| Message ID | 20230127101627.891614-1-ilias.apalodimas@linaro.org (mailing list archive) |
|---|---|
| State | Changes Requested |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | page_pool: add a comment explaining the fragment counter usage | expand |
On Fri, Jan 27, 2023 at 2:16 AM Ilias Apalodimas <ilias.apalodimas@linaro.org> wrote: > > When reading the page_pool code the first impression is that keeping > two separate counters, one being the page refcnt and the other being > fragment pp_frag_count, is counter-intuitive. > > However without that fragment counter we don't know when to reliably > destroy or sync the outstanding DMA mappings. So let's add a comment > explaining this part. > > Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org> > --- > include/net/page_pool.h | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/include/net/page_pool.h b/include/net/page_pool.h > index 813c93499f20..115dbce6d431 100644 > --- a/include/net/page_pool.h > +++ b/include/net/page_pool.h > @@ -277,6 +277,14 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page, > unsigned int dma_sync_size, > bool allow_direct); > > +/* pp_frag_count is our number of outstanding DMA maps. 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. > + * This isn't quite right. Basically each frag is writable by the holder of the frag. As such pp_frag_count represents the number of writers who could still update the page either in the form of updating skb->data or via DMA from the device. > + * When pp_frag_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); The rest of this looks good to me.
diff --git a/include/net/page_pool.h b/include/net/page_pool.h index 813c93499f20..115dbce6d431 100644 --- a/include/net/page_pool.h +++ b/include/net/page_pool.h @@ -277,6 +277,14 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page, unsigned int dma_sync_size, bool allow_direct); +/* pp_frag_count is our number of outstanding DMA maps. 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 + * 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);
When reading the page_pool code the first impression is that keeping two separate counters, one being the page refcnt and the other being fragment pp_frag_count, is counter-intuitive. However without that fragment counter we don't know when to reliably destroy or sync the outstanding DMA mappings. So let's add a comment explaining this part. Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org> --- include/net/page_pool.h | 8 ++++++++ 1 file changed, 8 insertions(+)