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

[Dragos Tatulea]

On 13.05.26 11:36, David Hildenbrand (Arm) wrote:
> 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 ...
> 
So do I get it right that the path forward here is to revert this commit [1]
and wait until the work from Zi Yan is ready?

[1] db359fccf212 ("mm: introduce a new page type for page pool in page type")

Thanks,
Dragos