linux-kernel - Re: [PATCH] mm: convert scan_control.priority int => byte

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Message-ID: <20180529025722.GA25784@bombadil.infradead.org>
Date:   Mon, 28 May 2018 19:57:22 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Greg Thelen <gthelen@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: convert scan_control.priority int => byte

On Mon, May 28, 2018 at 07:40:25PM -0700, Greg Thelen wrote:
> Reclaim priorities range from 0..12(DEF_PRIORITY).
> scan_control.priority is a 4 byte int, which is overkill.
> 
> Since commit 6538b8ea886e ("x86_64: expand kernel stack to 16K") x86_64
> stack overflows are not an issue.  But it's inefficient to use 4 bytes
> for priority.

If you're looking to shave a few more bytes, allocation order can fit
in a u8 too (can't be more than 6 bits, and realistically won't be more
than 4 bits).  reclaim_idx likewise will fit in a u8, and actually won't
be more than 3 bits.

I am sceptical that nr_to_reclaim should really be an unsigned long; I
don't think we should be trying to free 4 billion pages in a single call.
nr_scanned might be over 4 billion (!) but nr_reclaimed can probably
shrink to unsigned int along with nr_to_reclaim.