Re: [PATCH v4] mm: introduce a new page type for page pool in page type

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/13/26 11:26, Pedro Falcato wrote:
> On Wed, May 13, 2026 at 11:12:43AM +0200, Vlastimil Babka (SUSE) wrote:
>> On 5/13/26 11:00, Dragos Tatulea wrote:
>>>
>>>
>>>
>>> Seems like this patch broke tcp_mmap because
>>> validate_page_before_insert() returns -EINVAL due
>>> to a page having a type. Here's the full flow:
>>>
>>> getsockopt(TCP_ZEROCOPY_RECEIVE) returns -EINVAL because of the
>>> below flow in the kernel:
>>>
>>> tcp_zerocopy_receive()
>>> -> tcp_zerocopy_vm_insert_batch()
>>>   -> vm_insert_pages()
>>>     -> insert_pages()
>>>       -> insert_page_in_batch_locked()
>>>         -> validate_page_before_insert() returns -EINVAL
>>>            because page_has_type(page) is now true.
>>>
>>> The patch below fixes the issue. But is this a valid fix?
>>
>> Hmm the check traces back to commit 0ee930e6cafa0 "mm/memory.c: prevent
>> mapping typed pages to userspace"
>>
>>> Pages which use page_type must never be mapped to userspace as it would
>>> destroy their page type.  Add an explicit check for this instead of
>>> assuming that kernel drivers always get this right.
>>
>> So uh, this doesn't look good I think.
> 
> Yep, you fundamentally can't map a page with a type as page type aliases with
> mapcount. Even with the given diff, just mapping it will increment the mapcount
> and wreak havoc. I think we need to revert this patch for now.
> 
> I'm not sure what the long term plan for this would be. If page types are moved
> to memdesc types, then the two stop colliding and that could work. I don't know
> if that's Willy's plan, however.
> 
> (then there's the other question: are page pool pages really folios? not really.
> they are mappable, but they aren't part of the page cache, or anon, nor are
> they in the LRU or have rmap capabilities. perhaps we need a different memdesc
> for those. we're one step away from reinventing class polymorphism from first
> principles ;)

Zi Yan is working on this: non-folio pages would no longer mess with
rmap/mapcounts, and page table walking code will identify them to be non-folio
things to skip them.

It will take a while, though ...

-- 
Cheers,

David