linux-kernel - Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers

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Message-ID: <20170116214822.GB9382@dhcp22.suse.cz>
Date:   Mon, 16 Jan 2017 22:48:22 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Al Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Anatoly Stepanov <astepanov@...udlinux.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers

On Mon 16-01-17 13:15:08, John Hubbard wrote:
> 
> 
> On 01/16/2017 11:40 AM, Michal Hocko wrote:
> > On Mon 16-01-17 11:09:37, John Hubbard wrote:
> > > 
> > > 
> > > On 01/16/2017 12:47 AM, Michal Hocko wrote:
> > > > On Sun 15-01-17 20:34:13, John Hubbard wrote:
> > [...]
> > > > > Is that "Reclaim modifiers" line still true, or is it a leftover from an
> > > > > earlier approach? I am having trouble reconciling it with rest of the
> > > > > patchset, because:
> > > > > 
> > > > > a) the flags argument below is effectively passed on to either kmalloc_node
> > > > > (possibly adding, but not removing flags), or to __vmalloc_node_flags.
> > > > 
> > > > The above only says thos are _unsupported_ - in other words the behavior
> > > > is not defined. Even if flags are passed down to kmalloc resp. vmalloc
> > > > it doesn't mean they are used that way.  Remember that vmalloc uses
> > > > some hardcoded GFP_KERNEL allocations.  So while I could be really
> > > > strict about this and mask away these flags I doubt this is worth the
> > > > additional code.
> > > 
> > > I do wonder about passing those flags through to kmalloc. Maybe it is worth
> > > stripping out __GFP_NORETRY and __GFP_NOFAIL, after all. It provides some
> > > insulation from any future changes to the implementation of kmalloc, and it
> > > also makes the documentation more believable.
> > 
> > I am not really convinced that we should take an extra steps for these
> > flags. There are no existing users for those flags and new users should
> > follow the documentation.
> 
> OK, let's just fortify the documentation ever so slightly, then, so that
> users are more likely to do the right thing. How's this sound:
> 
> * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL are not supported. (Even
> * though the current implementation passes the flags on through to kmalloc and
> * vmalloc, that is done for efficiency and to avoid unnecessary code. The caller
> * should not pass in these flags.)
> *
> * __GFP_REPEAT is supported, but only for large (>64kB) allocations.
> 
> 
> ? Or is that documentation overkill?

Dunno, it sounds like an overkill to me. It is telling more than
necessary. If we want to be so vocal about gfp flags then we would have
to say much more I suspect. E.g. what about __GFP_HIGHMEM? This flag is
supported for vmalloc while unsupported for kmalloc. I am pretty sure
there would be other gfp flags to consider and then this would grow
borringly large and uninteresting to the point when people simply stop
reading it. Let's just be as simple as possible.
-- 
Michal Hocko
SUSE Labs