On 2023/11/23 10:25, Liang Chen wrote: > 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]. The renaming looks good to me, some minor nit. It is good to add a cover-letter using 'git format-patch --cover-letter' to explain the overall background or modifications this patchset make when there is more than one patch. > > [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; It seems that we may have 4 bytes available for 64 bit arch if we change the 'atomic_long_t' to 'refcount_t':) > }; > 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 Maybe "referenced by only 2 or 3 users" is more appropriate now? > * 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. Do we need to rename the page_pool_defrag_page() and page_pool_is_last_frag() too? > */ > if (unlikely(!ret)) > - atomic_long_set(&page->pp_frag_count, 1); > + atomic_long_set(&page->pp_ref_count, 1); > > return ret; > } >
