diff mbox series

[v3,1/2] mm/memory_hotplug: drain per-cpu pages again during memory offline

Message ID 20200904151448.100489-2-pasha.tatashin@soleen.com
State New
Headers show
Series drain pcp outside of page isolation | expand

Commit Message

Pavel Tatashin Sept. 4, 2020, 3:14 p.m. UTC
There is a race during page offline that can lead to infinite loop:
a page never ends up on a buddy list and __offline_pages() keeps
retrying infinitely or until a termination signal is received.

Thread#1 - a new process:

load_elf_binary
 begin_new_exec
  exec_mmap
   mmput
    exit_mmap
     tlb_finish_mmu
      tlb_flush_mmu
       release_pages
        free_unref_page_list
         free_unref_page_prepare
          set_pcppage_migratetype(page, migratetype);
             // Set page->index migration type below  MIGRATE_PCPTYPES

Thread#2 - hot-removes memory
__offline_pages
  start_isolate_page_range
    set_migratetype_isolate
      set_pageblock_migratetype(page, MIGRATE_ISOLATE);
        Set migration type to MIGRATE_ISOLATE-> set
        drain_all_pages(zone);
             // drain per-cpu page lists to buddy allocator.

Thread#1 - continue
         free_unref_page_commit
           migratetype = get_pcppage_migratetype(page);
              // get old migration type
           list_add(&page->lru, &pcp->lists[migratetype]);
              // add new page to already drained pcp list

Thread#2
Never drains pcp again, and therefore gets stuck in the loop.

The fix is to try to drain per-cpu lists again after
check_pages_isolated_cb() fails.

Fixes: c52e75935f8d ("mm: remove extra drain pages on pcp list")

Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: stable@vger.kernel.org
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/memory_hotplug.c | 14 ++++++++++++++
 mm/page_isolation.c |  8 ++++++++
 2 files changed, 22 insertions(+)

