linux-kernel - Re: [PATCH RFC 07/29] mm/migrate: rename isolate_movable_page() to isolate_movable_ops

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Message-ID: <nr4e7unt2dtfav5y5ckmsrehu37xejqonarlulzcn6mnhnnvxl@o3ppo34wqyyj>
Date: Mon, 30 Jun 2025 10:20:56 +1000
From: Alistair Popple <apopple@...dia.com>
To: "Huang, Ying" <ying.huang@...ux.alibaba.com>
Cc: David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>, 
	Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	linux-doc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, virtualization@...ts.linux.dev, 
	linux-fsdevel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Jonathan Corbet <corbet@....net>, Madhavan Srinivasan <maddy@...ux.ibm.com>, 
	Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, Jerrin Shaji George <jerrin.shaji-george@...adcom.com>, 
	Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez <eperezma@...hat.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
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	Johannes Weiner <hannes@...xchg.org>, Jason Gunthorpe <jgg@...pe.ca>, 
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	Naoya Horiguchi <nao.horiguchi@...il.com>, Oscar Salvador <osalvador@...e.de>, 
	Rik van Riel <riel@...riel.com>, Harry Yoo <harry.yoo@...cle.com>, 
	Qi Zheng <zhengqi.arch@...edance.com>, Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [PATCH RFC 07/29] mm/migrate: rename isolate_movable_page() to
 isolate_movable_ops_page()

On Sun, Jun 29, 2025 at 07:28:50PM +0800, Huang, Ying wrote:
> David Hildenbrand <david@...hat.com> writes:
> 
> > On 18.06.25 20:48, Zi Yan wrote:
> >> On 18 Jun 2025, at 14:39, Matthew Wilcox wrote:
> >> 
> >>> On Wed, Jun 18, 2025 at 02:14:15PM -0400, Zi Yan wrote:
> >>>> On 18 Jun 2025, at 13:39, David Hildenbrand wrote:
> >>>>
> >>>>> ... and start moving back to per-page things that will absolutely not be
> >>>>> folio things in the future. Add documentation and a comment that the
> >>>>> remaining folio stuff (lock, refcount) will have to be reworked as well.
> >>>>>
> >>>>> While at it, convert the VM_BUG_ON() into a WARN_ON_ONCE() and handle
> >>>>> it gracefully (relevant with further changes), and convert a
> >>>>> WARN_ON_ONCE() into a VM_WARN_ON_ONCE_PAGE().
> >>>>
> >>>> The reason is that there is no upstream code, which use movable_ops for
> >>>> folios? Is there any fundamental reason preventing movable_ops from
> >>>> being used on folios?
> >>>
> >>> folios either belong to a filesystem or they are anonymous memory, and
> >>> so either the filesystem knows how to migrate them (through its a_ops)
> >>> or the migration code knows how to handle anon folios directly.
> >
> > Right, migration of folios will be handled by migration core.
> >
> >> for device private pages, to support migrating >0 order anon or fs
> >> folios
> >> to device, how should we represent them for devices? if you think folio is
> >> only for anon and fs.
> >
> > I assume they are proper folios, so yes. Just like they are handled
> > today (-> folios)

Yes, they should be proper folios.

> > I was asking a related question at LSF/MM in Alistair's session: are
> > we sure these things will be folios even before they are assigned to a
> > filesystem? I recall the answer was "yes".
> >
> > So we don't (and will not) support movable_ops for folios.
> 
> Is it possible to use some device specific callbacks (DMA?) to copy
> from/to the device private folios (or pages) to/from the normal
> file/anon folios in the future?

I guess we could put such callbacks on the folio->pgmap, but I'm not sure why
we would want to. Currently all migration to/from device private (or coherent)
folios is managed by the device, which is one of the features of ZONE_DEVICE.
Did you have some particular reason/idea for why we might want to do this?

> ---
> Best Regards,
> Huang, Ying