Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page

[Andrew Morton]

On Tue, 23 Oct 2012 09:35:32 +0300 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote:

> On Fri, Oct 19, 2012 at 02:59:41AM +0300, Kirill A. Shutemov wrote:
> > On Thu, Oct 18, 2012 at 04:45:02PM -0700, Andrew Morton wrote:
> > > On Mon, 15 Oct 2012 09:00:59 +0300
> > > "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > > 
> > > > H. Peter Anvin doesn't like huge zero page which sticks in memory forever
> > > > after the first allocation. Here's implementation of lockless refcounting
> > > > for huge zero page.
> > > > 
> > > > We have two basic primitives: {get,put}_huge_zero_page(). They
> > > > manipulate reference counter.
> > > > 
> > > > If counter is 0, get_huge_zero_page() allocates a new huge page and
> > > > takes two references: one for caller and one for shrinker. We free the
> > > > page only in shrinker callback if counter is 1 (only shrinker has the
> > > > reference).
> > > > 
> > > > put_huge_zero_page() only decrements counter. Counter is never zero
> > > > in put_huge_zero_page() since shrinker holds on reference.
> > > > 
> > > > Freeing huge zero page in shrinker callback helps to avoid frequent
> > > > allocate-free.
> > > 
> > > I'd like more details on this please.  The cost of freeing then
> > > reinstantiating that page is tremendous, because it has to be zeroed
> > > out again.  If there is any way at all in which the kernel can be made
> > > to enter a high-frequency free/reinstantiate pattern then I expect the
> > > effects would be quite bad.
> > > 
> > > Do we have sufficient mechanisms in there to prevent this from
> > > happening in all cases?  If so, what are they, because I'm not seeing
> > > them?
> > 
> > We only free huge zero page in shrinker callback if nobody in the system
> > uses it. Never on put_huge_zero_page(). Shrinker runs only under memory
> > pressure or if user asks (drop_caches).
> > Do you think we need an additional protection mechanism?
> 
> Andrew?
> 

Well, how hard is it to trigger the bad behavior?  One can easily
create a situation in which that page's refcount frequently switches
from 0 to 1 and back again.  And one can easily create a situation in
which the shrinkers are being called frequently.  Run both at the same
time and what happens?
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