Date:	Thu, 23 Sep 2010 11:52:26 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
cc:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Jun'ichi Nomura <j-nomura@...jp.nec.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/10] hugetlb: hugepage migration core

On Wed, 8 Sep 2010, Naoya Horiguchi wrote:


> +static int hugetlbfs_migrate_page(struct address_space *mapping,
> +				struct page *newpage, struct page *page)
> +{
> +	int rc;
> +
> +	rc = migrate_huge_page_move_mapping(mapping, newpage, page);
> +	if (rc)
> +		return rc;
> +	migrate_page_copy(newpage, page);
> +
> +	return 0;
> +}
> +
>  static int hugetlbfs_statfs(struct dentry *dentry, struct kstatfs *buf)
>  {
>  	struct hugetlbfs_sb_info *sbinfo = HUGETLBFS_SB(dentry->d_sb);
> @@ -659,6 +673,7 @@ static const struct address_space_operations hugetlbfs_aops = {
>  	.write_begin	= hugetlbfs_write_begin,
>  	.write_end	= hugetlbfs_write_end,
>  	.set_page_dirty	= hugetlbfs_set_page_dirty,
> +	.migratepage    = hugetlbfs_migrate_page,
>  };

Very straightforward conversion of innermost piece to huge pages. Good.

If migrate_page_move_mapping would do huge pages like it seems
migrate_page_copy() does (a bit surprising) then we could save ourselves
the function?

> index 351f8d1..55f3e2d 100644
> --- v2.6.36-rc2/mm/hugetlb.c
> +++ v2.6.36-rc2/mm/hugetlb.c
> @@ -2217,6 +2217,19 @@ nomem:
>  	return -ENOMEM;
>  }
>
> +static int is_hugetlb_entry_migration(pte_t pte)
> +{
> +	swp_entry_t swp;
> +
> +	if (huge_pte_none(pte) || pte_present(pte))
> +		return 0;
> +	swp = pte_to_swp_entry(pte);
> +	if (non_swap_entry(swp) && is_migration_entry(swp)) {
> +		return 1;
> +	} else
> +		return 0;
> +}

Ok that implies that to some extend swap must be supported for this case
in the core vm?

> @@ -2651,7 +2664,10 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	ptep = huge_pte_offset(mm, address);
>  	if (ptep) {
>  		entry = huge_ptep_get(ptep);
> -		if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
> +		if (unlikely(is_hugetlb_entry_migration(entry))) {
> +			migration_entry_wait(mm, (pmd_t *)ptep, address);
> +			return 0;
> +		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
>  			return VM_FAULT_HWPOISON;

No we have here in hugetlb_fault() a copy of  the migration wait logic
from do_swap_page(). Hope the rest of the VM cannot inadvertantly
encounter such a migration entry elsewhere?


> diff --git v2.6.36-rc2/mm/migrate.c v2.6.36-rc2/mm/migrate.c

> @@ -130,10 +139,17 @@ static int remove_migration_pte(struct page *new, struct vm_area_struct *vma,
>  	pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
>  	if (is_write_migration_entry(entry))
>  		pte = pte_mkwrite(pte);
> +	if (PageHuge(new))
> +		pte = pte_mkhuge(pte);
>  	flush_cache_page(vma, addr, pte_pfn(pte));
>  	set_pte_at(mm, addr, ptep, pte);
>
> -	if (PageAnon(new))
> +	if (PageHuge(new)) {
> +		if (PageAnon(new))
> +			hugepage_add_anon_rmap(new, vma, addr);
> +		else
> +			page_dup_rmap(new);

dup_rmap? What are non anon huge pages use for? That is not a file backed
huge page right?


> @@ -276,11 +292,59 @@ static int migrate_page_move_mapping(struct address_space *mapping,
>  }
>
>  /*
> + * The expected number of remaining references is the same as that
> + * of migrate_page_move_mapping().
> + */
> +int migrate_huge_page_move_mapping(struct address_space *mapping,
> +				   struct page *newpage, struct page *page)
> +{
> +	int expected_count;
> +	void **pslot;
> +
> +	if (!mapping) {
> +		if (page_count(page) != 1)
> +			return -EAGAIN;
> +		return 0;
> +	}
> +
> +	spin_lock_irq(&mapping->tree_lock);
> +
> +	pslot = radix_tree_lookup_slot(&mapping->page_tree,
> +					page_index(page));
> +
> +	expected_count = 2 + page_has_private(page);
> +	if (page_count(page) != expected_count ||
> +	    (struct page *)radix_tree_deref_slot(pslot) != page) {
> +		spin_unlock_irq(&mapping->tree_lock);
> +		return -EAGAIN;
> +	}
> +
> +	if (!page_freeze_refs(page, expected_count)) {
> +		spin_unlock_irq(&mapping->tree_lock);
> +		return -EAGAIN;
> +	}
> +
> +	get_page(newpage);
> +
> +	radix_tree_replace_slot(pslot, newpage);
> +
> +	page_unfreeze_refs(page, expected_count);
> +
> +	__put_page(page);
> +
> +	spin_unlock_irq(&mapping->tree_lock);
> +	return 0;
> +}

Thats a pretty accurate copy of move_mapping(). Why are the counter
updates missing at the end? This also suggests that the two functions
could be merged into one.


