Re: [PATCH 5/9] migrate: enable migrate_pages() to migrate hugepage

[Naoya Horiguchi]

On Mon, Mar 18, 2013 at 04:40:57PM +0100, Michal Hocko wrote:
> On Thu 21-02-13 14:41:44, Naoya Horiguchi wrote:
> > This patch extends check_range() to handle vma with VM_HUGETLB set.
> > With this changes, we can migrate hugepage with migrate_pages(2).
> > Note that for larger hugepages (covered by pud entries, 1GB for
> > x86_64 for example), we simply skip it now.
> > 
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> > ---
> >  include/linux/hugetlb.h |  6 ++++--
> >  mm/hugetlb.c            | 10 ++++++++++
> >  mm/mempolicy.c          | 46 ++++++++++++++++++++++++++++++++++------------
> >  3 files changed, 48 insertions(+), 14 deletions(-)
> > 
> > diff --git v3.8.orig/include/linux/hugetlb.h v3.8/include/linux/hugetlb.h
> > index 8f87115..eb33df5 100644
> > --- v3.8.orig/include/linux/hugetlb.h
> > +++ v3.8/include/linux/hugetlb.h
> > @@ -69,6 +69,7 @@ void hugetlb_unreserve_pages(struct inode *inode, long offset, long freed);
> >  int dequeue_hwpoisoned_huge_page(struct page *page);
> >  void putback_active_hugepage(struct page *page);
> >  void putback_active_hugepages(struct list_head *l);
> > +void migrate_hugepage_add(struct page *page, struct list_head *list);
> >  void copy_huge_page(struct page *dst, struct page *src);
> >  
> >  extern unsigned long hugepages_treat_as_movable;
> > @@ -88,8 +89,8 @@ struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
> >  				pmd_t *pmd, int write);
> >  struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address,
> >  				pud_t *pud, int write);
> > -int pmd_huge(pmd_t pmd);
> > -int pud_huge(pud_t pmd);
> > +extern int pmd_huge(pmd_t pmd);
> > +extern int pud_huge(pud_t pmd);
> 
> extern is not needed here.

OK.

> >  unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
> >  		unsigned long address, unsigned long end, pgprot_t newprot);
> >  
> > @@ -134,6 +135,7 @@ static inline int dequeue_hwpoisoned_huge_page(struct page *page)
> >  
> >  #define putback_active_hugepage(p) 0
> >  #define putback_active_hugepages(l) 0
> > +#define migrate_hugepage_add(p, l) 0
> >  static inline void copy_huge_page(struct page *dst, struct page *src)
> >  {
> >  }
> > diff --git v3.8.orig/mm/hugetlb.c v3.8/mm/hugetlb.c
> > index cb9d43b8..86ffcb7 100644
> > --- v3.8.orig/mm/hugetlb.c
> > +++ v3.8/mm/hugetlb.c
> > @@ -3202,3 +3202,13 @@ void putback_active_hugepages(struct list_head *l)
> >  	list_for_each_entry_safe(page, page2, l, lru)
> >  		putback_active_hugepage(page);
> >  }
> > +
> > +void migrate_hugepage_add(struct page *page, struct list_head *list)
> > +{
> > +	VM_BUG_ON(!PageHuge(page));
> > +	get_page(page);
> > +	spin_lock(&hugetlb_lock);
> 
> Why hugetlb_lock? Comment for this lock says that it protects
> hugepage_freelists, nr_huge_pages, and free_huge_pages.

I think that this comment is out of date and hugepage_activelists,
which was introduced recently, should be protected because this
patchset adds is_hugepage_movable() which runs through the list.
So I'll update the comment in the next post.

> > +	list_move_tail(&page->lru, list);
> > +	spin_unlock(&hugetlb_lock);
> > +	return;
> > +}
> > diff --git v3.8.orig/mm/mempolicy.c v3.8/mm/mempolicy.c
> > index e2df1c1..8627135 100644
> > --- v3.8.orig/mm/mempolicy.c
> > +++ v3.8/mm/mempolicy.c
> > @@ -525,6 +525,27 @@ static int check_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> >  	return addr != end;
> >  }
> >  
> > +static void check_hugetlb_pmd_range(struct vm_area_struct *vma, pmd_t *pmd,
> > +		const nodemask_t *nodes, unsigned long flags,
> > +				    void *private)
> > +{
> > +#ifdef CONFIG_HUGETLB_PAGE
> > +	int nid;
> > +	struct page *page;
> > +
> > +	spin_lock(&vma->vm_mm->page_table_lock);
> > +	page = pte_page(huge_ptep_get((pte_t *)pmd));
> > +	spin_unlock(&vma->vm_mm->page_table_lock);
> 
> I am a bit confused why page_table_lock is used here and why it doesn't
> cover the page usage.

I expected this function to do the same for pmd as check_pte_range() does
for pte, but the above code didn't do it. I should've put spin_unlock
below migrate_hugepage_add(). Sorry for the confusion.

> > +	nid = page_to_nid(page);
> > +	if (node_isset(nid, *nodes) != !!(flags & MPOL_MF_INVERT)
> > +	    && ((flags & MPOL_MF_MOVE && page_mapcount(page) == 1)
> > +		|| flags & MPOL_MF_MOVE_ALL))
> > +		migrate_hugepage_add(page, private);
> > +#else
> > +	BUG();
> > +#endif
> > +}
> > +
> >  static inline int check_pmd_range(struct vm_area_struct *vma, pud_t *pud,
> >  		unsigned long addr, unsigned long end,
> >  		const nodemask_t *nodes, unsigned long flags,
> > @@ -536,6 +557,11 @@ static inline int check_pmd_range(struct vm_area_struct *vma, pud_t *pud,
> >  	pmd = pmd_offset(pud, addr);
> >  	do {
> >  		next = pmd_addr_end(addr, end);
> > +		if (pmd_huge(*pmd) && is_vm_hugetlb_page(vma)) {
> 
> Why an explicit check for is_vm_hugetlb_page here? Isn't pmd_huge()
> sufficient?

I think we need both check here because if we use only pmd_huge(),
pmd for thp goes into this branch wrongly. 

Thanks,
Naoya

> > +			check_hugetlb_pmd_range(vma, pmd, nodes,
> > +						flags, private);
> > +			continue;
> > +		}
> >  		split_huge_page_pmd(vma, addr, pmd);
> >  		if (pmd_none_or_trans_huge_or_clear_bad(pmd))
> >  			continue;
> [...]
> -- 
> Michal Hocko
> SUSE Labs
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