Re: [PATCH] vmalloc: Don't use flush flag when no exec perm

[Edgecombe, Rick P]

On Tue, 2019-05-28 at 22:51 -0700, Rick Edgecombe wrote:
> The addition of VM_FLUSH_RESET_PERMS for BPF JIT allocations was
> bisected to prevent boot on an UltraSparc III machine. It was found
> that
> sometime shortly after the TLB flush this flag does on vfree of the
> BPF
> program, the machine hung. Further investigation showed that before
> any of
> the changes for this flag were introduced, with
> CONFIG_DEBUG_PAGEALLOC
> configured (which does a similar TLB flush of the vmalloc range on
> every vfree), this machine also hung shortly after the first vmalloc
> unmap/free.
> 
> So the evidence points to there being some existing issue with the
> vmalloc TLB flushes, but it's still unknown exactly why these hangs
> are
> happening on sparc. It is also unknown when someone with this
> hardware
> could resolve this, and in the meantime using this flag on it turns a
> lurking behavior into something that prevents boot.

The sparc TLB flush issue has been bisected and is being worked on now,
so hopefully we won't need this patch:
https://marc.info/?l=linux-sparc&m=155915694304118&w=2