> @@ -724,6 +788,92 @@ move_newpage:
>  }
>
>  /*
> + * Counterpart of unmap_and_move_page() for hugepage migration.
> + *
> + * This function doesn't wait the completion of hugepage I/O
> + * because there is no race between I/O and migration for hugepage.
> + * Note that currently hugepage I/O occurs only in direct I/O
> + * where no lock is held and PG_writeback is irrelevant,
> + * and writeback status of all subpages are counted in the reference
> + * count of the head page (i.e. if all subpages of a 2MB hugepage are
> + * under direct I/O, the reference of the head page is 512 and a bit more.)
> + * This means that when we try to migrate hugepage whose subpages are
> + * doing direct I/O, some references remain after try_to_unmap() and
> + * hugepage migration fails without data corruption.
> + *
> + * There is also no race when direct I/O is issued on the page under migration,
> + * because then pte is replaced with migration swap entry and direct I/O code
> + * will wait in the page fault for migration to complete.
> + */
> +static int unmap_and_move_huge_page(new_page_t get_new_page,
> +				unsigned long private, struct page *hpage,
> +				int force, int offlining)
> +{
> +	int rc = 0;
> +	int *result = NULL;
> +	struct page *new_hpage = get_new_page(hpage, private, &result);
> +	int rcu_locked = 0;
> +	struct anon_vma *anon_vma = NULL;
> +
> +	if (!new_hpage)
> +		return -ENOMEM;
> +
> +	rc = -EAGAIN;
> +
> +	if (!trylock_page(hpage)) {
> +		if (!force)
> +			goto out;
> +		lock_page(hpage);
> +	}
> +
> +	if (PageAnon(hpage)) {
> +		rcu_read_lock();
> +		rcu_locked = 1;
> +
> +		if (page_mapped(hpage)) {
> +			anon_vma = page_anon_vma(hpage);
> +			atomic_inc(&anon_vma->external_refcount);
> +		}
> +	}
> +
> +	try_to_unmap(hpage, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
> +
> +	if (!page_mapped(hpage))
> +		rc = move_to_new_page(new_hpage, hpage, 1);
> +
> +	if (rc)
> +		remove_migration_ptes(hpage, hpage);
> +
> +	if (anon_vma && atomic_dec_and_lock(&anon_vma->external_refcount,
> +					    &anon_vma->lock)) {
> +		int empty = list_empty(&anon_vma->head);
> +		spin_unlock(&anon_vma->lock);
> +		if (empty)
> +			anon_vma_free(anon_vma);

Hmmm.. The anon_vma dropping looks different? Why cant we use
drop_anon_mva like in unmap_and_move? Also we do not take the root lock
like drop_anon_vma does.

> @@ -788,6 +938,52 @@ out:
>  	return nr_failed + retry;
>  }
>
> +int migrate_huge_pages(struct list_head *from,
> +		new_page_t get_new_page, unsigned long private, int offlining)
> +{
> +	int retry = 1;
> +	int nr_failed = 0;
> +	int pass = 0;
> +	struct page *page;
> +	struct page *page2;
> +	int rc;
> +
> +	for (pass = 0; pass < 10 && retry; pass++) {
> +		retry = 0;
> +
> +		list_for_each_entry_safe(page, page2, from, lru) {
> +			cond_resched();
> +
> +			rc = unmap_and_move_huge_page(get_new_page,
> +					private, page, pass > 2, offlining);
> +
> +			switch(rc) {
> +			case -ENOMEM:
> +				goto out;
> +			case -EAGAIN:
> +				retry++;
> +				break;
> +			case 0:
> +				break;
> +			default:
> +				/* Permanent failure */
> +				nr_failed++;
> +				break;
> +			}
> +		}
> +	}
> +	rc = 0;
> +out:
> +
> +	list_for_each_entry_safe(page, page2, from, lru)
> +		put_page(page);
> +
> +	if (rc)
> +		return rc;
> +
> +	return nr_failed + retry;
> +}

Copy of migrate_pages(). putback_lru_pages() omitted as also proposed for
upstream in a recent discussion.



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