Comments

Michal Hocko Sept. 7, 2020, 7:27 a.m. UTC | #1
On Fri 04-09-20 11:14:47, Pavel Tatashin wrote:
> There is a race during page offline that can lead to infinite loop:
> a page never ends up on a buddy list and __offline_pages() keeps
> retrying infinitely or until a termination signal is received.
> 
> Thread#1 - a new process:
> 
> load_elf_binary
>  begin_new_exec
>   exec_mmap
>    mmput
>     exit_mmap
>      tlb_finish_mmu
>       tlb_flush_mmu
>        release_pages
>         free_unref_page_list
>          free_unref_page_prepare
>           set_pcppage_migratetype(page, migratetype);
>              // Set page->index migration type below  MIGRATE_PCPTYPES
> 
> Thread#2 - hot-removes memory
> __offline_pages
>   start_isolate_page_range
>     set_migratetype_isolate
>       set_pageblock_migratetype(page, MIGRATE_ISOLATE);
>         Set migration type to MIGRATE_ISOLATE-> set
>         drain_all_pages(zone);
>              // drain per-cpu page lists to buddy allocator.
> 
> Thread#1 - continue
>          free_unref_page_commit
>            migratetype = get_pcppage_migratetype(page);
>               // get old migration type
>            list_add(&page->lru, &pcp->lists[migratetype]);
>               // add new page to already drained pcp list
> 
> Thread#2
> Never drains pcp again, and therefore gets stuck in the loop.
> 
> The fix is to try to drain per-cpu lists again after
> check_pages_isolated_cb() fails.
> 
> Fixes: c52e75935f8d ("mm: remove extra drain pages on pcp list")
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> Cc: stable@vger.kernel.org
> Acked-by: David Rientjes <rientjes@google.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Already acked the mmotm version but let's add it here as well.
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memory_hotplug.c | 14 ++++++++++++++
>  mm/page_isolation.c |  8 ++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index e9d5ab5d3ca0..b11a269e2356 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1575,6 +1575,20 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  		/* check again */
>  		ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
>  					    NULL, check_pages_isolated_cb);
> +		/*
> +		 * per-cpu pages are drained in start_isolate_page_range, but if
> +		 * there are still pages that are not free, make sure that we
> +		 * drain again, because when we isolated range we might
> +		 * have raced with another thread that was adding pages to pcp
> +		 * list.
> +		 *
> +		 * Forward progress should be still guaranteed because
> +		 * pages on the pcp list can only belong to MOVABLE_ZONE
> +		 * because has_unmovable_pages explicitly checks for
> +		 * PageBuddy on freed pages on other zones.
> +		 */
> +		if (ret)
> +			drain_all_pages(zone);
>  	} while (ret);
>  
>  	/* Ok, all of our target is isolated.
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 242c03121d73..63a3db10a8c0 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -170,6 +170,14 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
>   * pageblocks we may have modified and return -EBUSY to caller. This
>   * prevents two threads from simultaneously working on overlapping ranges.
>   *
> + * Please note that there is no strong synchronization with the page allocator
> + * either. Pages might be freed while their page blocks are marked ISOLATED.
> + * In some cases pages might still end up on pcp lists and that would allow
> + * for their allocation even when they are in fact isolated already. Depending
> + * on how strong of a guarantee the caller needs drain_all_pages might be needed
> + * (e.g. __offline_pages will need to call it after check for isolated range for
> + * a next retry).
> + *
>   * Return: the number of isolated pageblocks on success and -EBUSY if any part
>   * of range cannot be isolated.
>   */
> -- 
> 2.25.1
>
David Hildenbrand Sept. 8, 2020, 9:57 a.m. UTC | #2
On 04.09.20 17:14, Pavel Tatashin wrote:
> There is a race during page offline that can lead to infinite loop:
> a page never ends up on a buddy list and __offline_pages() keeps
> retrying infinitely or until a termination signal is received.
> 
> Thread#1 - a new process:
> 
> load_elf_binary
>  begin_new_exec
>   exec_mmap
>    mmput
>     exit_mmap
>      tlb_finish_mmu
>       tlb_flush_mmu
>        release_pages
>         free_unref_page_list
>          free_unref_page_prepare
>           set_pcppage_migratetype(page, migratetype);
>              // Set page->index migration type below  MIGRATE_PCPTYPES
> 
> Thread#2 - hot-removes memory
> __offline_pages
>   start_isolate_page_range
>     set_migratetype_isolate
>       set_pageblock_migratetype(page, MIGRATE_ISOLATE);
>         Set migration type to MIGRATE_ISOLATE-> set
>         drain_all_pages(zone);
>              // drain per-cpu page lists to buddy allocator.
> 
> Thread#1 - continue
>          free_unref_page_commit
>            migratetype = get_pcppage_migratetype(page);
>               // get old migration type
>            list_add(&page->lru, &pcp->lists[migratetype]);
>               // add new page to already drained pcp list
> 
> Thread#2
> Never drains pcp again, and therefore gets stuck in the loop.
> 
> The fix is to try to drain per-cpu lists again after
> check_pages_isolated_cb() fails.
> 
> Fixes: c52e75935f8d ("mm: remove extra drain pages on pcp list")
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> Cc: stable@vger.kernel.org
> Acked-by: David Rientjes <rientjes@google.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/memory_hotplug.c | 14 ++++++++++++++
>  mm/page_isolation.c |  8 ++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index e9d5ab5d3ca0..b11a269e2356 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1575,6 +1575,20 @@ static int __ref __offline_pages(unsigned long start_pfn,
>  		/* check again */
>  		ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
>  					    NULL, check_pages_isolated_cb);
> +		/*
> +		 * per-cpu pages are drained in start_isolate_page_range, but if
> +		 * there are still pages that are not free, make sure that we
> +		 * drain again, because when we isolated range we might
> +		 * have raced with another thread that was adding pages to pcp
> +		 * list.
> +		 *
> +		 * Forward progress should be still guaranteed because
> +		 * pages on the pcp list can only belong to MOVABLE_ZONE
> +		 * because has_unmovable_pages explicitly checks for
> +		 * PageBuddy on freed pages on other zones.
> +		 */
> +		if (ret)
> +			drain_all_pages(zone);
>  	} while (ret);
>  
>  	/* Ok, all of our target is isolated.
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index 242c03121d73..63a3db10a8c0 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -170,6 +170,14 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
>   * pageblocks we may have modified and return -EBUSY to caller. This
>   * prevents two threads from simultaneously working on overlapping ranges.
>   *
> + * Please note that there is no strong synchronization with the page allocator
> + * either. Pages might be freed while their page blocks are marked ISOLATED.
> + * In some cases pages might still end up on pcp lists and that would allow
> + * for their allocation even when they are in fact isolated already. Depending
> + * on how strong of a guarantee the caller needs drain_all_pages might be needed
> + * (e.g. __offline_pages will need to call it after check for isolated range for
> + * a next retry).
> + *
>   * Return: the number of isolated pageblocks on success and -EBUSY if any part
>   * of range cannot be isolated.
>   */
> 

Acked-by: David Hildenbrand <david@redhat.com>

As an easy stable fix to be improved in the near future.
diff mbox series

Patch

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index e9d5ab5d3ca0..b11a269e2356 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1575,6 +1575,20 @@  static int __ref __offline_pages(unsigned long start_pfn,
 		/* check again */
 		ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
 					    NULL, check_pages_isolated_cb);
+		/*
+		 * per-cpu pages are drained in start_isolate_page_range, but if
+		 * there are still pages that are not free, make sure that we
+		 * drain again, because when we isolated range we might
+		 * have raced with another thread that was adding pages to pcp
+		 * list.
+		 *
+		 * Forward progress should be still guaranteed because
+		 * pages on the pcp list can only belong to MOVABLE_ZONE
+		 * because has_unmovable_pages explicitly checks for
+		 * PageBuddy on freed pages on other zones.
+		 */
+		if (ret)
+			drain_all_pages(zone);
 	} while (ret);
 
 	/* Ok, all of our target is isolated.
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 242c03121d73..63a3db10a8c0 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -170,6 +170,14 @@  __first_valid_page(unsigned long pfn, unsigned long nr_pages)
  * pageblocks we may have modified and return -EBUSY to caller. This
  * prevents two threads from simultaneously working on overlapping ranges.
  *
+ * Please note that there is no strong synchronization with the page allocator
+ * either. Pages might be freed while their page blocks are marked ISOLATED.
+ * In some cases pages might still end up on pcp lists and that would allow
+ * for their allocation even when they are in fact isolated already. Depending
+ * on how strong of a guarantee the caller needs drain_all_pages might be needed
+ * (e.g. __offline_pages will need to call it after check for isolated range for
+ * a next retry).
+ *
  * Return: the number of isolated pageblocks on success and -EBUSY if any part
  * of range cannot be isolated.
  */