Re: [PATCH 5/8] hugetlb: call update_and_free_page without hugetlb_lock

[Michal Hocko]

On Thu 25-03-21 10:12:05, Mike Kravetz wrote:
> On 3/25/21 3:55 AM, Michal Hocko wrote:
> > On Wed 24-03-21 17:28:32, Mike Kravetz wrote:
> >> With the introduction of remove_hugetlb_page(), there is no need for
> >> update_and_free_page to hold the hugetlb lock.  Change all callers to
> >> drop the lock before calling.
> >>
> >> With additional code modifications, this will allow loops which decrease
> >> the huge page pool to drop the hugetlb_lock with each page to reduce
> >> long hold times.
> >>
> >> The ugly unlock/lock cycle in free_pool_huge_page will be removed in
> >> a subsequent patch which restructures free_pool_huge_page.
> >>
> >> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > 
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > 
> > One minor thing below
> > 
> > [...]
> >> @@ -2563,22 +2572,37 @@ static void try_to_free_low(struct hstate *h, unsigned long count,
> >>  						nodemask_t *nodes_allowed)
> >>  {
> >>  	int i;
> >> +	struct list_head page_list;
> >> +	struct page *page, *next;
> >>  
> >>  	if (hstate_is_gigantic(h))
> >>  		return;
> >>  
> >> +	/*
> >> +	 * Collect pages to be freed on a list, and free after dropping lock
> >> +	 */
> >> +	INIT_LIST_HEAD(&page_list);
> >>  	for_each_node_mask(i, *nodes_allowed) {
> >> -		struct page *page, *next;
> >>  		struct list_head *freel = &h->hugepage_freelists[i];
> >>  		list_for_each_entry_safe(page, next, freel, lru) {
> >>  			if (count >= h->nr_huge_pages)
> >> -				return;
> >> +				goto out;
> >>  			if (PageHighMem(page))
> >>  				continue;
> >>  			remove_hugetlb_page(h, page, false);
> >> -			update_and_free_page(h, page);
> >> +			INIT_LIST_HEAD(&page->lru);
> > 
> > What is the point of rhis INIT_LIST_HEAD? Page has been removed from the
> > list by remove_hugetlb_page so it can be added to a new one without any
> > reinitialization.
> 
> remove_hugetlb_page just does a list_del.  list_del will poison the
> pointers in page->lru.  The following list_add will then complain about
> list corruption.

Are you sure? list_del followed by list_add is a normal API usage
pattern AFAIK. INIT_LIST_HEAD is to do the first initialization before
first use.

-- 
Michal Hocko
SUSE Labs