Re: [RFC][alpha] saner vmalloc handling (was Re: [Bug report] hash_name() may cross page boundary and trigger sleep in RCU context)

[david laight]

On Sun, 30 Nov 2025 03:01:46 +0000
Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:

> On Sat, Nov 29, 2025 at 03:37:28AM +0000, Al Viro wrote:
> 
> > AFAICS, 32bit arm is similar to 32bit x86 in that respect; propagation
> > is lazier, though - there arch_sync_kernel_mappings() bumps a counter
> > in init_mm and context switches use that to check if propagation needs
> > to be done.  No idea how well does that work on vfree() side of things -
> > hadn't looked into that rabbit hole...  
> 
> BTW, speaking of vmalloc space - does anybody object against sorting
> CONFIG_ALPHA_LARGE_VMALLOC out, so that we wouldn't need to mess
> with that in alpha page fault handler?
> 
> Basically, do what amd64 does - something along the lines of (untested)
> patch below.  Comments?

How difficult would it be to allocate the pte for the next 8GB on demand
inside vmalloc(), and then propagate it to the per-task page tables.
That is a path than can sleep, so being slow if it needs to synchronise
with other cpu shouldn't matter - especially since it won't happen often.

That should be moderately generic code and would let the vmalloc limit
be 'soft'; perhaps based on physical memory size, and even be raisable
from a sysctl.

Likely more use for very large x86-64 and arm-64 systems than alpha.

	David