Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce wrprotect_tlb_flush_pending

[Andrea Arcangeli <aarcange@redhat.com>]

On Fri, Jan 08, 2021 at 11:25:21AM -0800, Linus Torvalds wrote:
> On Fri, Jan 8, 2021 at 9:53 AM Andrea Arcangeli <aarcange@redhat.com> wrote:
> >
> > Do you intend to eventually fix the zygote vmsplice case or not?
> > Because in current upstream it's not fixed currently using the
> > enterprise default config.
> 
> Is this the hugepage case? Neither of your patches actually touched
> that, so I've forgotten the details.

The two patches only fixed the TLB flushing deferral in clear_refs and
uffd-wp.

So I didn't actually try to fix the hugepage case by adding the
page_count checks there too. I could try to do that at least it'd be
consistent but I still would try to find an alternate solution later.

> > Irrelevant special case as in: long term GUP pin on the memory?
> 
> Irrelevant special case in that
> 
>  (a) an extra COW shouldn't be a correctness issue unless somebody
> does something horribly wrong (and obviously the code that hasn't
> taken the mmap_lock for writing are then examples of that)
> 
> and
> 
>  (b) it's not a performance issue either unless you can find a real
> load that does it.
> 
> Hmm?

For b) I don't have an hard time to imagine `ps` hanging for seconds,
if clear_refs is touched on a 4T mm, but b) is not the main
concern.

Having to rely on a) is the main concern and it's not about tlb
flushes but the long term GUP pins.

Thanks,
Andrea