Re: [PATCH 08/17] prmem: struct page: track vmap_area

[Igor Stoppa]

On 24/10/2018 06:12, Matthew Wilcox wrote:
> On Wed, Oct 24, 2018 at 12:34:55AM +0300, Igor Stoppa wrote:
> > The connection between each page and its vmap_area avoids more expensive
> > searches through the btree of vmap_areas.
>
> Typo -- it's an rbtree.

ack

> > +++ b/include/linux/mm_types.h
> > @@ -87,13 +87,24 @@ struct page {
> >   			/* See page-flags.h for PAGE_MAPPING_FLAGS */
> >   			struct address_space *mapping;
> >   			pgoff_t index;		/* Our offset within mapping. */
> > -			/**
> > -			 * @private: Mapping-private opaque data.
> > -			 * Usually used for buffer_heads if PagePrivate.
> > -			 * Used for swp_entry_t if PageSwapCache.
> > -			 * Indicates order in the buddy system if PageBuddy.
> > -			 */
> > -			unsigned long private;
> > +			union {
> > +				/**
> > +				 * @private: Mapping-private opaque data.
> > +				 * Usually used for buffer_heads if
> > +				 * PagePrivate.
> > +				 * Used for swp_entry_t if PageSwapCache.
> > +				 * Indicates order in the buddy system if
> > +				 * PageBuddy.
> > +				 */
> > +				unsigned long private;
> > +				/**
> > +				 * @area: reference to the containing area
> > +				 * For pages that are mapped into a virtually
> > +				 * contiguous area, avoids performing a more
> > +				 * expensive lookup.
> > +				 */
> > +				struct vmap_area *area;
> > +			};
>
> Not like this.  Make it part of a different struct in the existing union,
> not a part of the pagecache struct.  And there's no need to use ->private
> explicitly.

Ok, I'll have a look at the googledoc you made

> > @@ -1747,6 +1750,10 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
> >   	if (!addr)
> >   		return NULL;
> >   
> > +	va = __find_vmap_area((unsigned long)addr);
> > +	for (i = 0; i < va->vm->nr_pages; i++)
> > +		va->vm->pages[i]->area = va;
>
> I don't like it that you're calling this for _every_ vmalloc() caller
> when most of them will never use this.  Perhaps have page->va be initially
> NULL and then cache the lookup in it when it's accessed for the first time.

If __find_vmap_area() was part of the API, this loop could be left out 
from __vmalloc_node_range() and the user of the allocation could 
initialize the field, if needed.

What is the reason for keeping __find_vmap_area() private?

--
igor