Re: [PATCH v6 1/5] mm/zone_device: Reinitialize large zone device private folios

[Zi Yan <ziy@nvidia.com>]

On 20 Jan 2026, at 8:53, Jason Gunthorpe wrote:

> On Mon, Jan 19, 2026 at 09:50:16PM -0500, Zi Yan wrote:
>>>> I suppose we want some prep_single_page(page) and some reorg to share
>>>> code with the other prep function.
>>
>> This is just an unnecessary need due to lack of knowledge of/do not want
>> to investigate core MM page and folio initialization code.
>
> It will be better to keep this related code together, not spread all
> around.

Or clarify what code is for preparing pages, which would go away at memdesc
time, and what code is for preparing folios, which would stay.

>
>>>> I don't think so. It should do the above job efficiently and iterate
>>>> over the page list exactly once.
>>
>> folio initialization should not iterate over any page list, since folio is
>> supposed to be treated as a whole instead of individual pages.
>
> The tail pages need to have the right data in them or compound_head
> won't work.

That is done by set_compound_head() in prep_compound_tail().
prep_compound_page() take cares of it. As long as it is called, even if
the pages in that compound page have random states before, the compound
page should function correctly afterwards.

>
>> folio->mapping = NULL;
>> folio->memcg_data = 0;
>> folio->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
>>
>> should be enough.
>
> This seems believable to me for setting up an order 0 page.

It works for any folio, regardless of its order. fields used in second
or third subpages are all taken care of by prep_compound_page().

>
>> if (order)
>> 	folio_set_large_rmappable(folio);
>
> That one is in zone_device_folio_init()

Yes. And the code location looks right to me.

>
> And maybe the naming has got really confused if we have both functions
> now :\

Yes. One of the issues is that device private code used to only handles
order-0 pages and was converted to use high order folio directly without
using high order page (namely compound page) as an intermediate step.
This two-step-in-one caused confusion. But the key thing to avoid the
confusion is that to form a high order folio, a list of contiguous pages
would become a compound page by calling prep_compound_page(), then
the compound page becomes a folio by calling folio_set_large_rmappable().

BTW, the code in prep_compound_head() after folio_set_order(folio, order)
should belong to folio_set_large_rmappable() and they are causing confusion,
since they are only applicable to rmappable large folios. I am going to
send a patch to fix it.


Best Regards,
Yan, Zi