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

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Message-ID: <c3cb8f4f-e343-4109-b679-a6b37e18dd30@redhat.com>
Date: Mon, 23 Jun 2025 17:53:46 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>
Cc: 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,
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 Arnd Bergmann <arnd@...db.de>,
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 Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [PATCH RFC 07/29] mm/migrate: rename isolate_movable_page() to
 isolate_movable_ops_page()

On 23.06.25 17:42, Zi Yan wrote:
> On 23 Jun 2025, at 11:33, David Hildenbrand wrote:
> 
>> 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)
>>
>> 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.
> 
> Got it. (I was abusing it to help develop alloc_contig_range() at pageblock
> granularity, since it was easy to write a driver to allocate a compound page
> at a specific PFN and claim the page is movable, then do page online/offline
> the range containing the PFNs. :) )

Oh, interesting :)

Yeah, with a folio that will be a bit harder ...

> 
> For the patch, Reviewed-by: Zi Yan <ziy@...dia.com>

Thanks!

-- 
Cheers,

David / dhildenb