linux-kernel - Re: [PATCH] mm, memory_hotplug: Don't bail out in do_migrate

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Date:   Tue, 11 Dec 2018 10:35:53 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>, akpm@...ux-foundation.org
Cc:     mhocko@...e.com, pasha.tatashin@...een.com,
        dan.j.williams@...il.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm, memory_hotplug: Don't bail out in do_migrate_range
 prematurely

On 11.12.18 09:50, Oscar Salvador wrote:
> do_migrate_ranges() takes a memory range and tries to isolate the
> pages to put them into a list.
> This list will be later on used in migrate_pages() to know
> the pages we need to migrate.
> 
> Currently, if we fail to isolate a single page, we put all already
> isolated pages back to their LRU and we bail out from the function.
> This is quite suboptimal, as this will force us to start over again
> because scan_movable_pages will give us the same range.
> If there is no chance that we can isolate that page, we will loop here
> forever.
> 
> Issue debugged in 4d0c7db96 ("hwpoison, memory_hotplug: allow hwpoisoned
> pages to be offlined") has proved that.
> During the debugging of that issue, it was noticed that if
> do_migrate_ranges() fails to isolate a single page, we will
> just discard the work we have done so far and bail out, which means
> that scan_movable_pages() will find again the same set of pages.
> 
> Instead, we can just skip the error, keep isolating as much pages
> as possible and then proceed with the call to migrate_pages().
> This will allow us to do some work at least.
> 
> There is no danger in the skipped pages to be lost, because
> scan_movable_pages() will give us them as they could not
> be isolated and therefore migrated.
> 
> Although this patch has proved to be useful when dealing with
> 4d0c7db96 because it allows us to move forward as long as the
> page is not in LRU, we still need 4d0c7db96
> ("hwpoison, memory_hotplug: allow hwpoisoned pages to be offlined")
> to handle the LRU case and the unmapping of the page if needed.
> So, this is just a follow-up cleanup.

So if I get it right the behavior of the function is now

"If at least one page could be migrated, return 0. If no pages could be
migrated, return an error."

Don't we want that to be

"If all pages could be migrated, return 0. If only a subset could be
migrated, return -EAGAIN. If no pages could be migrated, return the
causing error."

So somehow remember if we had issues with one page and instead of
reporting 0, report e.g. -EAGAIN?

Meaning something like "If you get -EAGAIN, try again to get the real
reason".

> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>  mm/memory_hotplug.c | 19 +------------------
>  1 file changed, 1 insertion(+), 18 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 86ab673fc4e3..804d0280d2ab 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1339,7 +1339,6 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  	unsigned long pfn;
>  	struct page *page;
>  	int move_pages = NR_OFFLINE_AT_ONCE_PAGES;
> -	int not_managed = 0;
>  	int ret = 0;
>  	LIST_HEAD(source);
>  
> @@ -1395,25 +1394,9 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  				inc_node_page_state(page, NR_ISOLATED_ANON +
>  						    page_is_file_cache(page));
>  
> -		} else {
> -			pr_warn("failed to isolate pfn %lx\n", pfn);
> -			dump_page(page, "isolation failed");
> -			put_page(page);
> -			/* Because we don't have big zone->lock. we should
> -			   check this again here. */
> -			if (page_count(page)) {
> -				not_managed++;
> -				ret = -EBUSY;
> -				break;
> -			}
>  		}
>  	}
>  	if (!list_empty(&source)) {
> -		if (not_managed) {
> -			putback_movable_pages(&source);
> -			goto out;
> -		}
> -
>  		/* Allocate a new page from the nearest neighbor node */
>  		ret = migrate_pages(&source, new_node_page, NULL, 0,
>  					MIGRATE_SYNC, MR_MEMORY_HOTPLUG);
> @@ -1426,7 +1409,7 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  			putback_movable_pages(&source);
>  		}
>  	}
> -out:
> +
>  	return ret;
>  }
>  
> 


-- 

Thanks,

David / dhildenb