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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 8 Jun 2026 15:45:58 -0400

On Mon, Jun 08, 2026 at 11:53:40AM -0400, Gregory Price wrote:
> On Mon, Jun 08, 2026 at 12:23:07PM +0100, Lorenzo Stoakes wrote:
> > On Mon, Jun 08, 2026 at 04:36:38AM -0400, Michael S. Tsirkin wrote:
> > 
> > > +			 * user_addr != USER_ADDR_NONE implies sleepable
> > > +			 * context (user page fault).
> > 
> > Can you safely assume that? Also inferring which context we are in from this
> > parameter seems risky.
> > 
> > It seems to me that you're now making it such that kernel developers:
> > 
> > - Have to know when and when not to specify a user address, and under what
> >   circumstances we might consider that to be mapped.
> > 
> > - Need to know to do this correctly for aliasing architectures or have silent
> >   correctness issues.
> > 
> > - Need to take context into account when specifying this.
> > 
> > We definitely need to find a simpler way to do this!
> >
> 
> This feedback was poked at in earlier versions.  There's a tension
> between keeping the old interface as-is, having explicit interfaces
> for something like this, and the state of a page inside the
> allocator vs outside.
> 
> Double-plus complicated by the fact that we're trying to reason about
> two allocators at once:  host and guest.
> 
> It seems it has gotten a bit more complicated since then (I missed this
> "sleepable context" bit, not sure if it was there on prior versions).
> 
> 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.

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

-- 
MST