Re: [PATCH] KVM: Avoid wasting pages for small lpage_info arrays

[Avi Kivity <avi@redhat.com>]

On 05/15/2012 11:25 PM, Andrew Morton wrote:
> On Tue, 15 May 2012 11:02:17 +0300
> Avi Kivity <avi@redhat.com> wrote:
>
> > On 05/14/2012 04:29 PM, Takuya Yoshikawa wrote:
> > > On Sun, 13 May 2012 13:20:46 +0300
> > > Avi Kivity <avi@redhat.com> wrote:
> > >
> > > > I don't feel that the savings is worth the extra complication.  We save
> > > > two pages per memslot here.
> > >
> > > Using a 4KB vmalloced page for a 16B array is ...
> > >
> > > Actually I felt like you before and did not do this, but recently there
> > > was a talk about creating hundreds of memslots.
> > >
> > > > What about using kvmalloc() instead of vmalloc()?  It's in
> > > > security/apparmor now, but can be made generic.
> > >
> > > Andrew once, maybe some times, rejected making such an API generic saying
> > > that there should not be a generic criterion by which we can decide which
> > > function - vmalloc() or kmalloc() - to use.
> > >
> > > So each caller should decide by its own criteria.
> > >
> > > In this case, we need to implement kvm specific kvmalloc().
> > > BTW, we are already doing this for dirty_bitmap.
> > 
> > Okay, a local kvmalloc() is better than open-coding the logic.
> > 
> > Andrew, prepare yourself for some code duplication.
>
> There are reasons for avoiding vmalloc().
>
> The kernel does not run in a virtual memory environment.  It is a
> harsh, low-level environment and kernel code should be robust. 

This is about downgrading an existing vmalloc() to kmalloc(), when the
sizes permit, to reduce wastage.  Not about upgrading a kmalloc() to
vmalloc().

> Assuming that you can allocate vast amounts of contiguous memory is not
> robust.  Robust code will implement data structures which avoid this
> weakness.

This is true on some architectures.  On others vast amounts of
contiguous memory _are_ available, and implementing software radix trees
to replace the hardware radix trees is not going to improve things.

-- 
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