linux-kernel - Re: [PATCH] net/mlx5e: Make DEFAULT_FRAG

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Message-ID: <l3st5aik5jtsexq6yng5el5txeif4itbg35kl2ft32zhi3pmef@kn4x6bo4ws7s>
Date: Thu, 4 Sep 2025 16:31:38 +0000
From: Dragos Tatulea <dtatulea@...dia.com>
To: Mingrui Cui <mingruic@...look.com>, Saeed Mahameed <saeedm@...dia.com>, 
	Leon Romanovsky <leon@...nel.org>, Tariq Toukan <tariqt@...dia.com>, Mark Bloch <mbloch@...dia.com>, 
	Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, linux-rdma@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net/mlx5e: Make DEFAULT_FRAG_SIZE relative to page size

On Tue, Sep 02, 2025 at 09:00:16PM +0800, Mingrui Cui wrote:
> When page size is 4K, DEFAULT_FRAG_SIZE of 2048 ensures that with 3
> fragments per WQE, odd-indexed WQEs always share the same page with
> their subsequent WQE. However, this relationship does not hold for page
> sizes larger than 8K. In this case, wqe_index_mask cannot guarantee that
> newly allocated WQEs won't share the same page with old WQEs.
> 
> If the last WQE in a bulk processed by mlx5e_post_rx_wqes() shares a
> page with its subsequent WQE, allocating a page for that WQE will
> overwrite mlx5e_frag_page, preventing the original page from being
> recycled. When the next WQE is processed, the newly allocated page will
> be immediately recycled.
> 
> In the next round, if these two WQEs are handled in the same bulk,
> page_pool_defrag_page() will be called again on the page, causing
> pp_frag_count to become negative.
> 
> Fix this by making DEFAULT_FRAG_SIZE always equal to half of the page
> size.
>
Was there an actual encountered issue or is this a code clarity fix?

For 64K page size, linear mode will be used so the constant will not be
used for calculating the frag size.

Thanks,
Dragos