On Tue, 23 Oct 2012 09:35:32 +0300 "Kirill A. Shutemov" [off-list ref] wrote:

On Fri, Oct 19, 2012 at 02:59:41AM +0300, Kirill A. Shutemov wrote:

quoted

On Thu, Oct 18, 2012 at 04:45:02PM -0700, Andrew Morton wrote:

quoted

On Mon, 15 Oct 2012 09:00:59 +0300
"Kirill A. Shutemov" [off-list ref] wrote:

quoted

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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