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Date:   Mon, 14 Oct 2019 10:39:14 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        David Hildenbrand <david@redhat.com>,
        Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: memory offline infinite loop after soft offline
Message-ID: <20191014083914.GA317@dhcp22.suse.cz>
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On Fri 11-10-19 17:32:44, Qian Cai wrote:
> # /opt/ltp/runtest/bin/move_pages12
> move_pages12.c:263: INFO: Free RAM 258988928 kB
> move_pages12.c:281: INFO: Increasing 2048kB hugepages pool on node 0 to 4
> move_pages12.c:291: INFO: Increasing 2048kB hugepages pool on node 8 to 4
> move_pages12.c:207: INFO: Allocating and freeing 4 hugepages on node 0
> move_pages12.c:207: INFO: Allocating and freeing 4 hugepages on node 8
> move_pages12.c:197: PASS: Bug not reproduced
> move_pages12.c:197: PASS: Bug not reproduced
> 
> for mem in $(ls -d /sys/devices/system/memory/memory*); do
> ????????echo offline > $mem/state
> ????????echo online > $mem/state
> done
> 
> That LTP move_pages12 test will first madvise(MADV_SOFT_OFFLINE) for a range.
> Then, one of "echo offline" will trigger an infinite loop in __offline_pages()
> here,
> 
> 		/* check again */
> 		ret = walk_system_ram_range(start_pfn, end_pfn - start_pfn,
> 					????NULL, check_pages_isolated_cb);
> 	} while (ret);
> 
> because check_pages_isolated_cb() always return -EBUSY from
> test_pages_isolated(),
> 
> 
> 	pfn = __test_page_isolated_in_pageblock(start_pfn, end_pfn,
> 						skip_hwpoisoned_pages);
>         ...
>         return pfn < end_pfn ? -EBUSY : 0;
> 
> The root cause is in __test_page_isolated_in_pageblock() where "pfn" is always
> less than "end_pfn" because the associated page is not a PageBuddy.
> 
> 	while (pfn < end_pfn) {
> 	...
> 		else
> 			break;
> 
> 	return pfn;

Hmm, this is interesting. I would expect that this would hit the
previous branch
	if (skip_hwpoisoned_pages && PageHWPoison(page))
and skip over hwpoisoned page. But I cannot seem to find that we would
mark all tail pages HWPoison as well and so any tail page seem to
confuse __test_page_isolated_in_pageblock.

Oscar is rewriting the hwpoison implementation but I am not sure
how/whether he is handling this case differently. Naoya, shouldn't we do
the following at least?
---
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 89c19c0feadb..5fb3fee16fde 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -274,7 +274,7 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn,
 			 * simple way to verify that as VM_BUG_ON(), though.
 			 */
 			pfn += 1 << page_order(page);
-		else if (skip_hwpoisoned_pages && PageHWPoison(page))
+		else if (skip_hwpoisoned_pages && PageHWPoison(compound_head(page)))
 			/* A HWPoisoned page cannot be also PageBuddy */
 			pfn++;
 		else
-- 
Michal Hocko
SUSE Labs