Re: [Xen-devel] sparse M2P table

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 16.09.09 15:40 >>>
>Ah, I'd forgotten we guarantee that the M2P is made up of aligned 2MB
>extents. That given I don't think we should bother having mapping holes
>after all -- the cost of the extra page-directory entries is only ~4kB per
>1GB of M2P. What I would instead do is just alias the first 2MB extent of
>the M2P into every 'empty' 2MB extent of the M2P (this is just a handy
>'safe' 2MB piece of memory to map in, so that (a) tools requests to map the
>M2P just continue to work; and (b) so that we have no doubts about guests
>possibly not handling faults on accesses into the M2P). Aliasing the first
>M2P extent like that just avoids us burning another 2MB for no good reason,
>to map regions of the M2P which really only contain 'garbage'.
>
>Does that sound good?

Indeed, it doesn't really matter *what* gets mapped there. I wouldn't,
however, strictly stick to the first chunk - if there's a 1G chunk, and later
a 1G hole, I'd prefer using the existing 1G chunk there, and iteratively
populate it with 2M chunks only when no 1G chunk was previously
allocated. Perhaps (depending on what turns out simpler) I might also
just use the most recent chunk for sticking in a hole...

Jan


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