linux-kernel - Re: [PATCH V6 3/3] arm64/mm: Enable memory hot remove

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Message-ID: <20190625103128.GA12207@lakrids.cambridge.arm.com>
Date:   Tue, 25 Jun 2019 11:31:29 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Steve Capper <Steve.Capper@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <Will.Deacon@....com>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "ira.weiny@...el.com" <ira.weiny@...el.com>,
        "david@...hat.com" <david@...hat.com>, "cai@....pw" <cai@....pw>,
        "logang@...tatee.com" <logang@...tatee.com>,
        James Morse <James.Morse@....com>,
        "cpandya@...eaurora.org" <cpandya@...eaurora.org>,
        "arunks@...eaurora.org" <arunks@...eaurora.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "osalvador@...e.de" <osalvador@...e.de>,
        Ard Biesheuvel <Ard.Biesheuvel@....com>, nd <nd@....com>
Subject: Re: [PATCH V6 3/3] arm64/mm: Enable memory hot remove

On Tue, Jun 25, 2019 at 10:57:07AM +0530, Anshuman Khandual wrote:
> On 06/24/2019 10:22 PM, Mark Rutland wrote:
> > On Fri, Jun 21, 2019 at 03:35:53PM +0100, Steve Capper wrote:
> >> On Wed, Jun 19, 2019 at 09:47:40AM +0530, Anshuman Khandual wrote:
> >>> +static void free_hotplug_page_range(struct page *page, size_t size)
> >>> +{
> >>> +	WARN_ON(!page || PageReserved(page));
> >>> +	free_pages((unsigned long)page_address(page), get_order(size));
> >>> +}
> >>
> >> We are dealing with power of 2 number of pages, it makes a lot more
> >> sense (to me) to replace the size parameter with order.
> >>
> >> Also, all the callers are for known compile-time sizes, so we could just
> >> translate the size parameter as follows to remove any usage of get_order?
> >> PAGE_SIZE -> 0
> >> PMD_SIZE -> PMD_SHIFT - PAGE_SHIFT
> >> PUD_SIZE -> PUD_SHIFT - PAGE_SHIFT
> > 
> > Now that I look at this again, the above makes sense to me.
> > 
> > I'd requested the current form (which I now realise is broken), since
> > back in v2 the code looked like:
> > 
> > static void free_pagetable(struct page *page, int order)
> > {
> > 	...
> > 	free_pages((unsigned long)page_address(page), order);
> > 	...
> > }
> > 
> > ... with callsites looking like:
> > 
> > free_pagetable(pud_page(*pud), get_order(PUD_SIZE));
> > 
> > ... which I now see is off by PAGE_SHIFT, and we inherited that bug in
> > the current code, so the calculated order is vastly larger than it
> > should be. It's worrying that doesn't seem to be caught by anything in
> > testing. :/
> 
> get_order() returns the minimum page allocation order for a given size
> which already takes into account PAGE_SHIFT i.e get_order(PAGE_SIZE)
> returns 0.

Phew.

Let's leave this as is then -- it's clearer/simpler than using the SHIFT
constants, performance isn't a major concern in this path, and it's very
likely that GCC will inline and constant-fold this away regardless.

Sorry for the noise, and thanks for correcting me.

Mark.