linux-kernel - Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE

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Date:   Wed, 20 Apr 2022 11:28:07 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Song Liu <songliubraving@...com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "song@...nel.org" <song@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "pmladek@...e.com" <pmladek@...e.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "dborkman@...hat.com" <dborkman@...hat.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "bp@...en8.de" <bp@...en8.de>, "mbenes@...e.cz" <mbenes@...e.cz>,
        "imbrenda@...ux.ibm.com" <imbrenda@...ux.ibm.com>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE_VMAP

On Wed, Apr 20, 2022 at 02:42:37PM +0000, Song Liu wrote:
> For (b), we have seen direct map fragmentation causing visible
> performance drop for our major services. This is the shadow 
> production benchmark, so it is not possible to run it out of 
> our data centers. Tracing showed that BPF program was the top 
> trigger of these direct map splits. 

It's often not easy to reproduce issues like these, but I've
ran into that before for other Proof of Concept issues before
and the solution has been a Linux selftest. For instance a
"multithreaded" bombing for kmod can be triggered with
lib/test_kmod.c and tools/testing/selftests/kmod/kmod.sh

Would desinging a selftest to abuse eBPF JIT be a possible
way to reproduce the issue?

  Luis