netdev - Re: [net RFC] net/mlx4_en: Use frag stride in crossing page boundary condition

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Date:   Thu, 14 Jun 2018 20:47:59 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "kafai@...com" <kafai@...com>, Tariq Toukan <tariqt@...lanox.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>
Subject: Re: [net RFC] net/mlx4_en: Use frag stride in crossing page boundary
 condition

On Thu, 2018-06-14 at 12:12 -0700, Eric Dumazet wrote:
> 
> On 06/14/2018 11:56 AM, Saeed Mahameed wrote:
> 
> > Interestingly for this exact frag_stride we don't have an issue :)
> > since it goes through a different condition branch
> > (the page flipping thing):
> > 
> > if (frag_info->frag_stride == PAGE_SIZE / 2) {
> >     frags->page_offset ^= PAGE_SIZE / 2;
> > 	release = page_count(page) != 1 ||
> > 		  page_is_pfmemalloc(page) ||
> > 		  page_to_nid(page) != numa_mem_id();
> > 
> 
> I guess you forgot to test on PowerPC where PAGE_SIZE=65536 ?
> 
> On PowerPC, the first branch is never taken.
> 

This code is already in the driver, i guess as optimization for when a
page can be reused only twice (PAGE_SIZE=4K, strides_size = 2k).
frag_stride is never more than 2k.

for power PC we should always take the other branch which will reuse as
much 2k strides as possible in a page regardless of PAGE_SIZE.

so are we good with my change ?