linux-kernel - Re: [PATCH 5/6] mm, memory_hotplug: Provide argument for the pgprot_t in arch_add

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Date:   Wed, 11 Dec 2019 09:37:45 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-ia64@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Linux-sh <linux-sh@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 5/6] mm, memory_hotplug: Provide argument for the
 pgprot_t in arch_add_memory()

On Tue 10-12-19 16:52:31, Logan Gunthorpe wrote:
[...]
> In my opinion, having a coder and reviewer see PAGE_KERNEL and ask if
> that makes sense is a benefit. Having it hidden because we don't want
> people to think about it is worse, harder to understand and results in
> bugs that are more difficult to spot.

My experience would disagree here. We have several examples in the MM
where an overly complex and versatile APIs led to suble bugs, a lot of
copy&pasting and cargo cult programing (just look at the page allocator
as a shiny example - e.g. gfp_flags). So I am always trying to be
carefull here.

> Though, we may be overthinking this: arch_add_memory() is a low level
> non-exported API that's currently used in exactly two places.

This is a fair argument. Most users are and should be using
add_memory().

> I don't
> think there's going to be many, if any, valid new use cases coming up
> for it in the future. That's more what memremap_pages() is for.

OK, fair enough. If this is indeed the simplest way forward then I will
not stand in the way.
-- 
Michal Hocko
SUSE Labs