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.
>> 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.
>>> @@ -189,6 +198,8 @@ struct page {
>>> #endif
>>> #endif
>>> struct kmem_cache *slab_cache; /* SL[AU]B: Pointer to slab */
>>> + /* XXX: Sure page_pool will have no users of "private"? */
>>> + struct page_pool *pool;
>>> };
>>>
>>> #ifdef CONFIG_MEMCG
>
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