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page_pool: add a comment explaining the fragment counter usage

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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.

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>
This commit is contained in:
Ilias Apalodimas 2023-02-18 00:21:30 +02:00 committed by Jakub Kicinski
parent a00da30c05
commit 4d4266e3fd
1 changed files with 10 additions and 0 deletions
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10
include/net/page_pool.h
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@ -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);