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

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

On 6/9/26 00:28, Gregory Price wrote:
> 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@casper.infradead.org/
>> [2] https://lore.kernel.org/lkml/20260426165330-mutt-send-email-mst@kernel.org/
>>
> 
> Hm, rewinding on this back to v3 here:

I'm late ...

> https://lore.kernel.org/lkml/016cc5e5-044c-46c6-a668-200f90a64d85@kernel.org/
> 
> 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.

Exactly.

> 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

Yes. And the hugetlb stuff should just be left alone for now. We don't encourage
new hugetlb features, and the same should apply to new optimizations that are
not straight forward.

> 
> 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's a bit related to your private node work .... if we have an interface that
consumes an alloc_context, we could just forward the address easily.

Now, there are different ways of doing it, but having folio allocation sometimes
use GFP_ZERO and sometimes not is just nasty. It's all a big hack.

> 
> 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.

Yes. And once there is only one user remaining, we could inline it and get rid
of this horrible helper :)

-- 
Cheers,

David