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

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Date:   Mon, 28 Aug 2017 14:35:42 +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 Fri 25-08-17 23:39:36, Pavel Machek wrote:
> On Fri 2017-08-25 10:04:42, Michal Hocko wrote:
> > On Fri 25-08-17 09:28:19, Pavel Machek wrote:
> > > On Fri 2017-08-25 08:35:46, Michal Hocko wrote:
> > > > On Wed 23-08-17 19:57:09, Pavel Machek wrote:
> > [...]
> > > > > 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.
> > > 
> > > It has real meaning.
> > 
> > Which is?
> 
> "This allocation is temporary. It lasts milliseconds, not hours."

And why would such a semantic make any sense what so ever? We certainly
do not try to wait for a pinned memory for $TIMEOUT when somebody really
needs a larger memory block and there is a temporary allocation standing
in the way. We simply do not know that an object is a temporary one.

> > > You can define more exact meaning, and then adjust the usage. But
> > > there's no need to do treewide replacement...
> > 
> > I have checked most of them and except for the initially added onces the
> > large portion where added without a good reasons or even break an
> > intuitive meaning by taking locks.
> 
> I don't see it. kmalloc() itself takes locks. Of course everyone takes
> locks. I don't think that's intuitive meaning.

I was talking about users of the flag. I have seen some to take a lock
right after they allocated GFP_TEMPORARY object.

> > Seriously, if we need a short term semantic it should be clearly defined
> > first.
> 
> "milliseconds, not hours."
> 
> > Is there any specific case why you think this patch is in a wrong
> > direction? E.g. a measurable regression?
> 
> Not playing that game. You should argue why it is improvement. And I
> don't believe you did.

Please read the whole changelog where I was quite verbose about how the
current flag is abused and how its semantic is weak and encourages a
wrong usage pattern. Moreover it is not even clear whether it helps
anything. I haven't seen any actual counter argument from you other than
"milliseconds not hours" without actually explaining how that would be
useful for any decisions done in the core MM layer.
-- 
Michal Hocko
SUSE Labs