diff options
| author | Ilias Apalodimas <ilias.apalodimas@linaro.org> | 2023-02-18 01:21:30 +0300 |
|---|---|---|
| committer | Jakub Kicinski <kuba@kernel.org> | 2023-02-21 20:15:39 +0300 |
| commit | 4d4266e3fd321fadb628ce02de641b129522c39c (patch) | |
| tree | 3a3b01a058c6b6deb48ef8f14630d1eae7bef589 /include/net | |
| parent | a00da30c052f07d67da56efd6a4f1fc85956c979 (diff) | |
| download | linux-4d4266e3fd321fadb628ce02de641b129522c39c.tar.xz | |
page_pool: add a comment explaining the fragment counter usage
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.
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/r/20230217222130.85205-1-ilias.apalodimas@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Diffstat (limited to 'include/net')
| -rw-r--r-- | include/net/page_pool.h | 10 |
1 files changed, 10 insertions, 0 deletions
diff --git a/include/net/page_pool.h b/include/net/page_pool.h index 34bf531ffc8d..ddfa0b328677 100644 --- a/include/net/page_pool.h +++ b/include/net/page_pool.h @@ -277,6 +277,16 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page, unsigned int dma_sync_size, bool allow_direct); +/* pp_frag_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 + * 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); |