linux-kernel - Re: [RFC PATCH v2 12/26] KVM: arm64: Introduce a Hyp buddy page allocator

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Date:   Thu, 4 Feb 2021 18:13:18 +0000
From:   Will Deacon <will@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, android-kvm@...gle.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        Fuad Tabba <tabba@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        David Brazdil <dbrazdil@...gle.com>
Subject: Re: [RFC PATCH v2 12/26] KVM: arm64: Introduce a Hyp buddy page
 allocator

On Thu, Feb 04, 2021 at 06:01:12PM +0000, Quentin Perret wrote:
> On Thursday 04 Feb 2021 at 17:48:49 (+0000), Will Deacon wrote:
> > On Thu, Feb 04, 2021 at 02:52:52PM +0000, Quentin Perret wrote:
> > > On Thursday 04 Feb 2021 at 14:31:08 (+0000), Will Deacon wrote:
> > > > On Wed, Feb 03, 2021 at 06:33:30PM +0000, Quentin Perret wrote:
> > > > > On Tuesday 02 Feb 2021 at 18:13:08 (+0000), Will Deacon wrote:
> > > > > > On Fri, Jan 08, 2021 at 12:15:10PM +0000, Quentin Perret wrote:
> > > > > > > + *   __find_buddy(pool, page 0, order 0) => page 1
> > > > > > > + *   __find_buddy(pool, page 0, order 1) => page 2
> > > > > > > + *   __find_buddy(pool, page 1, order 0) => page 0
> > > > > > > + *   __find_buddy(pool, page 2, order 0) => page 3
> > > > > > > + */
> > > > > > > +static struct hyp_page *__find_buddy(struct hyp_pool *pool, struct hyp_page *p,
> > > > > > > +				     unsigned int order)
> > > > > > > +{
> > > > > > > +	phys_addr_t addr = hyp_page_to_phys(p);
> > > > > > > +
> > > > > > > +	addr ^= (PAGE_SIZE << order);
> > > > > > > +	if (addr < pool->range_start || addr >= pool->range_end)
> > > > > > > +		return NULL;
> > > > > > 
> > > > > > Are these range checks only needed because the pool isn't required to be
> > > > > > an exact power-of-2 pages in size? If so, maybe it would be more
> > > > > > straightforward to limit the max order on a per-pool basis depending upon
> > > > > > its size?
> > > > > 
> > > > > More importantly, it is because pages outside of the pool are not
> > > > > guaranteed to be covered by the hyp_vmemmap, so I really need to make
> > > > > sure I don't dereference them.
> > > > 
> > > > Wouldn't having a per-pool max order help with that?
> > > 
> > > The issue is, I have no alignment guarantees for the pools, so I may end
> > > up with max_order = 0 ...
> > 
> > Yeah, so you would still need the range tracking,
> 
> Hmm actually I don't think I would, but that would essentially mean the
> 'buddy' allocator is now turned into a free list of single pages
> (because we cannot create pages of order 1).

Right, I'm not suggesting we do that.

> > but it would at least help
> > to reduce HYP_MAX_ORDER failed searches each time. Still, we can always do
> > that later.
> 
> Sorry but I am not following. In which case do we have HYP_MAX_ORDER
> failed searches?

I was going from memory, but the loop in __hyp_alloc_pages() searches up to
HYP_MAX_ORDER, whereas this is _never_ going to succeed beyond some per-pool
order determined by the size of the pool. But I doubt it matters -- I
thought we did more than just check a list.

Will