Re: [tip:core/memblock] x86, memblock: Fix crashkernel allocation

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Oct 06, 2010 at 04:09:29PM -0700, H. Peter Anvin wrote:
> On 10/06/2010 03:47 PM, Vivek Goyal wrote:
> > 
> > I really don't mind fixing the things properly in long term, just that I am
> > running out of ideas regarding how to fix it in proper way.
> > 
> > To me the best thing would be that this whole allocation thing be dyanmic
> > from user space where kexec will run, determine what it is loading, 
> > determine what are the memory contstraints on these segments (min, upper
> > limit, alignment etc), and then ask kernel for reserving contiguous
> > memory. This kind of dynamic reservation will remove lot of problems
> > associated with crashkernel= reservations.
> > 
> > But I am not aware of anyway of doing dynamic allocation and it certainly
> > does not seem to be easy to be able to allocated 128M of memory contiguously.
> > 
> > Because we don't have a way to reserve memory dynamically later, we end up
> > doing a big chunk of reservation using kernel command line and later
> > figure out what to load where. Now with this approach kexec has not even run
> > so how it can tell you what are the memory constraints.
> > 
> > So to me one of the ways of properly fixing is adding some kind of
> > capability to reserve the memory dynamically (may be using sys_kexec())
> > and get rid of this notion of reserving memory at boot time.
> 
> The problem, of course, will allocating very large chunks of memory at
> runtime is that there are going to be some number of non-movable and
> non-evictable pages that are going to break up the contiguous ranges.
> However, the mm recently added support for moving most pages, which
> should make that kind of allocation a lot more feasible.  I haven't
> experimented how well it works in practice, but I rather suspect that as
> long as the crashkernel is installed sufficiently early in the boot
> process it should have a very good probability of success.

Ok.

>  Another
> option, although one which has its own hackiness issues, is to do a
> conservative allocation at boot time in preparation of the kexec call,
> which is then freed.  This doesn't really address the issue of location,
> though, which is part of the problem here.
> 
> > The other concern you raised is hiding constraints from kernel. At this
> > point of time the only problem with crashkernel=X@0 syntax is that it
> > does not tell you whether to look for memory bottom up or top down. How
> > about if we specify it explicitly in the syntax so that kernel does not
> > have to assume things?
> 
> See below.
> 
> > In fact the initial crashkernel syntax was. crashkernel=X@Y. This meant
> > allocated X amount of memory at location Y. This left no ambiguity and
> > kernel did not have to assume things. It had the problem though that 
> > we might not have physical RAM at location Y. So I think that's when
> > somebody came up with the idea of crashkernel=X@0 so that we ideally
> > want memory at location 0, but if you can't provide that, then provide
> > anything available next scanning bottom up. 
> > 
> > So the only part missing from syntax is explicitly speicifying "next
> > available location scanning bottom up". If we add that to syntax then
> > kernel does not have to make assumptions. (except the alignment part).
> > 
> > So how about modifying syntax to crashkernel=X@Y#BU.
> > 
> > The "#BU" part can be optional and in that case kernel is free to allocate
> > memory either top down or bottom up.
> > 
> > Or any other string which can communicate the bottom up part in a more 
> > intutive manner.
> 
> The whole problem here is that "bottoms up" isn't the true constraint --
> it's a proxy for "this chunk needs < address X, this chunk needs <
> address Y, ..." which is the real issue.  This is particularly messy
> since low memory is a (sometimes very) precious resource that is used by
> a lot of things (BIOS stubs, DMA-mask-limited hardware devices, and
> perhaps especially 1:1 mappable pages on 32 bits, and so on), and one of
> the major reasons we want to switch to a top-down allocation scheme is
> to not waste a precious resource when we don't have to.
> 
> The one improvement one could to the crashkernel= syntax is perhaps
> "crashkernel=X<Y" meaning "allocate entirely below Y", since that is (at
> least in part) the real constraint.  It could even be extended to
> multiple segments: "crashkernel=X<Y,Z<W,..." if we really need to...
> that way you have your preallocation.

Ok, I was browsing through kexec-tools, x86 bzImage code and trying to
refresh my memory what segments were being loaded and what were memory
address concerns.

- relocatable bzImage (max addr 0x37ffffff, 896MB). 
	Though I don't know/understand where that 896MB come from.

- initrd (max addr 0x37ffffff, 896MB)
	Don't know why 896MB as upper limit

- Purgatory (max addr 2G)

- A segment to keep elf headers (no limit)
	These are accessed when second kernel as fully booted so can be
	addressed in higher addresses.

- A backup segment to copy first 640K of memory (not aware of any limit)
- Setup/parameter segment (no limit)
	- We don't really execute anything here and just access it for
  	  command line.

So atleast for bzImage it looks that if we specify crashkernel=128M<896M, it
will work.

So I am fine with above additional syntax for crashkernel=. May be we shall
have to the deprecate the crashkernel=X<@0 syntax.

CCing kexec list, in case others have any comments.

Thanks
Vivek

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