Re: [PATCH resend v6 03/30] mm: thread user_addr through page allocator for cache-friendly zeroing

[Gregory Price]

On Mon, May 11, 2026 at 05:59:23PM -0400, Michael S. Tsirkin wrote:
> On Mon, May 11, 2026 at 12:52:10PM -0400, Gregory Price wrote:
> > On Mon, May 11, 2026 at 11:55:40AM -0400, Michael S. Tsirkin wrote:
> > > On Mon, May 11, 2026 at 11:37:37AM -0400, Gregory Price wrote:
> > > > On Mon, May 11, 2026 at 05:01:55AM -0400, Michael S. Tsirkin wrote:
> > > > 
> > > > > +/*
> > > > > + * Sentinel for user_addr: indicates a non-user allocation.
> > > > > + * Cannot use 0 because address 0 is a valid userspace mapping.
> > > > > + */
> > > > > +#define USER_ADDR_NONE	((unsigned long)-1)
> > > > 
> > > > Ehm, hm.  Does -1 hold as a non-user address across all architectures?
> > > > 
> > > > What about in linear addressing / no VM mode?
> > > 
> > > this is used on a fault. I don't think there are any faults then?
> > > But maybe FAULT_ADDR_NONE would be clearer.
> > >
> > 
> > Meh, naming here is less relevant than the sentinel correctness.
> > 
> > My only concern is really whether -1 could end up being a valid address
> > in some horrid future timeline and this all going belly up.
> > 
> > Is why I asked about whether this is correct on all architectures.
> 
> I think the answer is yes: on all architectures Linux supports, the last
> page of the address space is never a valid user mapping. The kernel
> enforces this -- mmap will not create a mapping whose end wraps past -1.
> 
>   if (addr > TASK_SIZE - len)
>       return -ENOMEM;
> 
> 
> So the maximum vm_end = addr + len <= TASK_SIZE.
> On every architecture TASK_SIZE <= (unsigned long)-1, so vm_end can
> never wrap around to 0 and address -1 can never be within any
> VMA.
> 

That is worth documenting at least in the changelog if not the comment
with the define there's a clue if anything ever changes.

~Gregory