Excessive TLB flush ranges

[Baoquan He]

On 05/16/23 at 08:40am, Thomas Gleixner wrote:
> On Tue, May 16 2023 at 10:26, Baoquan He wrote:
> > On 05/15/23 at 08:17pm, Uladzislau Rezki wrote:
> >> For systems which lack a full TLB flush and to flush a long range is
> >> a problem(it takes time), probably we can flush VA one by one. Because
> >> currently we calculate a flush range [min:max] and that range includes
> >> the space that might not be mapped at all. Like below:
> >
> > It's fine if we only calculate a flush range of [min:max] with VA. In
> > vm_reset_perms(), it calculates the flush range with the impacted direct
> > mapping range, then merge it with VA's range. That looks really strange
> > and surprising. If the vm->pages[] are got from a lower part of physical
> > memory, the final merged flush will span tremendous range. Wondering why
> > we need merge the direct map range with VA range, then do flush. Not
> > sure if I misunderstand it.
> 
> So what happens on this BPF teardown is:
> 
> The vfree(8k) ends up flushing 3 entries. The actual vmalloc part (2) and
> one extra which is in the direct map. I haven't verified that yet, but I
> assume it's the alias of one of the vmalloc'ed pages.

It looks like the reason. As Uladzislau pointed out, ARCH-es may
have full TLB flush, so won't get trouble from the merged flush
in the calculated [min:max] way, e.g arm64 and x86's flush_tlb_kernel_range().
However, arm32 seems lacking the ability of full TLB flash. If agreed, I
can make a draft patch to do the flush for direct map and VA seperately,
see if it works.