linux-kernel - Re: [RFC] free

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Date:	Tue, 5 Jan 2016 15:26:02 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Michal Hocko <mhocko@...nel.org>
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, 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.  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...

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())

> > 4) if it's very early in bootstrap, alloc_bootmem() and friends
> > may be the only option.  Rule of the thumb: if it's already printed
> > Memory: ...../..... available.....
> > you shouldn't be using that one.  Allocations are physically contiguous
> > and at that point large physically contiguous allocations are still OK.

Probably needs at least some discussion of memblock vs. bootmem APIs.
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