linux-kernel - Re: strange PAGE_ALLOC_COSTLY_ORDER usage in xgbe_map_rx

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Date:   Fri, 2 Jun 2017 09:20:54 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: strange PAGE_ALLOC_COSTLY_ORDER usage in xgbe_map_rx_buffer

On 5/31/2017 11:04 AM, Michal Hocko wrote:
> Hi Tom,

Hi Michal,

> I have stumbled over the following construct in xgbe_map_rx_buffer
> 	order = max_t(int, PAGE_ALLOC_COSTLY_ORDER - 1, 0);
> which looks quite suspicious. Why does it PAGE_ALLOC_COSTLY_ORDER - 1?
> And why do you depend on PAGE_ALLOC_COSTLY_ORDER at all?
> 

The driver tries to allocate a number of pages to be used as receive
buffers.  Based on what I could find in documentation, the value of
PAGE_ALLOC_COSTLY_ORDER is the point at which order allocations
(could) get expensive.  So I decrease by one the order requested. The
max_t test is just to insure that in case PAGE_ALLOC_COSTLY_ORDER ever
gets defined as 0, 0 would be used.

I believe there have been some enhancements relative to speed in
allocating 0-order pages recently that may make this unnecessary. I
haven't run any performance tests yet to determine if I can just go to
a 0-order allocation, though.

Thanks,
Tom

> Thanks!
>