Re: [PATCH v10 12/37] mm: use folio_zero_user for user pages in post_alloc_hook

[Gregory Price]

On Mon, Jun 08, 2026 at 03:45:58PM -0400, Michael S. Tsirkin wrote:
> On Mon, Jun 08, 2026 at 11:53:40AM -0400, Gregory Price wrote:
> > 
> > If `user_addr` is now implying anything other than exactly: "This needs
> > to be zeroed / caches flushed", then this is bad.
> > 
> > ~Gregory
> 
> Well if you do folio_zero_user in a non sleepable context then things
> are not going to work. So combining e.g. GFP_ATOMIC and GFP_ZERO and
> user_addr all together is not a good idea.
>

Can you say whether (GFP_ATOMIC | GFP_ZERO) w/o user_addr has the same
issue?  If not, then this subtle complexity is now a tripping hazard.

Is there some combination of arguments here that should just outright
fail if a user attempts it?

>
> You are saying it's bad? It's pretty fundamental to the idea of moving
> zeroing into the allocator, I feel.
> 

I'm saying having to infer that safety state from the cobbling of those
things together is not a good pattern (at least as-is).

If the introduction of user_addr into the mix is the thing that causes
us to have to infer safety, then there's an argument the page allocator
shouldn't handle that operation (in this case: user_addr cache flush).

Please consider that this is arguably the most fundamental interface in
in all of mm/.  All we're doing is going through the process of figuring
out what changes here are reasonable while trying to meet your goal.

~Gregory