So (says the person who only ever uses i386 and ia64), does an arch exist
which needs highmem/kmap, but only ever gives 1 kmap in the pool?
> > This works for us because we are the only consumer of huge amounts of kmaps
> > on our systems, but it would be nice to have a generic interface to do that
> > so that multiple apps don't deadlock against each other (e.g. NFS + Lustre).
> This isn't the problem, if NFS wouldn't be broken it couldn't deadlock
> against Lustre even with your design (assuming you don't fall in the two
> problems mentioned above). But still your design is more fragile and
> less scalable, especially for a generic implementation where you don't
> know how many pages you'll reserve in mean, and you don't know how many
> kmaps entries the architecture can provide to you. But of course with
> kmap_nonblock you'll have to fallback submitting single pages if it
> fails, it's a bit more difficult but it's more robust and optimized IMHO.
In our case, Lustre (well Portals really, the underlying network protocol)
always knows in advance the number of pages that it will need to kmap
because the client needs to tell the server in advance how much bulk data
is going to send. This is required for being able to do RDMA. It might
be possible to have the server do the transfer in multiple parts if
kmap_nonblock() failed, but that is not how things are currently set up,
which is why we block in advance until we know we can get enough pages.
This is very similar to ext3 journaling, which requests in advance the
maximum number of journal blocks it might need, and blocks until it can
get them all.
The only problem happens when other parts of the kernel start acquiring
multiple kmaps without using the same reservation/accounting system as us.
Each works fine in isolation, but in combination it fails.
-- Andreas Dilger http://sourceforge.net/projects/ext2resize/ http://www-mddsp.enel.ucalgary.ca/People/adilger/
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