To support multiple users referencing the same fragment, pp_frag_count is renamed to pp_ref_count to better reflect its actual meaning based on the suggestion from [1]. [1] http://lore.kernel.org/netdev/f71d9448-70c8-8793-dc9a-0eb48a570300@xxxxxxxxxx Signed-off-by: Liang Chen <liangchen.linux@xxxxxxxxx> --- include/linux/mm_types.h | 2 +- include/net/page_pool/helpers.h | 31 ++++++++++++++++++------------- 2 files changed, 19 insertions(+), 14 deletions(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 957ce38768b2..64e4572ef06d 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -125,7 +125,7 @@ struct page { struct page_pool *pp; unsigned long _pp_mapping_pad; unsigned long dma_addr; - atomic_long_t pp_frag_count; + atomic_long_t pp_ref_count; }; struct { /* Tail pages of compound page */ unsigned long compound_head; /* Bit zero is set */ diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h index 4ebd544ae977..a6dc9412c9ae 100644 --- a/include/net/page_pool/helpers.h +++ b/include/net/page_pool/helpers.h @@ -29,7 +29,7 @@ * page allocated from page pool. Page splitting enables memory saving and thus * avoids TLB/cache miss for data access, but there also is some cost to * implement page splitting, mainly some cache line dirtying/bouncing for - * 'struct page' and atomic operation for page->pp_frag_count. + * 'struct page' and atomic operation for page->pp_ref_count. * * The API keeps track of in-flight pages, in order to let API users know when * it is safe to free a page_pool object, the API users must call @@ -214,61 +214,66 @@ inline enum dma_data_direction page_pool_get_dma_dir(struct page_pool *pool) return pool->p.dma_dir; } -/* pp_frag_count represents the number of writers who can update the page +/* pp_ref_count represents the number of writers who can update the page * either by updating skb->data or via DMA mappings for the device. * We can't rely on the page refcnt for that as we don't know who might be * holding page references and we can't reliably destroy or sync DMA mappings * of the fragments. * - * When pp_frag_count reaches 0 we can either recycle the page if the page + * pp_ref_count initially corresponds to the number of fragments. However, + * when multiple users start to reference a single fragment, for example in + * skb_try_coalesce, the pp_ref_count will become greater than the number of + * fragments. + * + * When pp_ref_count reaches 0 we can either recycle the page if the page * refcnt is 1 or return it back to the memory allocator and destroy any * mappings we have. */ static inline void page_pool_fragment_page(struct page *page, long nr) { - atomic_long_set(&page->pp_frag_count, nr); + atomic_long_set(&page->pp_ref_count, nr); } static inline long page_pool_defrag_page(struct page *page, long nr) { long ret; - /* If nr == pp_frag_count then we have cleared all remaining + /* If nr == pp_ref_count then we have cleared all remaining * references to the page: * 1. 'n == 1': no need to actually overwrite it. * 2. 'n != 1': overwrite it with one, which is the rare case - * for pp_frag_count draining. + * for pp_ref_count draining. * * The main advantage to doing this is that not only we avoid a atomic * update, as an atomic_read is generally a much cheaper operation than * an atomic update, especially when dealing with a page that may be - * partitioned into only 2 or 3 pieces; but also unify the pp_frag_count + * partitioned into only 2 or 3 pieces; but also unify the pp_ref_count * handling by ensuring all pages have partitioned into only 1 piece * initially, and only overwrite it when the page is partitioned into * more than one piece. */ - if (atomic_long_read(&page->pp_frag_count) == nr) { + if (atomic_long_read(&page->pp_ref_count) == nr) { /* As we have ensured nr is always one for constant case using * the BUILD_BUG_ON(), only need to handle the non-constant case - * here for pp_frag_count draining, which is a rare case. + * here for pp_ref_count draining, which is a rare case. */ BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1); if (!__builtin_constant_p(nr)) - atomic_long_set(&page->pp_frag_count, 1); + atomic_long_set(&page->pp_ref_count, 1); return 0; } - ret = atomic_long_sub_return(nr, &page->pp_frag_count); + ret = atomic_long_sub_return(nr, &page->pp_ref_count); WARN_ON(ret < 0); - /* We are the last user here too, reset pp_frag_count back to 1 to + /* We are the last user here too, reset pp_ref_count back to 1 to * ensure all pages have been partitioned into 1 piece initially, * this should be the rare case when the last two fragment users call * page_pool_defrag_page() currently. */ if (unlikely(!ret)) - atomic_long_set(&page->pp_frag_count, 1); + atomic_long_set(&page->pp_ref_count, 1); return ret; } -- 2.31.1
