Re: [PATCH v3 2/3] mm: hugetlb: introduce PageHugeActive flag

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Tue, Mar 31, 2015 at 02:06:10PM -0700, Andrew Morton wrote:
> On Tue, 31 Mar 2015 08:50:46 +0000 Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> wrote:
> 
> > We are not safe from calling isolate_huge_page() on a hugepage concurrently,
> > which can make the victim hugepage in invalid state and results in BUG_ON().
> > 
> > The root problem of this is that we don't have any information on struct page
> > (so easily accessible) about hugepages' activeness. Note that hugepages'
> > activeness means just being linked to hstate->hugepage_activelist, which is
> > not the same as normal pages' activeness represented by PageActive flag.
> > 
> > Normal pages are isolated by isolate_lru_page() which prechecks PageLRU before
> > isolation, so let's do similarly for hugetlb with a new PageHugeActive flag.
> > 
> > Set/ClearPageHugeActive should be called within hugetlb_lock. But hugetlb_cow()
> > and hugetlb_no_page() don't do this, being justified because in these function
> > SetPageHugeActive is called right after the hugepage is allocated and no other
> > thread tries to isolate it.
> > 
> > ...
> >
> > +/*
> > + * Page flag to show that the hugepage is "active/in-use" (i.e. being linked to
> > + * hstate->hugepage_activelist.)
> > + *
> > + * This function can be called for tail pages, but never returns true for them.
> > + */
> > +int PageHugeActive(struct page *page)
> > +{
> > +	VM_BUG_ON_PAGE(!PageHuge(page), page);
> > +	return PageHead(page) && PagePrivate(&page[1]);
> > +}
> 
> This is not a "page flag".  It is a regular old C function which tests
> for a certain page state.  Yes, it kind of pretends to act like a
> separate page flag but its use of the peculiar naming convention is
> rather misleading.
> 
> I mean, if you see
> 
> 	SetPageHugeActive(page);
> 
> then you expect that to be doing set_bit(PG_hugeactive, &page->flags)
> but that isn't what it does, so the name is misleading.
> 
> Agree?

Yes I agree, page_huge_active() is fine for me.

> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: mm-hugetlb-introduce-pagehugeactive-flag-fix
> 
> s/PageHugeActive/page_huge_active/, make it return bool
> 
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Michal Hocko <mhocko@suse.cz>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: David Rientjes <rientjes@google.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  mm/hugetlb.c |   22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff -puN mm/hugetlb.c~mm-hugetlb-introduce-pagehugeactive-flag-fix mm/hugetlb.c
> --- a/mm/hugetlb.c~mm-hugetlb-introduce-pagehugeactive-flag-fix
> +++ a/mm/hugetlb.c
> @@ -925,25 +925,25 @@ struct hstate *size_to_hstate(unsigned l
>  }
>  
>  /*
> - * Page flag to show that the hugepage is "active/in-use" (i.e. being linked to
> - * hstate->hugepage_activelist.)
> + * Test to determine whether the hugepage is "active/in-use" (i.e. being linked
> + * to hstate->hugepage_activelist.)
>   *
>   * This function can be called for tail pages, but never returns true for them.
>   */
> -int PageHugeActive(struct page *page)
> +bool page_huge_active(struct page *page)
>  {
>  	VM_BUG_ON_PAGE(!PageHuge(page), page);
>  	return PageHead(page) && PagePrivate(&page[1]);
>  }
>  
>  /* never called for tail page */
> -void SetPageHugeActive(struct page *page)
> +void set_page_huge_active(struct page *page)
>  {
>  	VM_BUG_ON_PAGE(!PageHeadHuge(page), page);
>  	SetPagePrivate(&page[1]);
>  }
>  
> -void ClearPageHugeActive(struct page *page)
> +void clear_page_huge_active(struct page *page)
>  {
>  	VM_BUG_ON_PAGE(!PageHeadHuge(page), page);
>  	ClearPagePrivate(&page[1]);
> @@ -977,7 +977,7 @@ void free_huge_page(struct page *page)
>  		restore_reserve = true;
>  
>  	spin_lock(&hugetlb_lock);
> -	ClearPageHugeActive(page);
> +	clear_page_huge_active(page);
>  	hugetlb_cgroup_uncharge_page(hstate_index(h),
>  				     pages_per_huge_page(h), page);
>  	if (restore_reserve)
> @@ -2998,7 +2998,7 @@ retry_avoidcopy:
>  	copy_user_huge_page(new_page, old_page, address, vma,
>  			    pages_per_huge_page(h));
>  	__SetPageUptodate(new_page);
> -	SetPageHugeActive(new_page);
> +	set_page_huge_active(new_page);
>  
>  	mmun_start = address & huge_page_mask(h);
>  	mmun_end = mmun_start + huge_page_size(h);
> @@ -3111,7 +3111,7 @@ retry:
>  		}
>  		clear_huge_page(page, address, pages_per_huge_page(h));
>  		__SetPageUptodate(page);
> -		SetPageHugeActive(page);
> +		set_page_huge_active(page);
>  
>  		if (vma->vm_flags & VM_MAYSHARE) {
>  			int err;
> @@ -3946,11 +3946,11 @@ bool isolate_huge_page(struct page *page
>  
>  	VM_BUG_ON_PAGE(!PageHead(page), page);
>  	spin_lock(&hugetlb_lock);
> -	if (!PageHugeActive(page) || !get_page_unless_zero(page)) {
> +	if (!page_huge_active(page) || !get_page_unless_zero(page)) {
>  		ret = false;
>  		goto unlock;
>  	}
> -	ClearPageHugeActive(page);
> +	clear_page_huge_active(page);
>  	list_move_tail(&page->lru, list);
>  unlock:
>  	spin_unlock(&hugetlb_lock);
> @@ -3961,7 +3961,7 @@ void putback_active_hugepage(struct page
>  {
>  	VM_BUG_ON_PAGE(!PageHead(page), page);
>  	spin_lock(&hugetlb_lock);
> -	SetPageHugeActive(page);
> +	set_page_huge_active(page);
>  	list_move_tail(&page->lru, &(page_hstate(page))->hugepage_activelist);
>  	spin_unlock(&hugetlb_lock);
>  	put_page(page);
> 
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