On 6/30/20 7:26 AM, David Hildenbrand wrote:
> Right now, if we have two isolations racing, we might trigger the
> WARN_ON_ONCE() and to dump_page(NULL), dereferencing NULL. Let's just
> return directly.
Just curious, what call path has the WARN_ON_ONCE()/dump_page(NULL)?
>
> In the future, we might want to report -EAGAIN to the caller instead, as
> this could indicate a temporary isolation failure only.
>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Hi David,
That 'return -EAGAIN' was added as a sort of synchronization mechanism.
See commit message for 2c7452a075d4d. Before adding the 'return -EAGAIN',
I could create races which would abandon isolated pageblocks. Repeating
those races over and over would result in a good chunk of system memory
being isolated and unusable.
Admittedly, these races are rare and I had to work really hard to produce
them. I'll try to find my testing mechanism. My concern is reintroducing
this abandoning of pageblocks. I have not looked further in your series
to see if this potentially addressed later. If not, then we should not
remove the return code.
--
Mike Kravetz
> ---
> mm/page_isolation.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index f6d07c5f0d34d..553b49a34cf71 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -29,10 +29,12 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
> /*
> * We assume the caller intended to SET migrate type to isolate.
> * If it is already set, then someone else must have raced and
> - * set it before us. Return -EBUSY
> + * set it before us.
> */
> - if (is_migrate_isolate_page(page))
> - goto out;
> + if (is_migrate_isolate_page(page)) {
> + spin_unlock_irqrestore(&zone->lock, flags);
> + return -EBUSY;
> + }
>
> /*
> * FIXME: Now, memory hotplug doesn't call shrink_slab() by itself.
>
