netdev - Re: [PATCH net-next] net/rds: Use DMA memory pool allocation for rds

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Message-Id: <20191001.101615.1260420946739435364.davem@davemloft.net>
Date:   Tue, 01 Oct 2019 10:16:15 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     ka-cheong.poon@...cle.com
Cc:     netdev@...r.kernel.org, santosh.shilimkar@...cle.com,
        rds-devel@....oracle.com
Subject: Re: [PATCH net-next] net/rds: Use DMA memory pool allocation for
 rds_header

From: Ka-Cheong Poon <ka-cheong.poon@...cle.com>
Date: Mon, 30 Sep 2019 02:08:00 -0700

> Currently, RDS calls ib_dma_alloc_coherent() to allocate a large piece
> of contiguous DMA coherent memory to store struct rds_header for
> sending/receiving packets.  The memory allocated is then partitioned
> into struct rds_header.  This is not necessary and can be costly at
> times when memory is fragmented.  Instead, RDS should use the DMA
> memory pool interface to handle this.
> 
> Suggested-by: Håkon Bugge <haakon.bugge@...cle.com>
> Signed-off-by: Ka-Cheong Poon <ka-cheong.poon@...cle.com>

This is trading a one-time overhead for extra levels of dereferencing
on every single descriptor access in the fast paths.

I do not agree with this tradeoff, please implement this more
reasonably.