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

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Message-Id: <20190109105652.40e24fa969a2bb7a58e097a8@linux-foundation.org>
Date:   Wed, 9 Jan 2019 10:56:52 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Arun KS <arunks@...eaurora.org>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        arunks.linux@...il.com, 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 Jan 2019 16:36:36 +0530 Arun KS <arunks@...eaurora.org> wrote:

> On 2019-01-09 16:27, Michal Hocko wrote:
> > On Wed 09-01-19 16:12:48, Arun KS wrote:
> > [...]
> >> It will be called once per online of a section and the arg value is 
> >> always
> >> set to 0 while entering online_pages_range.
> > 
> > You rare right that this will be the case in the most simple scenario.
> > But the point is that the callback can be called several times from
> > walk_system_ram_range and then your current code wouldn't work 
> > properly.
> 
> Thanks. Will use +=

The v8 patch
https://lore.kernel.org/lkml/1547032395-24582-1-git-send-email-arunks@codeaurora.org/T/#u

(which you apparently sent 7 minutes after typing the above) still has

 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);


Even then the code makes no sense.

static int online_pages_range(unsigned long start_pfn, unsigned long nr_pages,
			void *arg)
{
	unsigned long onlined_pages = *(unsigned long *)arg;

	if (PageReserved(pfn_to_page(start_pfn)))
		onlined_pages += online_pages_blocks(start_pfn, nr_pages);

	online_mem_sections(start_pfn, start_pfn + nr_pages);

	*(unsigned long *)arg += onlined_pages;
	return 0;
}

Either the final assignment should be

	*(unsigned long *)arg = onlined_pages;

or the initialization should be

	unsigned long onlined_pages = 0;



This is becoming a tad tiresome and I'd prefer not to have to check up
on such things.  Can we please get this right?