Re: [RFC PATCH 3/3] mm: unclutter THP migration

[Zi Yan]

On 8 Dec 2017, at 11:15, Michal Hocko wrote:

> From: Michal Hocko <mhocko@xxxxxxxx>
>
> THP migration is hacked into the generic migration with rather
> surprising semantic. The migration allocation callback is supposed to
> check whether the THP can be migrated at once and if that is not the
> case then it allocates a simple page to migrate. unmap_and_move then
> fixes that up by spliting the THP into small pages while moving the
> head page to the newly allocated order-0 page. Remaning pages are moved
> to the LRU list by split_huge_page. The same happens if the THP
> allocation fails. This is really ugly and error prone [1].
>
> I also believe that split_huge_page to the LRU lists is inherently
> wrong because all tail pages are not migrated. Some callers will just
> work around that by retrying (e.g. memory hotplug). There are other
> pfn walkers which are simply broken though. e.g. madvise_inject_error
> will migrate head and then advances next pfn by the huge page size.
> do_move_page_to_node_array, queue_pages_range (migrate_pages, mbind),
> will simply split the THP before migration if the THP migration is not
> supported then falls back to single page migration but it doesn't handle
> tail pages if the THP migration path is not able to allocate a fresh
> THP so we end up with ENOMEM and fail the whole migration which is
> a questionable behavior. Page compaction doesn't try to migrate large
> pages so it should be immune.
>
> This patch tries to unclutter the situation by moving the special THP
> handling up to the migrate_pages layer where it actually belongs. We
> simply split the THP page into the existing list if unmap_and_move fails
> with ENOMEM and retry. So we will _always_ migrate all THP subpages and
> specific migrate_pages users do not have to deal with this case in a
> special way.
>
> [1] https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Flkml.kernel.org%2Fr%2F20171121021855.50525-1-zi.yan%40sent.com&data=02%7C01%7Czi.yan%40cs.rutgers.edu%7Cfbb3ed29196a430d9c7808d53e5703cc%7Cb92d2b234d35447093ff69aca6632ffe%7C1%7C0%7C636483465807198257&sdata=1jHMT9Nsyfc7xiMpjy05vYHrY9DCV4Z9LlOSsaJFdBY%3D&reserved=0
>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  include/linux/migrate.h |  4 ++--
>  mm/huge_memory.c        |  6 ++++++
>  mm/memory_hotplug.c     |  2 +-
>  mm/mempolicy.c          | 31 +++----------------------------
>  mm/migrate.c            | 29 +++++++++++++++++++----------
>  5 files changed, 31 insertions(+), 41 deletions(-)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index e5d99ade2319..0c6fe904bc97 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -42,9 +42,9 @@ static inline struct page *new_page_nodemask(struct page *page,
>  		return alloc_huge_page_nodemask(page_hstate(compound_head(page)),
>  				preferred_nid, nodemask);
>
> -	if (thp_migration_supported() && PageTransHuge(page)) {
> -		order = HPAGE_PMD_ORDER;
> +	if (PageTransHuge(page)) {
>  		gfp_mask |= GFP_TRANSHUGE;
> +		order = HPAGE_PMD_ORDER;
>  	}
>
>  	if (PageHighMem(page) || (zone_idx(page_zone(page)) == ZONE_MOVABLE))
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 7544ce4ef4dc..8865906c248c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2425,6 +2425,12 @@ static void __split_huge_page_tail(struct page *head, int tail,
>
>  	page_tail->index = head->index + tail;
>  	page_cpupid_xchg_last(page_tail, page_cpupid_last(head));
> +
> +	/*
> +	 * always add to the tail because some iterators expect new
> +	 * pages to show after the currently processed elements - e.g.
> +	 * migrate_pages
> +	 */
>  	lru_add_page_tail(head, page_tail, lruvec, list);
>  }
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index d865623edee7..442e63a2cf72 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1390,7 +1390,7 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  			if (isolate_huge_page(page, &source))
>  				move_pages -= 1 << compound_order(head);
>  			continue;
> -		} else if (thp_migration_supported() && PageTransHuge(page))
> +		} else if (PageTransHuge(page))
>  			pfn = page_to_pfn(compound_head(page))
>  				+ hpage_nr_pages(page) - 1;
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 4d849d3098e5..b6f4fcf9df64 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -446,15 +446,6 @@ static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
>  		__split_huge_pmd(walk->vma, pmd, addr, false, NULL);
>  		goto out;
>  	}
> -	if (!thp_migration_supported()) {
