[v2] mm/hotplug: fix offline undo_isolate_page_range()
diff mbox series

Message ID 20190313143133.46200-1-cai@lca.pw
State New
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Series
  • [v2] mm/hotplug: fix offline undo_isolate_page_range()
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Commit Message

Qian Cai March 13, 2019, 2:31 p.m. UTC
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")
Signed-off-by: Qian Cai <cai@lca.pw>
---

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(-)

Comments

Michal Hocko March 14, 2019, 8:05 a.m. UTC | #1
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)
Oscar Salvador March 14, 2019, 8:09 a.m. UTC | #2
On Wed, Mar 13, 2019 at 10:31:33AM -0400, Qian Cai wrote:
> 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.

Well, x86_64's memblocks can be up to 2GB depending on the memory we got.
Plus the fact that alloc_contig_range can be used to isolate 16GB-hugetlb
pages on powerpc, and that could be a lot of pageblocks.

While an "int" could still hold, I think we should use "long" just to be
more future-proof.

> 
> Fixes: 2ce13640b3f4 ("mm: __first_valid_page skip over offline pages")
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
> 
> 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;

I would rather use a meaningful name here, like "nr_isolated_pageblocks" or
something like that.
Oscar Salvador March 14, 2019, 8:10 a.m. UTC | #3
On Wed, Mar 13, 2019 at 10:31:33AM -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. 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")
> Signed-off-by: Qian Cai <cai@lca.pw>

Forgot it:

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

> ---
> 
> 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 March 14, 2019, 8:22 a.m. UTC | #4
On Thu 14-03-19 09:09:22, Oscar Salvador wrote:
> On Wed, Mar 13, 2019 at 10:31:33AM -0400, Qian Cai wrote:
> > 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.
> 
> Well, x86_64's memblocks can be up to 2GB depending on the memory we got.
> Plus the fact that alloc_contig_range can be used to isolate 16GB-hugetlb
> pages on powerpc, and that could be a lot of pageblocks.
> 
> While an "int" could still hold, I think we should use "long" just to be
> more future-proof.

I would rather not touch that for now. If we really need to change the
type then do it when it is really needed. I pressume more things would
have to be changed anyway.

Patch
diff mbox series

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;