[Xen-devel] RE: [RFC PATCH 0/4] (Take 2): transcendent memory ("tmem") for Linux

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Rik van Riel [mailto:riel@xxxxxxxxxx]

> Dan Magenheimer wrote:
> > "Preswap" IS persistent, but for various reasons may not always be
> > available for use, again due to factors that may not be 
> visible to the
> > kernel (but, briefly, if the kernel is being "good" and has 
> shared its
> > resources nicely, then it will be able to use preswap, else 
> it will not).
> > Once a page is put, a get on the page will always succeed. 
> 
> What happens when all of the free memory on a system
> has been consumed by preswap by a few guests?
> Will the system be unable to start another guest,

The default policy (and only policy implemented as of now) is
that no guest is allowed to use more than max_mem for the
sum of directly-addressable memory (e.g. RAM) and persistent
tmem (e.g. preswap).  So if a guest is using its default
memory==max_mem and is doing no ballooning, nothing can
be put in preswap by that guest.
 
> or is there some way to free the preswap memory?

Yes and no.  There is no way externally to free preswap
memory, but an in-guest userland root service can write to sysfs
to affect preswap size.  This essentially does a partial
swapoff on preswap if there is sufficient (directly addressable)
guest RAM available.  (I have this prototyped as part of
the xenballoond self-ballooning service in xen-unstable.)

Dan

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