Re: [PATCH RFC 07/29] mm/migrate: rename isolate_movable_page() to isolate_movable_ops_page()

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Alistair Popple <apopple@nvidia.com> writes:

> On Sun, Jun 29, 2025 at 07:28:50PM +0800, Huang, Ying wrote:
>> David Hildenbrand <david@redhat.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.

So, folios include file cache, anonymous, and some device private.

>> > 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.

Yes.  The is the current behavior per my understanding too.

> Did you have some particular reason/idea for why we might want to do this?

No.  Just want to check whether there are some requirements for that.  I
think that it's just another way to organize code.

---
Best Regards,
Huang, Ying