On Mon, 9 Jan 2017 11:43:39 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote:
> On 01/04/2017 12:00 PM, Jesper Dangaard Brouer wrote:
> >
> > On Tue, 3 Jan 2017 17:07:49 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote:
> >
> >> On 12/20/2016 02:28 PM, Jesper Dangaard Brouer wrote:
> >>> The focus in this patch is getting the API around page_pool figured out.
> >>>
> >>> The internal data structures for returning page_pool pages is not optimal.
> >>> This implementation use ptr_ring for recycling, which is known not to scale
> >>> in case of multiple remote CPUs releasing/returning pages.
> >>
> >> Just few very quick impressions...
> >>
> >>> A bulking interface into the page allocator is also left for later. (This
> >>> requires cooperation will Mel Gorman, who just send me some PoC patches for this).
> >>> ---
> > [...]
> >>> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >>> index 4424784ac374..11b4d8fb280b 100644
> >>> --- a/include/linux/mm.h
> >>> +++ b/include/linux/mm.h
> > [...]
> >>> @@ -765,6 +766,11 @@ static inline void put_page(struct page *page)
> >>> {
> >>> page = compound_head(page);
> >>>
> >>> + if (PagePool(page)) {
> >>> + page_pool_put_page(page);
> >>> + return;
> >>> + }
> >>
> >> Can't say I'm thrilled about a new page flag and a test in put_page().
> >
> > In patch 4/4, I'm scaling this back. Avoiding to modify the inlined
> > put_page(), by letting refcnt reach zero and catching pages belonging to
> > a page_pool in __free_pages_ok() and free_hot_cold_page(). (Result
> > in being more dependent on page-refcnt and loosing some performance).
> >
> > Still needing a new page flag, or some other method of identifying when
> > a page belongs to a page_pool.
>
> I see. I guess if all page pool pages were order>0 compound pages, you
> could hook this to the existing compound_dtor functionality instead.
The page_pool will support order>0 pages, but it is the order-0 case
that is optimized for.
> >> I don't know the full life cycle here, but isn't it that these pages
> >> will be specifically allocated and used in page pool aware drivers,
> >> so maybe they can be also specifically freed there without hooking to
> >> the generic page refcount mechanism?
> >
> > Drivers are already manipulating refcnt, to "splitup" the page (to
> > save memory) for storing more RX frames per page. Which is something
> > the page_pool still need to support. (XDP can request one page per
> > packet and gain the direct recycle optimization and instead waste mem).
> >
> > Notice, a page_pool aware driver doesn't handle the "free-side". Free
> > happens when the packet/page is being consumed, spliced or transmitted
> > out another non-page_pool-aware NIC driver. An interresting case is
> > packet-page waiting for DMA TX completion (on another NIC), thus need
> > to async-store info on page_pool and DMA-addr.
> >
> > Could extend the SKB (with a page_pool pointer)... BUT it defeats the
> > purpose of avoiding to allocate the SKB. E.g. in the cases where XDP
> > takes the route-decision and transmit/forward the "raw"-page (out
> > another NIC or into a "raw" socket), then we don't have a meta-data
> > structure to store this info in. Thus, this info is stored in struct
> > page.
>
> OK.
>
> >>> + */
> >>> struct address_space *mapping; /* If low bit clear, points to
> >>> * inode address_space, or NULL.
> >>> * If page mapped as anonymous
> >>> @@ -63,6 +69,7 @@ struct page {
> >>> union {
> >>> pgoff_t index; /* Our offset within mapping. */
> >>> void *freelist; /* sl[aou]b first free object */
> >>> + dma_addr_t dma_addr; /* used by page_pool */
> >>> /* page_deferred_list().prev -- second tail page */
> >>> };
> >>>
> >>> @@ -117,6 +124,8 @@ struct page {
> >>> * avoid collision and false-positive PageTail().
> >>> */
> >>> union {
> >>> + /* XXX: Idea reuse lru list, in page_pool to align with PCP */
> >>> +
> >>> struct list_head lru; /* Pageout list, eg. active_list
> >>> * protected by zone_lru_lock !
> >>> * Can be used as a generic list
> >
> > Guess, I can move it here, as the page cannot be on the LRU-list, while
> > being used (or VMA mapped). Right?
>
> Well typically the VMA mapped pages are those on the LRU list (anonymous
> or file). But I don't suppose you will want memory reclaim to free your
> pages, so seems lru field should be reusable for you.
Thanks for the info.
So, LRU-list area could be reusable, but I does not align so well with
the bulking API Mel just introduced/proposed, but still doable.
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>