linux-kernel - Re: [PATCH v7] mm/page_alloc.c: memory_hotplug: free pages as higher order

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Message-ID: <20190109084031.GN31793@dhcp22.suse.cz>
Date:   Wed, 9 Jan 2019 09:40:31 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Arun KS <arunks@...eaurora.org>
Cc:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        arunks.linux@...il.com, akpm@...ux-foundation.org, vbabka@...e.cz,
        osalvador@...e.de, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, getarunks@...il.com
Subject: Re: [PATCH v7] mm/page_alloc.c: memory_hotplug: free pages as higher
 order

On Wed 09-01-19 13:58:50, Arun KS wrote:
> On 2019-01-09 13:07, Michal Hocko wrote:
> > On Wed 09-01-19 11:28:52, Arun KS wrote:
> > > On 2019-01-08 23:43, Michal Hocko wrote:
> > > > On Tue 08-01-19 09:56:09, Alexander Duyck wrote:
> > > > > On Fri, 2019-01-04 at 10:31 +0530, Arun KS wrote:
> > > > [...]
> > > > > >  static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
> > > > > >  			void *arg)
> > > > > >  {
> > > > > > -	unsigned long i;
> > > > > >  	unsigned long onlined_pages = *(unsigned long *)arg;
> > > > > > -	struct page *page;
> > > > > >
> > > > > >  	if (PageReserved(pfn_to_page(start_pfn)))
> > > > > > -		for (i = 0; i < nr_pages; i++) {
> > > > > > -			page = pfn_to_page(start_pfn + i);
> > > > > > -			(*online_page_callback)(page);
> > > > > > -			onlined_pages++;
> > > > > > -		}
> > > > > > +		onlined_pages = online_pages_blocks(start_pfn, nr_pages);
> > > > >
> > > > > Shouldn't this be a "+=" instead of an "="? It seems like you are
> > > > > going
> > > > > to lose your count otherwise.
> > > >
> > > > You are right of course. I should have noticed during the review.
> > > > Thanks!
> > > 
> > > I think we don't need to. The caller function is setting
> > > onlined_pages = 0
> > > before calling online_pages_range().
> > > And there are no other reference to online_pages_range other than from
> > > online_pages().
> > 
> > Are you missing that we accumulate onlined_pages via
> > 	*(unsigned long *)arg = onlined_pages;
> > in online_pages_range?
> 
> In my testing I didn't find any problem. To match the code being replaced
> and to avoid any corner cases, it is better to use +=
> Will update the patch.

Have you checked that the number of present pages both in the zone and
the node is correct because I fail to see how that would be possible.

-- 
Michal Hocko
SUSE Labs