linux-ext4 - Re: [RFC PATCH] mm: introduce kv[mz]alloc helpers

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Message-ID: <20161220135016.GH3769@dhcp22.suse.cz>
Date:   Tue, 20 Dec 2016 14:50:16 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Anatoly Stepanov <astepanov@...udlinux.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Theodore Ts'o <tytso@....edu>, kvm@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-security-module@...r.kernel.org,
        Dave Chinner <david@...morbit.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Joe Perches <joe@...ches.com>
Subject: Re: [RFC PATCH] mm: introduce kv[mz]alloc helpers

On Wed 14-12-16 09:59:16, Michal Hocko wrote:
> On Tue 13-12-16 14:07:33, Joe Perches wrote:
> > On Tue, 2016-12-13 at 11:14 +0100, Michal Hocko wrote:
> > > Are there any more comments or objections to this patch? Is this a good
> > > start or kv[mz]alloc has to provide a way to cover GFP_NOFS users as
> > > well in the initial version.
> > 
> > Did Andrew Morton ever comment on this?
> > I believe he was the primary objector in the past.
> > 
> > Last I recollect was over a year ago:
> > 
> > https://lkml.org/lkml/2015/7/7/1050
> 
> Let me quote:
> : Sigh.  We've resisted doing this because vmalloc() is somewhat of a bad
> : thing, and we don't want to make it easy for people to do bad things.
> : 
> : And vmalloc is bad because a) it's slow and b) it does GFP_KERNEL
> : allocations for page tables and c) it is susceptible to arena
> : fragmentation.
> : 
> : We'd prefer that people fix their junk so it doesn't depend upon large
> : contiguous allocations.  This isn't userspace - kernel space is hostile
> : and kernel code should be robust.
> : 
> : So I dunno.  Should we continue to make it a bit more awkward to use
> : vmalloc()?  Probably that tactic isn't being very successful - people
> : will just go ahead and open-code it.  And given the surprising amount
> : of stuff you've placed in kvmalloc_node(), they'll implement it
> : incorrectly...
> : 
> : How about we compromise: add kvmalloc_node(), but include a BUG_ON("you
> : suck") to it?
> 
> While I agree with some of those points, the reality really sucks,
> though. We have tried the same tactic with __GFP_NOFAIL and failed as
> well. I guess we should just bite the bullet and provide an api which is
> so common that people keep reinventing their own ways around that, many
> times wrongly or suboptimally. BUG_ON("you suck") is just not going to
> help much I am afraid.
> 
> What do you think Andrew?

So what are we going to do about this patch?
-- 
Michal Hocko
SUSE Labs
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