Re: [PATCH mm/bpf v2] mm: bpf: add find_vma_no_check() without lockdep_assert on mm->mmap_lock

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Sep 08, 2021 at 06:43:49PM +0000, Liam Howlett wrote:
> * Alexei Starovoitov <alexei.starovoitov@gmail.com> [210908 14:03]:
> > On Wed, Sep 08, 2021 at 10:52:59AM -0700, Andrew Morton wrote:
> > > On Wed, 8 Sep 2021 10:21:18 -0700 Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> > > 
> > > > > Again I am ignorant on the details so if you can clarify the following
> > > > > it may help me and others to better understand the problem:
> > > > > 
> > > > > 1. Peter's patch appears to just take the same "fallback" path
> > > > >    that would be taken if the trylock failed.
> > > > >    Is this really a breakage or just loss of performance ?
> > > > >    I would expect the latter, since it is called "fallback".
> > > > 
> > > > As Yonghong explained it's a user space breakage.
> > > > User space tooling expects build_id to be available 99.999% of the time
> > > > and that's what users observed in practice.
> > > > They've built a bunch of tools on top of this feature.
> > > > The data from these tools goes into various datacenter tables
> > > > and humans analyze it later.
> > > > So Peter's proposal is not acceptable. We don't want to get yelled at.
> > > > 
> > > 
> > > I'm not understanding.  Peter said "this patch merely removes a
> > > performance tweak" and you and Yonghong said "it breaks userspace". 
> > > These assertions are contradictory!
> > 
> > Peter said:
> > "The only sane approach is making the vma tree lockless, but so far the
> >  bpf people have resisted doing the right thing because they've been
> >  allowed to get away with these atrocities.
> > "
> > which is partially true.
> > bpf folks didn't resist it. There is work ongoing to make it lockless.
> > It just takes an long time. I don't see how bpf folks can speed it up
> > any further.
> 
> What work are you doing on a lockless vma tree?  I've been working on
> the maple tree and would like to hear what you've come up with.

Mainly cheering Michel and Paul from sidelines.
imo any approach would be better than the current state.