linux-kernel - Re: [RFC PATCH] treewide: remove GFP

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Message-ID: <20170825063545.GA25498@dhcp22.suse.cz>
Date:   Fri, 25 Aug 2017 08:35:46 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Pavel Machek <pavel@....cz>
Cc:     linux-mm@...ck.org, Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Neil Brown <neilb@...e.de>,
        Theodore Ts'o <tytso@....edu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] treewide: remove GFP_TEMPORARY allocation flag

On Wed 23-08-17 19:57:09, Pavel Machek wrote:
> Hi!
> 
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > GFP_TEMPORARY has been introduced by e12ba74d8ff3 ("Group short-lived
> > and reclaimable kernel allocations") along with __GFP_RECLAIMABLE. It's
> > primary motivation was to allow users to tell that an allocation is
> > short lived and so the allocator can try to place such allocations close
> > together and prevent long term fragmentation. As much as this sounds
> > like a reasonable semantic it becomes much less clear when to use the
> > highlevel GFP_TEMPORARY allocation flag. How long is temporary? Can
> > the context holding that memory sleep? Can it take locks? It seems
> > there is no good answer for those questions.
> > 
> > The current implementation of GFP_TEMPORARY is basically
> > GFP_KERNEL | __GFP_RECLAIMABLE which in itself is tricky because
> > basically none of the existing caller provide a way to reclaim the
> > allocated memory. So this is rather misleading and hard to evaluate for
> > any benefits.
> > 
> > I have checked some random users and none of them has added the flag
> > with a specific justification. I suspect most of them just copied from
> > other existing users and others just thought it might be a good idea
> > to use without any measuring. This suggests that GFP_TEMPORARY just
> > motivates for cargo cult usage without any reasoning.
> > 
> > I believe that our gfp flags are quite complex already and especially
> > those with highlevel semantic should be clearly defined to prevent from
> > confusion and abuse. Therefore I propose dropping GFP_TEMPORARY and
> > replace all existing users to simply use GFP_KERNEL. Please note that
> > SLAB users with shrinkers will still get __GFP_RECLAIMABLE heuristic
> > and so they will be placed properly for memory fragmentation prevention.
> > 
> > I can see reasons we might want some gfp flag to reflect shorterm
> > allocations but I propose starting from a clear semantic definition and
> > only then add users with proper justification.
> 
> Dunno. < 1msec probably is temporary, 1 hour probably is not. If it causes
> problems, can you just #define GFP_TEMPORARY GFP_KERNEL ? Treewide replace,
> and then starting again goes not look attractive to me.

I do not think we want a highlevel GFP_TEMPORARY without any meaning.
This just supports spreading the flag usage without a clear semantic
and it will lead to even bigger mess. Once we can actually define what
the flag means we can also add its users based on that new semantic.
-- 
Michal Hocko
SUSE Labs