Re: [PATCH v10 07/37] mm: thread user_addr through page allocator for cache-friendly zeroing

[Gregory Price]

On Mon, Jun 08, 2026 at 11:51:47PM +0200, David Hildenbrand (Arm) wrote:
> On 6/8/26 23:16, Zi Yan wrote:
> 
> There was Willy's comment in RFC v3 [1], which had 19 patches. Unfortunately, he
> no longer followed up to my initial push back and Michael's question later [2].
> 
> That would have probably been the right time to wait for more discussion.
> 
> RFC v4 had 22 patches with little replies.
> v5 had 28 patches with little replies.
> v6 had 30 patches with no replies.
> v7 had 31 patches with little replies.
> v8 had 37 patches with no replies.
> 
> [1] https://lore.kernel.org/lkml/aeu5P1bZW3yEH54t@xxxxxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/20260426165330-mutt-send-email-mst@xxxxxxxxxx/
>

Hm, rewinding on this back to v3 here:
https://lore.kernel.org/lkml/016cc5e5-044c-46c6-a668-200f90a64d85@xxxxxxxxxx/

You said:

  ```
  Exactly, that's why I am saying that vma_alloc_folio() is the only
  external interface people should be using with a user address.
  ```

Going through the list of folio_zero_user references:

Called unconditionally if a folio is acquired:
   fs/hugetlbfs/inode.c:   folio_zero_user(folio, addr);
   mm/hugetlb.c:           folio_zero_user(folio, vmf->real_address);
   mm/memfd.c:             folio_zero_user(folio, 0);

Called when user_alloc_needs_zeroing() and charging passes:
   mm/huge_memory.c:       folio_zero_user(folio, addr);
   mm/memory.c:            folio_zero_user(folio, vmf->address);

No one outside mm/ should know about this interface at all.
Arguably none of these should know about this interface either.

The appropriate place for this logic appears to be:
    vma_alloc_folio
    alloc_hugetlb_folio
    alloc_hugetlb_folio_reserve

The reason to sink it into the post_alloc_hook is to let the buddy
decide whether the page actually needs to be zeroed (like the virtio
situation) based on PG_zeroed or whatever.

It seems like at a minimum moving the logic all the way into
post_alloc_hook lets us actually delete folio_zero_user() as a published
interface and move it entirely within page_alloc.c.

The catch is user_alloc_needs_zeroing() coming along with it.

~Gregory