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

[Dragos Tatulea]

On 24.02.26 06:13, Byungchul Park wrote:
> Currently, the condition 'page->pp_magic == PP_SIGNATURE' is used to
> determine if a page belongs to a page pool.  However, with the planned
> removal of @pp_magic, we should instead leverage the page_type in struct
> page, such as PGTY_netpp, for this purpose.
> 
> Introduce and use the page type APIs e.g. PageNetpp(), __SetPageNetpp(),
> and __ClearPageNetpp() instead, and remove the existing APIs accessing
> @pp_magic e.g. page_pool_page_is_pp(), netmem_or_pp_magic(), and
> netmem_clear_pp_magic().
> 
> Plus, add @page_type to struct net_iov at the same offset as struct page
> so as to use the page_type APIs for struct net_iov as well.  While at it,
> reorder @type and @owner in struct net_iov to avoid a hole and
> increasing the struct size.
> 
> This work was inspired by the following link:
> 
>   https://lore.kernel.org/all/582f41c0-2742-4400-9c81-0d46bf4e8314@xxxxxxxxx/
> 
> While at it, move the sanity check for page pool to on the free path.
> 
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Co-developed-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
> Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
> Signed-off-by: Byungchul Park <byungchul@xxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Acked-by: Zi Yan <ziy@xxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> ---

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?

diff --git a/mm/memory.c b/mm/memory.c
index ea6568571131..4cb12673f450 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2326,7 +2326,7 @@ static int validate_page_before_insert(struct vm_area_struct *vma,
                        return -EINVAL;
                return 0;
        }
-       if (folio_test_anon(folio) || page_has_type(page))
+       if (folio_test_anon(folio) || (page_has_type(page) && !PageNetpp(page)))
                return -EINVAL;
        flush_dcache_folio(folio);
        return 0;

Thanks,
Dragos