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

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Date:   Wed, 20 Apr 2022 14:42:37 +0000
From:   Song Liu <songliubraving@...com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "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>
Subject: Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE_VMAP

Hi Linus,

> On Apr 19, 2022, at 7:18 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> On Tue, Apr 19, 2022 at 7:03 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
>> 
>> Here is the quote from Song's cover letter for bpf_prog_pack series:
> 
> I care about performance as much as the next person, but I care about
> correctness too.
> 
> That large-page code was a disaster, and was buggy and broken.
> 
> And even with those four patches, it's still broken.
> 
> End result: there's no way that thigh gets re-enabled without the
> correctness being in place.
> 
> At a minimum, to re-enable it, it needs (a) that zeroing and (b)
> actual numbers on real loads. (not some artificial benchmark).
> 
> Because without (a) there's no way in hell I'll enable it.
> 
> And without (b), "performance" isn't actually an argument.

I will send patch to do (a) later this week. 

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. 

Thanks,
Song