Alistair Popple [off-list ref] writes:

On Sun, Jun 29, 2025 at 07:28:50PM +0800, Huang, Ying wrote:

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David Hildenbrand [off-list ref] writes:

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On 18.06.25 20:48, Zi Yan wrote:

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On 18 Jun 2025, at 14:39, Matthew Wilcox wrote:

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On Wed, Jun 18, 2025 at 02:14:15PM -0400, Zi Yan wrote:

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On 18 Jun 2025, at 13:39, David Hildenbrand wrote:

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... 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.

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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.

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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