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

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Message-ID: <20170728102743.GI2274@dhcp22.suse.cz>
Date:   Fri, 28 Jul 2017 12:27:43 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     linux-mm@...ck.org, 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 Fri 28-07-17 10:52:49, Mel Gorman wrote:
> On Fri, Jul 28, 2017 at 11:19:04AM +0200, Michal Hocko wrote:
> > 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.
> > 
> 
> At the time of the introduction, the users were all very short-lived
> where short was for operations such as reading a proc file that discarded
> buffers afterwards.

Maybe we can add a special slab cache for those?

> However, it does seem to have misused over the last
> few years and it was too easy to confuse "temporary" with "short lived"
> and too easy to get confused about "how short lived is short lived?". On
> that basis;
> 
> Acked-by: Mel Gorman <mgorman@...e.de>

Thanks!
-- 
Michal Hocko
SUSE Labs