[PATCH v3] page_pool: add a comment explaining the fragment counter usage

[PATCH v3] page_pool: add a comment explaining the fragment counter usage

[Ilias Apalodimas]

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>
---
Changes since v2:
 - Removed a uneeded commas on the comment
Changes since v1:
 - Update the comment withe the correct description for pp_frag_count
 include/net/page_pool.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

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);
--
2.38.1

Re: [PATCH v3] page_pool: add a comment explaining the fragment counter usage

[Jesper Dangaard Brouer]

On 17/02/2023 23.21, Ilias Apalodimas 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.
> 
> Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>


> ---
> Changes since v2:
>   - Removed a uneeded commas on the comment
> Changes since v1:
>   - Update the comment withe the correct description for pp_frag_count
>   include/net/page_pool.h | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> 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);
> --
> 2.38.1
> 

Re: [PATCH v3] page_pool: add a comment explaining the fragment counter usage

[Paolo Abeni]

On Sat, 2023-02-18 at 00:21 +0200, Ilias Apalodimas 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.
> 
> Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

# Form letter - net-next is closed

The merge window for v6.3 has begun and therefore net-next is closed
for new drivers, features, code refactoring and optimizations.
We are currently accepting bug fixes only.

Please repost when net-next reopens after Mar 6th.

RFC patches sent for review only are obviously welcome at any time.

Re: [PATCH v3] page_pool: add a comment explaining the fragment counter usage

[Jakub Kicinski]

On Sat, 18 Feb 2023 00:21:30 +0200 Ilias Apalodimas 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.

I discussed with Paolo off-list, since it's just a comment change 
I'll push it in.

Re: [PATCH v3] page_pool: add a comment explaining the fragment counter usage

[Ilias Apalodimas]

On Tue, 21 Feb 2023 at 19:15, Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Sat, 18 Feb 2023 00:21:30 +0200 Ilias Apalodimas 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.
>
> I discussed with Paolo off-list, since it's just a comment change
> I'll push it in.

Fair enough. Thanks Jakub.

Regards
/Ilias

Re: [PATCH v3] page_pool: add a comment explaining the fragment counter usage

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (master)
by Jakub Kicinski <kuba@kernel.org>:

On Sat, 18 Feb 2023 00:21:30 +0200 you 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.
> 
> [...]

Here is the summary with links:
  - [v3] page_pool: add a comment explaining the fragment counter usage
    https://git.kernel.org/netdev/net-next/c/4d4266e3fd32

You are awesome, thank you!
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