Re: [PATCH v2 2/4] mm, page_owner: record page owner for each subpage

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Sep 24, 2019 at 05:10:59PM +0200, Vlastimil Babka wrote:
> On 9/24/19 1:31 PM, Kirill A. Shutemov wrote:
> > On Tue, Aug 20, 2019 at 03:18:26PM +0200, Vlastimil Babka wrote:
> >> Currently, page owner info is only recorded for the first page of a high-order
> >> allocation, and copied to tail pages in the event of a split page. With the
> >> plan to keep previous owner info after freeing the page, it would be benefical
> >> to record page owner for each subpage upon allocation. This increases the
> >> overhead for high orders, but that should be acceptable for a debugging option.
> >>
> >> The order stored for each subpage is the order of the whole allocation. This
> >> makes it possible to calculate the "head" pfn and to recognize "tail" pages
> >> (quoted because not all high-order allocations are compound pages with true
> >> head and tail pages). When reading the page_owner debugfs file, keep skipping
> >> the "tail" pages so that stats gathered by existing scripts don't get inflated.
> >>
> >> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> >> ---
> >>  mm/page_owner.c | 40 ++++++++++++++++++++++++++++------------
> >>  1 file changed, 28 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/mm/page_owner.c b/mm/page_owner.c
> >> index addcbb2ae4e4..813fcb70547b 100644
> >> --- a/mm/page_owner.c
> >> +++ b/mm/page_owner.c
> >> @@ -154,18 +154,23 @@ static noinline depot_stack_handle_t save_stack(gfp_t flags)
> >>  	return handle;
> >>  }
> >>  
> >> -static inline void __set_page_owner_handle(struct page_ext *page_ext,
> >> -	depot_stack_handle_t handle, unsigned int order, gfp_t gfp_mask)
> >> +static inline void __set_page_owner_handle(struct page *page,
> >> +	struct page_ext *page_ext, depot_stack_handle_t handle,
> >> +	unsigned int order, gfp_t gfp_mask)
> >>  {
> >>  	struct page_owner *page_owner;
> >> +	int i;
> >>  
> >> -	page_owner = get_page_owner(page_ext);
> >> -	page_owner->handle = handle;
> >> -	page_owner->order = order;
> >> -	page_owner->gfp_mask = gfp_mask;
> >> -	page_owner->last_migrate_reason = -1;
> >> +	for (i = 0; i < (1 << order); i++) {
> >> +		page_owner = get_page_owner(page_ext);
> >> +		page_owner->handle = handle;
> >> +		page_owner->order = order;
> >> +		page_owner->gfp_mask = gfp_mask;
> >> +		page_owner->last_migrate_reason = -1;
> >> +		__set_bit(PAGE_EXT_OWNER, &page_ext->flags);
> >>  
> >> -	__set_bit(PAGE_EXT_OWNER, &page_ext->flags);
> >> +		page_ext = lookup_page_ext(page + i);
> > 
> > Isn't it off-by-one? You are calculating page_ext for the next page,
> > right?
> 
> You're right, thanks!
> 
> > And cant we just do page_ext++ here instead?
> 
> Unfortunately no, as that implies sizeof(page_ext), which only declares
> unsigned long flags; and the rest is runtime-determined.
> Perhaps I could add a wrapper named e.g. page_ext_next() that would use
> get_entry_size() internally and hide the necessary casts to void * and back?

Yeah, it looks less costly than calling lookup_page_ext() on each
iteration. And looks like it can be inlined if we make 'extra_mem' visible
(under different name) outside page_ext.c.

-- 
 Kirill A. Shutemov