linux-kernel - Re: [RESEND PATCH] mm/hotplug: don't reset pagetype flags for offline

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Date:   Tue, 12 Mar 2019 16:35:06 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com, vbabka@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH] mm/hotplug: don't reset pagetype flags for offline

On Sun, Mar 10, 2019 at 04:01:02PM -0400, Qian Cai wrote:
> The commit f1dd2cd13c4b ("mm, memory_hotplug: do not associate hotadded
> memory to zones until online") introduced move_pfn_range_to_zone() which
> calls memmap_init_zone() during onlining a memory block.
> memmap_init_zone() will reset pagetype flags and makes migrate type to
> be MOVABLE.
> 
> However, in __offline_pages(), it also call undo_isolate_page_range()
> after offline_isolated_pages() to do the same thing. Due to
> the commit 2ce13640b3f4 ("mm: __first_valid_page skip over offline
> pages") changed __first_valid_page() to skip offline pages,
> undo_isolate_page_range() here just waste CPU cycles looping around the
> offlining PFN range while doing nothing, because __first_valid_page()
> will return NULL as offline_isolated_pages() has already marked all
> memory sections within the pfn range as offline via
> offline_mem_sections().
> 
> Also, after calling the "useless" undo_isolate_page_range() here, it
> reaches the point of no returning by notifying MEM_OFFLINE. Those pages
> will be marked as MIGRATE_MOVABLE again once onlining. In addition, fix
> an incorrect comment along the way.
> 
> Signed-off-by: Qian Cai <cai@....pw>

Reviewed-by: Oscar Salvador <osalvador@...e.de>

One thing I noticed when looking at start_isolate_page_range and 
undo_isolate_page_range functions, is that start_isolate_page_range increments
the number of isolated pageblocks, while undo_isolate_page_range does the counter
part.
Since undo_isolate_page_range is really never called during offlining,
we leave zone->nr_isolate_pageblock with a stale value.

I __think__  this does not matter much.
We only get to check whether a zone got isolated pageblocks in
has_isolate_pageblock(), and this is called from:

free_one_page
free_pcppages_bulk
__free_one_page

With a quick glance, the only difference in has_isolate_pageblock() returning
true or false, seems to be that those functions perform some extra checks in
case the zone reports to have isolated pageblocks.

I wonder if we should set nr_isolate_pageblock back to its original value
before start_isolate_page_range.

> ---
>  mm/memory_hotplug.c | 2 --
>  mm/sparse.c         | 2 +-
>  2 files changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 6b05576fb4ec..46017040b2f8 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1655,8 +1655,6 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  	/* Ok, all of our target is isolated.
>  	   We cannot do rollback at this point. */
>  	offline_isolated_pages(start_pfn, end_pfn);
> -	/* reset pagetype flags and makes migrate type to be MOVABLE */
> -	undo_isolate_page_range(start_pfn, end_pfn, MIGRATE_MOVABLE);
>  	/* removal success */
>  	adjust_managed_page_count(pfn_to_page(start_pfn), -offlined_pages);
>  	zone->present_pages -= offlined_pages;
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 77a0554fa5bd..b3771f35a0ed 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -556,7 +556,7 @@ void online_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>  }
>  
>  #ifdef CONFIG_MEMORY_HOTREMOVE
> -/* Mark all memory sections within the pfn range as online */
> +/* Mark all memory sections within the pfn range as offline */
>  void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
>  {
>  	unsigned long pfn;
> -- 
> 2.17.2 (Apple Git-113)
> 

-- 
Oscar Salvador
SUSE L3