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Wed, 08 Sep 2021 12:42:15 -0700 (PDT) Received: from ast-mbp.dhcp.thefacebook.com ([2620:10d:c090:500::4:97c5]) by smtp.gmail.com with ESMTPSA id h8sm17894pfr.219.2021.09.08.12.42.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 08 Sep 2021 12:42:14 -0700 (PDT) Date: Wed, 8 Sep 2021 12:42:12 -0700 From: Alexei Starovoitov To: Liam Howlett Cc: Andrew Morton , Luigi Rizzo , Yonghong Song , Peter Zijlstra , Jason Gunthorpe , Daniel Borkmann , Michel Lespinasse , bpf , "linux-mm@kvack.org" , Alexei Starovoitov , Andrii Nakryiko , "kernel-team@fb.com" Subject: Re: [PATCH mm/bpf v2] mm: bpf: add find_vma_no_check() without lockdep_assert on mm->mmap_lock Message-ID: <20210908194212.ku7ysko4e4ecca2t@ast-mbp.dhcp.thefacebook.com> References: <20210908135326.GZ1200268@ziepe.ca> <20210908151230.m2zyslt4qrufm4bv@revolver> <20210908172118.n2f4w7epm6hh62zf@ast-mbp.dhcp.thefacebook.com> <20210908105259.c47dcc4e4371ebb5e147ee6e@linux-foundation.org> <20210908180258.yjh62e5oouckar5b@ast-mbp.dhcp.thefacebook.com> <20210908184342.r3bwp7v24a6tnslg@revolver> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210908184342.r3bwp7v24a6tnslg@revolver> X-Stat-Signature: dpyezpbsyd4whghxdadpbuftfyd4qb4e Authentication-Results: imf09.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=Ej3KOl9A; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf09.hostedemail.com: domain of alexei.starovoitov@gmail.com designates 209.85.216.48 as permitted sender) smtp.mailfrom=alexei.starovoitov@gmail.com X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: E75723000103 X-HE-Tag: 1631130135-981874 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 08, 2021 at 06:43:49PM +0000, Liam Howlett wrote: > * Alexei Starovoitov [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 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.