linux-kernel - Re: [patch] mm, thp: add new background defrag option

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Message-ID: <alpine.DEB.2.10.1701051446140.19790@chino.kir.corp.google.com>
Date:   Thu, 5 Jan 2017 14:54:07 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch] mm, thp: add new background defrag option

On Thu, 5 Jan 2017, Vlastimil Babka wrote:

> Hmm that's probably why it's hard to understand, because "madvise
> request" is just setting a vma flag, and the THP allocation (and defrag)
> still happens at fault.
> 
> I'm not a fan of either name, so I've tried to implement my own
> suggestion. Turns out it was easier than expected, as there's no kernel
> boot option for "defer", just for "enabled", so that particular worry
> was unfounded.
> 
> And personally I think that it's less confusing when one can enable defer
> and madvise together (and not any other combination), than having to dig
> up the difference between "defer" and "background".
> 

I think allowing only two options to be combined amongst four available 
solo options is going to be confusing and then even more difficult for the 
user to understand what happens when they are combined.  Thus, I think 
these options should only have one settable mode as they have always done.

The kernel implementation takes less of a priority to userspace 
simplicitly, imo, and my patch actually cleans up much of the existing 
code and ends up adding fewer lines that yours.  I consider it an 
improvement in itself.  I don't see the benefit of allowing combined 
options.