Commit 4d4266e3 authored by Ilias Apalodimas's avatar Ilias Apalodimas Committed by Jakub Kicinski

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: default avatarAlexander Duyck <alexanderduyck@fb.com>
Signed-off-by: default avatarIlias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/r/20230217222130.85205-1-ilias.apalodimas@linaro.orgSigned-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent a00da30c
...@@ -277,6 +277,16 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page, ...@@ -277,6 +277,16 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page,
unsigned int dma_sync_size, unsigned int dma_sync_size,
bool allow_direct); 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) 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_frag_count, nr);
......
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