netdev - RE: [PATCH v4] bnx2x: Alloc 4k fragment for each rx ring buffer element

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Message-ID: <B5657A6538887040AD3A81F1008BEC63DA8E49@avmb3.qlogic.org>
Date:	Thu, 28 May 2015 07:24:22 +0000
From:	Yuval Mintz <Yuval.Mintz@...gic.com>
To:	Michal Schmidt <mschmidt@...hat.com>,
	Gabriel Krisman Bertazi <krisman@...ux.vnet.ibm.com>
CC:	David Miller <davem@...emloft.net>,
	"LinoSanfilippo@....de" <LinoSanfilippo@....de>,
	Ariel Elior <Ariel.Elior@...gic.com>,
	netdev <netdev@...r.kernel.org>,
	"cascardo@...cardo.eti.br" <cascardo@...cardo.eti.br>,
	"brking@...ux.vnet.ibm.com" <brking@...ux.vnet.ibm.com>
Subject: RE: [PATCH v4] bnx2x: Alloc 4k fragment for each rx ring buffer
 element

>> +struct bnx2x_alloc_pool {
>> +     struct page     *page;
>> +     dma_addr_t      dma;
>> +     u8              offset;
>> +     u8              frag_count;
>> +};
>...
>>  static int bnx2x_alloc_rx_sge(struct bnx2x *bp, struct bnx2x_fastpath *fp,
>>                             u16 index, gfp_t gfp_mask)
>>  {
>        ...
>> +     pool->offset += SGE_PAGE_SIZE;
>> +     pool->frag_count--;
>> +
>>       return 0;
>>  }

> One SGE_PAGE_SIZE is already bigger than representable by u8, so offset
> will overflow.

Thanks for the catch Michal.

Actually, this upsets me greatly. We didn't see it on a system with 4KB
pages, but this means you've actually tried to 'sell' us a fastpath fix that
was never tested on machines for which it was meant as an improvement.

Dave - if possible, please wait with accepting any further fixes for this
issue until we [qlogic] manage to prepare a test environment
where we can properly test this with 64KB page size architecture.

Thanks,
Yuval--
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