linux-kernel - Re: [RFC] free

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Message-ID: <20160105154207.GF15324@dhcp22.suse.cz>
Date:	Tue, 5 Jan 2016 16:42:07 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org
Subject: Re: [RFC] free_pages stuff

On Tue 05-01-16 15:26:02, Al Viro wrote:
> On Tue, Jan 05, 2016 at 02:59:03PM +0100, Michal Hocko wrote:
> 
> > > 3) vmalloc() is for large allocations.  They will be page-aligned,
> > > but *not* physically contiguous.  OTOH, large physically contiguous
> > > allocations are generally a bad idea.  Unlike other allocators, there's
> > > no variant that could be used in interrupt; freeing is possible there,
> > > but allocation is not.  Note that non-blocking variant *does* exist -
> > > __vmalloc(size, GFP_ATOMIC, PAGE_KERNEL) can be used in atomic
> > > contexts; it's the interrupt ones that are no-go.
> 
> The last sentence I'd put into that part was complete crap...
> 
> > It is also hardcoded GFP_KERNEL context so a usage from NOFS context
> > needs a special treatment.
> 
> ... in part because of this.  GFP_ATOMIC __vmalloc() will be anything but,
> and the only caller passing that is almost certainly bogus.

Agreed as just replied in the other email thread which I have noticed
only now.

> As for NOFS/NOIO,
> I wonder if we should apply that special treatment inside __vmalloc_area_node
> rather than in callers; see the current thread on linux-mm for details...

That would make a lot of sense to me. Spreading the _special_ treatment
all over the kernel is certainly worse.
 
> Another interesting issue is __GFP_HIGHMEM meaning for kmalloc and __vmalloc
> resp. (should never be passed to kmalloc, should almost always be passed
> to __vmalloc - the former needs pages mapped in kernel space, the latter
> probably never needs a separate kernel alias for the data pages, to such
> degree that I'm not sure if we shouldn't _force_ __GFP_HIGHMEM for data pages
> allocation in __vmalloc_area_node())

I would have to think about this one some more. Let's not fragment the
discussion and continue in that email thread:
http://lkml.kernel.org/r/20160103071246.GK9938%40ZenIV.linux.org.uk

-- 
Michal Hocko
SUSE Labs
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