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SubjectRe: [PATCH v5 08/11] mm: hwpoison: soft offline supports thp migration


Anshuman Khandual wrote:
> On 04/21/2017 02:17 AM, Zi Yan wrote:
>> From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>>
>> This patch enables thp migration for soft offline.
>>
>> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>>
>> ChangeLog: v1 -> v5:
>> - fix page isolation counting error
>>
>> Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
>> ---
>> mm/memory-failure.c | 35 ++++++++++++++---------------------
>> 1 file changed, 14 insertions(+), 21 deletions(-)
>>
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 9b77476ef31f..23ff02eb3ed4 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -1481,7 +1481,17 @@ static struct page *new_page(struct page *p, unsigned long private, int **x)
>> if (PageHuge(p))
>> return alloc_huge_page_node(page_hstate(compound_head(p)),
>> nid);
>> - else
>> + else if (thp_migration_supported() && PageTransHuge(p)) {
>> + struct page *thp;
>> +
>> + thp = alloc_pages_node(nid,
>> + (GFP_TRANSHUGE | __GFP_THISNODE) & ~__GFP_RECLAIM,
>
> Why not __GFP_RECLAIM ? Its soft offline path we wait a bit before
> declaring that THP page cannot be allocated and hence should invoke
> reclaim methods as well.

I am not sure how much effort the kernel wants to put here to soft
offline a THP. Naoya knows more here.


>
>> + HPAGE_PMD_ORDER);
>> + if (!thp)
>> + return NULL;
>> + prep_transhuge_page(thp);
>> + return thp;
>> + } else
>> return __alloc_pages_node(nid, GFP_HIGHUSER_MOVABLE, 0);
>> }
>>
>> @@ -1665,8 +1675,8 @@ static int __soft_offline_page(struct page *page, int flags)
>> * cannot have PAGE_MAPPING_MOVABLE.
>> */
>> if (!__PageMovable(page))
>> - inc_node_page_state(page, NR_ISOLATED_ANON +
>> - page_is_file_cache(page));
>> + mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
>> + page_is_file_cache(page), hpage_nr_pages(page));
>> list_add(&page->lru, &pagelist);
>> ret = migrate_pages(&pagelist, new_page, NULL, MPOL_MF_MOVE_ALL,
>> MIGRATE_SYNC, MR_MEMORY_FAILURE);
>> @@ -1689,28 +1699,11 @@ static int __soft_offline_page(struct page *page, int flags)
>> static int soft_offline_in_use_page(struct page *page, int flags)
>> {
>> int ret;
>> - struct page *hpage = compound_head(page);
>> -
>> - if (!PageHuge(page) && PageTransHuge(hpage)) {
>> - lock_page(hpage);
>> - if (!PageAnon(hpage) || unlikely(split_huge_page(hpage))) {
>> - unlock_page(hpage);
>> - if (!PageAnon(hpage))
>> - pr_info("soft offline: %#lx: non anonymous thp\n", page_to_pfn(page));
>> - else
>> - pr_info("soft offline: %#lx: thp split failed\n", page_to_pfn(page));
>> - put_hwpoison_page(hpage);
>> - return -EBUSY;
>> - }
>> - unlock_page(hpage);
>> - get_hwpoison_page(page);
>> - put_hwpoison_page(hpage);
>> - }
>>
>> if (PageHuge(page))
>> ret = soft_offline_huge_page(page, flags);
>> else
>> - ret = __soft_offline_page(page, flags);
>> + ret = __soft_offline_page(compound_head(page), flags);
>
> Hmm, what if the THP allocation fails in the new_page() path and
> we fallback for general page allocation. In that case we will
> always be still calling with the head page ? Because we dont
> split the huge page any more.

This could be a problem if the user wants to offline a TailPage but due
to THP allocation failure, the HeadPage is offlined.

It may be better to only soft offline THPs if page ==
compound_head(page). If page != compound_head(page), we still split THPs
like before.

Because in migrate_pages(), we cannot guarantee any TailPages in that
THP are migrated (1. THP allocation failure causes THP splitting, then
only HeadPage is going to be migrated; 2. even if we change existing
migrate_pages() implementation to add all TailPages to migration list
instead of LRU list, we still cannot guarantee the TailPage we want to
migrate is migrated.).

Naoya, what do you think?

--
Best Regards,
Yan Zi

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