Re: [PATCH mm-unstable v5 01/10] mm: add folio dtor and order setter functions

[Mike Kravetz]

On 12/07/22 12:11, Muchun Song wrote:
> 
> 
> > On Dec 7, 2022, at 11:42, Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> > 
> > On 12/07/22 11:34, Muchun Song wrote:
> >> 
> >> 
> >>> On Nov 30, 2022, at 06:50, Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx> wrote:
> >>> 
> >>> Add folio equivalents for set_compound_order() and set_compound_page_dtor().
> >>> 
> >>> Also remove extra new-lines introduced by mm/hugetlb: convert
> >>> move_hugetlb_state() to folios and mm/hugetlb_cgroup: convert
> >>> hugetlb_cgroup_uncharge_page() to folios.
> >>> 
> >>> Suggested-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> >>> Suggested-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >>> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
> >>> ---
> >>> include/linux/mm.h | 16 ++++++++++++++++
> >>> mm/hugetlb.c       |  4 +---
> >>> 2 files changed, 17 insertions(+), 3 deletions(-)
> >>> 
> >>> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >>> index a48c5ad16a5e..2bdef8a5298a 100644
> >>> --- a/include/linux/mm.h
> >>> +++ b/include/linux/mm.h
> >>> @@ -972,6 +972,13 @@ static inline void set_compound_page_dtor(struct page *page,
> >>> page[1].compound_dtor = compound_dtor;
> >>> }
> >>> 
> >>> +static inline void folio_set_compound_dtor(struct folio *folio,
> >>> + enum compound_dtor_id compound_dtor)
> >>> +{
> >>> + VM_BUG_ON_FOLIO(compound_dtor >= NR_COMPOUND_DTORS, folio);
> >>> + folio->_folio_dtor = compound_dtor;
> >>> +}
> >>> +
> >>> void destroy_large_folio(struct folio *folio);
> >>> 
> >>> static inline int head_compound_pincount(struct page *head)
> >>> @@ -987,6 +994,15 @@ static inline void set_compound_order(struct page *page, unsigned int order)
> >>> #endif
> >>> }
> >>> 
> >>> +static inline void folio_set_compound_order(struct folio *folio,
> >>> + unsigned int order)
> >>> +{
> >>> + folio->_folio_order = order;
> >>> +#ifdef CONFIG_64BIT
> >>> + folio->_folio_nr_pages = order ? 1U << order : 0;
> >> 
> >> It seems that you think the user could pass 0 to order. However,
> >> ->_folio_nr_pages and ->_folio_order fields are invalid for order-0 pages.
> >> You should not touch it. So this should be:
> >> 
> >> static inline void folio_set_compound_order(struct folio *folio,
> >>     unsigned int order)
> >> {
> >> 	if (!folio_test_large(folio))
> >> 		return;
> >> 
> >> 	folio->_folio_order = order;
> >> #ifdef CONFIG_64BIT
> >> 	folio->_folio_nr_pages = 1U << order;
> >> #endif
> >> }
> > 
> > I believe this was changed to accommodate the code in
> > __destroy_compound_gigantic_page().  It is used in a subsequent patch.
> > Here is the v6.0 version of the routine.
> 
> Thanks for your clarification.
> 
> > 
> > static void __destroy_compound_gigantic_page(struct page *page,
> > unsigned int order, bool demote)
> > {
> > 	int i;
> > 	int nr_pages = 1 << order;
> > 	struct page *p = page + 1;
> > 
> > 	atomic_set(compound_mapcount_ptr(page), 0);
> > 	atomic_set(compound_pincount_ptr(page), 0);
> > 
> > 	for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
> > 		p->mapping = NULL;
> > 		clear_compound_head(p);
> > 		if (!demote)
> > 			set_page_refcounted(p);
> > 	}
> > 
> > 	set_compound_order(page, 0);
> > #ifdef CONFIG_64BIT
> > 	page[1].compound_nr = 0;
> > #endif
> > 	__ClearPageHead(page);
> > }
> > 
> > 
> > Might have been better to change this set_compound_order call to
> > folio_set_compound_order in this patch.
> > 
> 
> Agree. It has confused me a lot. I suggest changing the code to the
> followings. The folio_test_large() check is still to avoid unexpected
> users for OOB.
> 
> static inline void folio_set_compound_order(struct folio *folio,
> 					    unsigned int order)
> {
> 	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> 	// or
> 	// if (!folio_test_large(folio))
> 	// 	return;
> 
> 	folio->_folio_order = order;
> #ifdef CONFIG_64BIT
> 	folio->_folio_nr_pages = order ? 1U << order : 0;
> #endif
> }

I think the VM_BUG_ON_FOLIO is appropriate as it would at least flag
data corruption.

Thinking about this some more, it seems that hugetlb is the only caller
that abuses folio_set_compound_order (and previously set_compound_order)
by passing in a zero order.  Since it is unlikely that anyone knows of
this abuse, it might be good to add a comment to the routine to note
why it handles the zero case.  This might help prevent changes which
would potentially break hugetlb.
-- 
Mike Kravetz