On Thu, Nov 23, 2023 at 2:18 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote: > > 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. > Thanks for the suggestion. A cover-letter will be provided for the next version. > > > > [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? > Sure. > > * 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? > Yeah, I think so. Once a pp page is drained, its management shifts to being primarily governed by pp_ref_count, and there's no longer a need to consider fragmenting. will be done in the next iteration. > > */ > > if (unlikely(!ret)) > > - atomic_long_set(&page->pp_frag_count, 1); > > + atomic_long_set(&page->pp_ref_count, 1); > > > > return ret; > > } > >
