netdev - Re: [PATCH net-next v4 1/5] page_pool: frag API support for 32-bit arch with 64-bit DMA

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Date: Tue, 13 Jun 2023 21:09:06 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Yunsheng Lin <linyunsheng@...wei.com>
Cc: <davem@...emloft.net>, <pabeni@...hat.com>, <netdev@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, Lorenzo Bianconi <lorenzo@...nel.org>,
 Alexander Duyck <alexander.duyck@...il.com>, Saeed Mahameed
 <saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>, Eric Dumazet
 <edumazet@...gle.com>, Jesper Dangaard Brouer <hawk@...nel.org>, Ilias
 Apalodimas <ilias.apalodimas@...aro.org>, <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH net-next v4 1/5] page_pool: frag API support for 32-bit
 arch with 64-bit DMA

On Mon, 12 Jun 2023 21:02:52 +0800 Yunsheng Lin wrote:
> Currently page_pool_alloc_frag() is not supported in 32-bit
> arch with 64-bit DMA, which seems to be quite common, see
> [1], which means driver may need to handle it when using
> page_pool_alloc_frag() API.
> 
> In order to simplify the driver's work for supporting page
> frag, this patch allows page_pool_alloc_frag() to call
> page_pool_alloc_pages() to return a big page frag without

it returns an entire (potentially compound) page, not a frag.
AFAICT

> page splitting because of overlap issue between pp_frag_count
> and dma_addr_upper in 'struct page' for those arches.

These two lines seem to belong in the first paragraph,

> As page_pool_create() with PP_FLAG_PAGE_FRAG is supported in

"is" -> "will now be"

> 32-bit arch with 64-bit DMA now, mlx5 calls page_pool_create()
> with PP_FLAG_PAGE_FRAG and manipulate the page->pp_frag_count
> directly using the page_pool_defrag_page(), so add a checking
> for it to aoivd writing to page->pp_frag_count that may not
> exist in some arch.

This paragraph needs some proof reading :(

> Note that it may aggravate truesize underestimate problem for
> skb as there is no page splitting for those pages, if driver
> need a accuate truesize, it may calculate that according to

accurate

> frag size, page order and PAGE_POOL_DMA_USE_PP_FRAG_COUNT
> being true or not. And we may provide a helper for that if it
> turns out to be helpful.
> 
> 1. https://lore.kernel.org/all/20211117075652.58299-1-linyunsheng@huawei.com/

> +		/* Return error here to avoid writing to page->pp_frag_count in
> +		 * mlx5e_page_release_fragmented() for page->pp_frag_count is

I don't see any direct access to pp_frag_count anywhere outside of
page_pool.h in net-next. PAGE_POOL_DMA_USE_PP_FRAG_COUNT sounds like 
an internal flag, drivers shouldn't be looking at it, IMO.