[Xen-devel] RE: [PATCH] less verbose tmem, was: tmem_relinquish_page: failing order=<n>

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Jan Beulich [mailto:JBeulich@xxxxxxxxxx]
> Subject: Re: [PATCH] less verbose tmem, was: tmem_relinquish_page:
> failing order=<n>
> 
> >>> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> 06.01.10 18:13 >>>
> >The message is relevant if any code calling alloc_heap_pages()
> >for order>0 isn't able to fallback to order=0.  All usages today
> >in Xen can fallback, but future calls may not, so I'd prefer
> >to keep the printk there at least in xen-unstable.  BUT there's
> 
> What makes you think so? Iirc there are several xmalloc()-s of more
> than a page in size (which ultimately will call alloc_heap_pages()),
> and those usually don't have a fallback.

I don't know this for a fact, but had discussed it in xen-devel
some time ago (maybe over a year ago).   Since tmem doesn't do
anything until at least one tmem-modified guest uses it, the only
issue is if launching a subsequent domain requires an allocation
of order>0.  Since Xen doesn't have any kind of memory defragmenter,
any such requirement is perilous at best.

> >no reason for the message to be logged in a released Xen
> >so here's a patch to ifndef NDEBUG the printk.
> 
> Even in a debug build this may be really annoying: On NUMA machines,
> the dma_bitsize mechanism (to avoid exhausting DMA memory) can
> cause close to 2000 of these messages when allocating Dom0's
> initial memory (in the absence of a severely restricting dom0_mem,
> and with node 0 spanning 4G or more). This makes booting
> unacceptably slow, especially when using a graphical console mode.

OK, I can take the patch one step further by only doing the
printk if tmem has been used at least once.  Let me take a
look at that.

Dan

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