Re: [Xen-devel] [RFC/PATCH v2] XENMEM_claim_pages (subop of existing) hypercall

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2012-11-20 at 08:16 +0000, Jan Beulich wrote:
> >>> On 19.11.12 at 21:53, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> wrote:
> >>  From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx]
> >> Subject: Re: [Xen-devel] [RFC/PATCH v2] XENMEM_claim_pages (subop of 
> > existing) hypercall
> > 
> > Hi Ian --
> > 
> >> On Thu, 2012-11-15 at 19:15 +0000, Dan Magenheimer wrote:
> >> > > From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx]
> >> > > Subject: Re: [Xen-devel] [RFC/PATCH v2] XENMEM_claim_pages (subop of 
> > existing) hypercall
> >> > >
> >> > > On Wed, 2012-11-14 at 23:55 +0000, Dan Magenheimer wrote:
> >> > >
> >> > > How does this interact with the feature which lets you create PV guests
> >> > > using only superpages? I believe is something Oracle added and still
> >> > > maintains (Dave added to the CC).
> >> > >
> >> > > Also doesn't this fail to make any sort of guarantee if you are 
> >> > > building
> >> > > a 32 bit PV guest, since they require memory under a certain host
> >> > > address limit (160GB IIRC)?
> >> > >
> >> > > Basically neither of those cases benefit from this hypercall at all? I
> >> > > don't know what the usecase for the superpage PV guests is (but I
> >> > > suppose it is important to Oracle, at least). The 32 bit PV guest use
> >> > > case is still a pretty significant one which I think ought to be
> >> > > handled, otherwise any system built on top of this functionality will
> >> > > only work reliably in a subset of cases.
> >> >
> >> > The claim mechanism will not benefit PV superpages.  IIUC, the
> >> > design of the PV superpages will cause a domain launch to fail
> >> > if it requests 10000 superpages but Xen can only successfully
> >> > allocate 9999.  That's already very fragile.  Since the only
> >> > way currently to find out if there are 10000 superpages available
> >> > is to start allocating them, claim can't really help.
> >> 
> >> Well, you could always account the number of free superpages in the
> >> system, which would allow you to cover this case too.
> > 
> > Because of the nature of the buddy allocator (i.e. 4MB chunks are
> > kept separately from 2MB chunks even though a 4MB page contains
> > two 2MB pages), I don't think this is trivial.
> 
> This ought to be as simple as adding respective accounting
> when
> - splitting a chunk in alloc_heap_pages(), crossing the super page
>   size boundary, and
> - merging chunks in free_heap_pages(), crossing the super page
>   size boundary.

That was my first thought too.

Ian.


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