> -		get_page(page);
> -		spin_unlock(ptl);
> -		lock_page(page);
> -		ret = split_huge_page(page);
> -		unlock_page(page);
> -		put_page(page);
> -		goto out;
> -	}
>  	if (!queue_pages_required(page, qp)) {
>  		ret = 1;
>  		goto unlock;
> @@ -495,7 +486,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
>
>  	if (pmd_trans_unstable(pmd))
>  		return 0;
> -retry:
> +
>  	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
>  	for (; addr != end; pte++, addr += PAGE_SIZE) {
>  		if (!pte_present(*pte))
> @@ -511,22 +502,6 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
>  			continue;
>  		if (!queue_pages_required(page, qp))
>  			continue;
> -		if (PageTransCompound(page) && !thp_migration_supported()) {
> -			get_page(page);
> -			pte_unmap_unlock(pte, ptl);
> -			lock_page(page);
> -			ret = split_huge_page(page);
> -			unlock_page(page);
> -			put_page(page);
> -			/* Failed to split -- skip. */
> -			if (ret) {
> -				pte = pte_offset_map_lock(walk->mm, pmd,
> -						addr, &ptl);
> -				continue;
> -			}
> -			goto retry;
> -		}
> -
>  		migrate_page_add(page, qp->pagelist, flags);
>  	}
>  	pte_unmap_unlock(pte - 1, ptl);
> @@ -948,7 +923,7 @@ struct page *alloc_new_node_page(struct page *page, unsigned long node)
>  	if (PageHuge(page))
>  		return alloc_huge_page_node(page_hstate(compound_head(page)),
>  					node);
> -	else if (thp_migration_supported() && PageTransHuge(page)) {
> +	else if (PageTransHuge(page)) {
>  		struct page *thp;
>
>  		thp = alloc_pages_node(node,
> @@ -1124,7 +1099,7 @@ static struct page *new_page(struct page *page, unsigned long start)
>  	if (PageHuge(page)) {
>  		BUG_ON(!vma);
>  		return alloc_huge_page_noerr(vma, address, 1);
> -	} else if (thp_migration_supported() && PageTransHuge(page)) {
> +	} else if (PageTransHuge(page)) {
>  		struct page *thp;
>
>  		thp = alloc_hugepage_vma(GFP_TRANSHUGE, vma, address,
> diff --git a/mm/migrate.c b/mm/migrate.c
> index f9235f0155a4..dc5df5fe5c82 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1138,6 +1138,9 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>  	int rc = MIGRATEPAGE_SUCCESS;
>  	struct page *newpage;
>
> +	if (!thp_migration_supported() && PageTransHuge(page))
> +		return -ENOMEM;
> +
>  	newpage = get_new_page(page, private);
>  	if (!newpage)
>  		return -ENOMEM;
> @@ -1159,14 +1162,6 @@ static ICE_noinline int unmap_and_move(new_page_t get_new_page,
>  		goto out;
>  	}
>
> -	if (unlikely(PageTransHuge(page) && !PageTransHuge(newpage))) {
> -		lock_page(page);
> -		rc = split_huge_page(page);
> -		unlock_page(page);
> -		if (rc)
> -			goto out;
> -	}
> -
>  	rc = __unmap_and_move(page, newpage, force, mode);
>  	if (rc == MIGRATEPAGE_SUCCESS)
>  		set_page_owner_migrate_reason(newpage, reason);
> @@ -1381,6 +1376,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  		retry = 0;
>
>  		list_for_each_entry_safe(page, page2, from, lru) {
> +retry:
>  			cond_resched();
>
>  			if (PageHuge(page))
> @@ -1394,6 +1390,21 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>
>  			switch(rc) {
>  			case -ENOMEM:
> +				/*
> +				 * THP migration might be unsupported or the
> +				 * allocation could've failed so we should
> +				 * retry on the same page with the THP split
> +				 * to base pages.
> +				 */
> +				if (PageTransHuge(page)) {
> +					lock_page(page);
> +					rc = split_huge_page_to_list(page, from);
> +					unlock_page(page);
> +					if (!rc) {
> +						list_safe_reset_next(page, page2, lru);
> +						goto retry;
> +					}
> +				}

The hunk splits the THP and adds all tail pages at the end of the list âfromâ.
Why do we need âlist_safe_reset_next(page, page2, lru);â here, when page2 is not changed here?

And it seems a little bit strange to only re-migrate the head page, then come back to all tail
pages after migrating the rest of pages in the list âfromâ. Is it better to split the THP into
a list other than âfromâ and insert the list after âpageâ, then retry from the split âpageâ?
Thus, we attempt to migrate all sub pages of the THP after it is split.


>  				nr_failed++;
>  				goto out;
>  			case -EAGAIN:
> @@ -1480,8 +1491,6 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
>
>  	/* FOLL_DUMP to ignore special (like zero) pages */
>  	follflags = FOLL_GET | FOLL_DUMP;
> -	if (!thp_migration_supported())
> -		follflags |= FOLL_SPLIT;
>  	page = follow_page(vma, addr, follflags);
>
>  	err = PTR_ERR(page);
> -- 
> 2.15.0


â
Best Regards,
Yan Zi

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