Re: [PATCH v2] mm/hotplug: fix offline undo_isolate_page_range()

[Michal Hocko <mhocko@kernel.org>]

On Wed 13-03-19 10:31:33, 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. The only thing
> left to do is to decrease the number of isolated pageblocks zone
> counter which would make some paths of the page allocation slower that
> the above commit introduced. A memory block is usually at most 1GiB in
> size, so an "int" should be enough to represent the number of pageblocks
> in a block. Fix an incorrect comment along the way.
> 
> Fixes: 2ce13640b3f4 ("mm: __first_valid_page skip over offline pages")

Cc: stable # 4.13+

> Signed-off-by: Qian Cai <cai@lca.pw>

Yes, this looks good.

Acked-by: Michal Hocko <mhocko@suse.com>

I would just ask you to add a return value of start_isolate_page_range documentation.

Thanks!
> ---
> 
> v2: return the nubmer of isolated pageblocks in start_isolate_page_range() per
>     Oscar; take the zone lock when undoing zone->nr_isolate_pageblock per
>     Michal.
> 
>  mm/memory_hotplug.c | 17 +++++++++++++----
>  mm/page_alloc.c     |  2 +-
>  mm/page_isolation.c | 16 ++++++++++------
>  mm/sparse.c         |  2 +-
>  4 files changed, 25 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index cd23c081924d..8ffe844766da 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1580,7 +1580,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  {
>  	unsigned long pfn, nr_pages;
>  	long offlined_pages;
> -	int ret, node;
> +	int ret, node, count;
>  	unsigned long flags;
>  	unsigned long valid_start, valid_end;
>  	struct zone *zone;
> @@ -1606,10 +1606,11 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  	ret = start_isolate_page_range(start_pfn, end_pfn,
>  				       MIGRATE_MOVABLE,
>  				       SKIP_HWPOISON | REPORT_FAILURE);
> -	if (ret) {
> +	if (ret < 0) {
>  		reason = "failure to isolate range";
>  		goto failed_removal;
>  	}
> +	count = ret;
>  
>  	arg.start_pfn = start_pfn;
>  	arg.nr_pages = nr_pages;
> @@ -1661,8 +1662,16 @@ 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);
> +
> +	/*
> +	 * Onlining will reset pagetype flags and makes migrate type
> +	 * MOVABLE, so just need to decrease the number of isolated
> +	 * pageblocks zone counter here.
> +	 */
> +	spin_lock_irqsave(&zone->lock, flags);
> +	zone->nr_isolate_pageblock -= count;
> +	spin_unlock_irqrestore(&zone->lock, flags);
> +
>  	/* removal success */
>  	adjust_managed_page_count(pfn_to_page(start_pfn), -offlined_pages);
>  	zone->present_pages -= offlined_pages;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 03fcf73d47da..d96ca5bc555b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8233,7 +8233,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  
>  	ret = start_isolate_page_range(pfn_max_align_down(start),
>  				       pfn_max_align_up(end), migratetype, 0);
> -	if (ret)
> +	if (ret < 0)
>  		return ret;
>  
>  	/*
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index ce323e56b34d..bf67b63227ca 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -172,7 +172,8 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
>   * future will not be allocated again.
>   *
>   * start_pfn/end_pfn must be aligned to pageblock_order.
> - * Return 0 on success and -EBUSY if any part of range cannot be isolated.
> + * Return the number of isolated pageblocks on success and -EBUSY if any part of
> + * range cannot be isolated.
>   *
>   * There is no high level synchronization mechanism that prevents two threads
>   * from trying to isolate overlapping ranges.  If this happens, one thread
> @@ -188,6 +189,7 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>  	unsigned long pfn;
>  	unsigned long undo_pfn;
>  	struct page *page;
> +	int count = 0;
>  
>  	BUG_ON(!IS_ALIGNED(start_pfn, pageblock_nr_pages));
>  	BUG_ON(!IS_ALIGNED(end_pfn, pageblock_nr_pages));
> @@ -196,13 +198,15 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>  	     pfn < end_pfn;
>  	     pfn += pageblock_nr_pages) {
>  		page = __first_valid_page(pfn, pageblock_nr_pages);
> -		if (page &&
> -		    set_migratetype_isolate(page, migratetype, flags)) {
> -			undo_pfn = pfn;
> -			goto undo;
> +		if (page) {
> +			if (set_migratetype_isolate(page, migratetype, flags)) {
> +				undo_pfn = pfn;
> +				goto undo;
> +			}
> +			count++;
>  		}
>  	}
> -	return 0;
> +	return count;
>  undo:
>  	for (pfn = start_pfn;
>  	     pfn < undo_pfn;
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 69904aa6165b..56e057c432f9 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -567,7 +567,7 @@ 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)

-- 
Michal Hocko
SUSE Labs