Re: [PATCH v1 20/29] mm: convert "movable" flag in page->mapping to a page flag

David Hildenbrand
2025-07-02T12:01:43Z

On 02.07.25 13:54, Harry Yoo wrote:
> On Mon, Jun 30, 2025 at 03:00:01PM +0200, David Hildenbrand wrote:
>> Instead, let's use a page flag. As the page flag can result in
>> false-positives, glue it to the page types for which we
>> support/implement movable_ops page migration.
>>
>> The flag reused by PageMovableOps() might be sued by other pages, so
>> warning in case it is set in page_has_movable_ops() might result in
>> false-positive warnings.
>>
>> Reviewed-by: Zi Yan <ziy@nvidia.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>
> LGTM,
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
>
> With a question: is there any reason to change the page flag
> operations to use atomic bit ops?

As we have the page lock in there, it's complicated. I thought about
this when writing that code, and was not able to convince myself that it
is safe.

But that was when I was prototyping and reshuffling patches, and we
would still have code that would clear the flag.

Given that we only allow setting the flag, it might be okay to use the
non-atomic variant as long as there can be nobody racing with us when
modifying flags. Especially trying to lock the folio concurrently is the
big problem.

In isolate_movable_ops_page(), there is a comment about checking the
flag before grabbing the page lock, so that should be handled.

I'll have to check some other cases in balloon/zsmalloc code.

--
Cheers,

David / dhildenb