopple@...dia.com>, Donet Tom
	<donettom@...ux.ibm.com>, Ralph Campbell <rcampbell@...dia.com>, Mika
 Penttilä <mpenttil@...hat.com>, Francois Dugast
	<francois.dugast@...el.com>
Subject: Re: [v2 01/11] mm/zone_device: support large zone device private
 folios

On Tue, Aug 05, 2025 at 02:58:42PM +0200, David Hildenbrand wrote:
> On 05.08.25 13:01, Balbir Singh wrote:
> > On 8/5/25 20:57, David Hildenbrand wrote:
> > > On 05.08.25 06:22, Balbir Singh wrote:
> > > > On 7/30/25 19:50, David Hildenbrand wrote:
> > > > 
> > > > > I think I asked that already but maybe missed the reply: Should these folios ever be added to the deferred split queue and is there any value in splitting them under memory pressure in the shrinker?
> > > > > 
> > > > > My gut feeling is "No", because the buddy cannot make use of these folios, but maybe there is an interesting case where we want that behavior?
> > > > > 
> > > > 
> > > > I realized I did not answer this
> > > > 
> > > > deferred_split() is the default action when partial unmaps take place. Anything that does
> > > > folio_rmap_remove_ptes can cause the folio to be deferred split if it gets partially
> > > > unmapped.
> > > 
> > > Right, but it's easy to exclude zone-device folios here. So the real question is: do you want to deal with deferred splits or not?
> > > 
> > > If not, then just disable it right from the start.
> > > 
> > 
> > I agree, I was trying to avoid special casing device private folios unless needed to the extent possible
> 
> By introducing a completely separate split logic :P
> 
> Jokes aside, we have plenty of zone_device special-casing already, no harm
> in adding one more folio_is_zone_device() there.
> 
> Deferred splitting is all weird already that you can call yourself fortunate
> if you don't have to mess with that for zone-device folios.
> 
> Again, unless there is a benefit in having it.

+1 on no deferred split for device folios.

Matt

> 
> -- 
> Cheers,
> 
> David / dhildenb
>