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

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Message-Id: <xr93sh69wu2w.fsf@gthelen.svl.corp.google.com>
Date:   Tue, 29 May 2018 23:12:07 -0700
From:   Greg Thelen <gthelen@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
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

Matthew Wilcox <willy@...radead.org> wrote:

> 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.

Nod.  Good tip.  Included in ("[PATCH v2] mm: condense scan_control").

> 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.

Agreed.  For patch simplicity, I'll pass on this for now.