[Xen-devel] tmem - really default to on?

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

Dan,

after tmem was turned on by default we're getting reports of domain
creation failures which go away as soon as tmem=0 gets specified. In
particular we see this happen even when there are several Gb of
freeable memory reported. As I understand it this is in particular with
respect to multi page allocations not being handled in tmem code, and
specifically shadow code tries to allocate a non negligible amount of
order-2 pages (and iirc iommu code also continues to require - at least
on large systems - higher order allocations). Unless I'm
misunderstanding something, this is a design limitation that can only
be overcome by eliminating all post-boot non-order-0 allocations that
cannot fall back to order-0 ones, and hence defaulting tmem to on
should be reconsidered.

Besides that, in trying to reproduce this in some way I also get the
impression that tmem's memory consumption may significantly
depend on the type of file system used - on my test box (using
reiserfs) I cannot get tmem to consume any memory. Any
explanation for this (I did verify that there are pools for each of
the partitions)?

Thanks, Jan


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