Re: [RFC] respect the referenced bit of KVM guest pages?

[Avi Kivity <avi@redhat.com>]

On 08/06/2009 01:08 PM, Andrea Arcangeli wrote:
> After some conversation it seems reactivating on large systems
> generates troubles to the VM as young bit have excessive time to be
> reactivated, giving troubles to shrink active list. I see that, so
> then the check should be still nuked, but the unconditional
> deactivation should happen instead. Otherwise it's trivial to put the
> VM to its knees and DoS it with a simple mmap of a file with MAP_EXEC
> as parameter of mmap. My whole point is that deciding if activating or
> deactivating pages can't be in function  of VM_EXEC, and clearly it
> helps on desktops but then it probably is a signal that the VM isn't
> good enough by itself to identify the important working set using
> young bits and stuff on desktop systems, and if there's a good reason
> to not activate, we shouldn't activate the VM_EXEC either as anything
> and anybody can generate a file mapping with VM_EXEC set...
>    

Reasonable; if you depend on a hint from userspace, that hint can be 
used against you.

> Likely we need a cut-off point, if we detect it takes more than X
> seconds to scan the whole active list, we start ignoring young bits,
> as young bits don't provide any meaningful information then and they
> just hang the VM in preventing it to shrink active list and looping
> over it endlessy with million pages inside that list. But on small
> systems if inactive list is short it may be too quick to just clear
> the young bit and only giving it time to be re-enabled in inactive
> list. That may be the source of the problem. Actually I'm speculating
> here, because I barely understood that this is swapin... not sure
> exactly what this regression is about but testing the patch posted is
> good idea and it will tell us if we just need to dynamically
> differentiating the algorithm between large and small systems and start
> ignoring young bits only at some point.
>    

How about, for every N pages that you scan, evict at least 1 page, 
regardless of young bit status?  That limits overscanning to a N:1 
ratio.  With N=250 we'll spend at most 25 usec in order to locate one 
page to evict.


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