Messages in this thread Patch in this message | | | From | Ilias Apalodimas <> | Subject | [PATCH] page_pool: add a comment explaining the fragment counter usage | Date | Fri, 27 Jan 2023 12:16:27 +0200 |
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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. + * + * 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); -- 2.38.1
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