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[209.85.220.65]) by mx.google.com with SMTPS id y4sor6424454ybp.147.2019.02.26.09.03.43 for (Google Transport Security); Tue, 26 Feb 2019 09:03:43 -0800 (PST) Received-SPF: pass (google.com: domain of dennisszhou@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; spf=pass (google.com: domain of dennisszhou@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=dennisszhou@gmail.com; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=kernel.org X-Google-Smtp-Source: AHgI3IbqGExLy62XrfPHcYlcJbEhfIUGhyPvfdWuLAX37FLp24NgOgc5mpag2drjeqrr1nXDyPE1Cg== X-Received: by 2002:a25:d44c:: with SMTP id m73mr18018663ybf.349.1551200623043; Tue, 26 Feb 2019 09:03:43 -0800 (PST) Received: from dennisz-mbp.dhcp.thefacebook.com ([2620:10d:c091:200::2:7f17]) by smtp.gmail.com with ESMTPSA id 207sm3527372ywm.67.2019.02.26.09.03.41 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 26 Feb 2019 09:03:42 -0800 (PST) Date: Tue, 26 Feb 2019 12:03:39 -0500 From: Dennis Zhou To: Christopher Lameter Cc: Peng Fan , "tj@kernel.org" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , "van.freenix@gmail.com" Subject: Re: [PATCH 1/2] percpu: km: remove SMP check Message-ID: <20190226170339.GB47262@dennisz-mbp.dhcp.thefacebook.com> References: <20190224132518.20586-1-peng.fan@nxp.com> <20190225151330.GA49611@dennisz-mbp.dhcp.thefacebook.com> <010001692a612815-46229701-ea3f-4a89-8f88-0c74194ba257-000000@email.amazonses.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <010001692a612815-46229701-ea3f-4a89-8f88-0c74194ba257-000000@email.amazonses.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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 Tue, Feb 26, 2019 at 03:16:44PM +0000, Christopher Lameter wrote: > On Mon, 25 Feb 2019, Dennis Zhou wrote: > > > > @@ -27,7 +27,7 @@ > > > * chunk size is not aligned. percpu-km code will whine about it. > > > */ > > > > > > -#if defined(CONFIG_SMP) && defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK) > > > +#if defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK) > > > #error "contiguous percpu allocation is incompatible with paged first chunk" > > > #endif > > > > > > -- > > > 2.16.4 > > > > > > > Hi, > > > > I think keeping CONFIG_SMP makes this easier to remember dependencies > > rather than having to dig into the config. So this is a NACK from me. > > But it simplifies the code and makes it easier to read. > > I think the check isn't quite right after looking at it a little longer. Looking at x86, I believe you can compile it with !SMP and CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK will still be set. This should still work because x86 has an MMU. I think more correctly it would be something like below, but I don't have the time to fully verify it right now. Thanks, Dennis --- diff --git a/mm/percpu-km.c b/mm/percpu-km.c index 0f643dc2dc65..69ccad7d9807 100644 --- a/mm/percpu-km.c +++ b/mm/percpu-km.c @@ -27,7 +27,7 @@ * chunk size is not aligned. percpu-km code will whine about it. */ -#if defined(CONFIG_SMP) && defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK) +#if !defined(CONFIG_MMU) && defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK) #error "contiguous percpu allocation is incompatible with paged first chunk" #endif