linux-kernel - Re: [PATCH 1/3] mm: vmalloc: Let user to control huge vmalloc default behavior

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Message-ID: <YckwuA8CWcQpQdGO@casper.infradead.org>
Date:   Mon, 27 Dec 2021 03:19:20 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 1/3] mm: vmalloc: Let user to control huge vmalloc
 default behavior

On Mon, Dec 27, 2021 at 09:44:24AM +0800, Kefeng Wang wrote:
> 
> On 2021/12/27 1:36, Christophe Leroy wrote:
> > 
> > Le 26/12/2021 à 09:39, Kefeng Wang a écrit :
> > > Add HUGE_VMALLOC_DEFAULT_ENABLED to let user to choose whether or
> > > not enable huge vmalloc mappings by default, and this could make
> > > more architectures to enable huge vmalloc mappings feature but
> > > don't want to enable it by default.
> > > 
> > > Add hugevmalloc=on/off parameter to enable or disable this feature
> > > at boot time, nohugevmalloc is still supported and equivalent to
> > > hugevmalloc=off.
> > 
> > Is there a real added value to have the user be able to select that ?
> > 
> > If the architecture supports it, is there any good reason to not use it ?
> 
> There are some disadvantages[1],  one of the main concerns is the possible
> 
> memory waste, we have backported this feature to our kernel 5.10, but our
> 
> downstream in our some scenario(especially in embedded), they don't want
> 
> it enabled by default, and others want it, this is why patch1 comes.
> 
> > 
> > Why not just do like PPC and enable it by default ? Why should it be
> > enabled by default on PPC but disabled by default on ARM64 and X86 ?
> 
> The PPC is default enabled, we don't changes this behavior.
> 
> Maybe upstream is not care about this, as I said in cover-letter, if
> arm64/x86
> 
> don't want patch1, we could only just select config to enable it.
> 
> Let's wait for more feedback.

We should not have different defaults by architecture.  Either we change
the default for PPC, or x86 & arm should have the same default as PPC.