On Wed, Jul 18, 2018 at 02:27:31PM -0400, Jeff Moyer wrote: > Hi, Dave, > > Dave Jiang <dave.jiang@xxxxxxxxx> writes: > > > When pmem namespaces created are smaller than section size, this can cause > > issue during removal and gpf was observed: > > > > Add code to check whether we have mapping already in the same section and > > prevent additional mapping from created if that is the case. > > > > Signed-off-by: Dave Jiang <dave.jiang@xxxxxxxxx> > > --- > > > > v2: Change dev_warn() to dev_WARN() to provide helpful backtrace. (Robert E) > > OK, I can reproduce the issue. What I don't like about your patch is > that you can still get yourself into trouble. Just create a namespace > with a size that isn't aligned to 128MB, and then all further > create-namespace operations will fail. The only "fix" is to delete the > odd-sized namespace and try again. And that warning message doesn't > really help the administrator to figure this out. > > Why can't we simply round up to the next section automatically? Either > that, or have the kernel export a minimum namespace size of 128MB, and > have ndctl enforce it? I know we had some requests for 4MB namespaces, > but it doesn't sound like those will be very useful if they're going to > waste 124MB of space. > > Or, we could try to fix this problem of having multiple namespace > co-exist in the same memblock section. That seems like the most obvious > fix, but there must be a reason you didn't pursue it. > > Dave, what do you think is the most viable option? Just as a reminder, the desire for small pmem devices comes from cloud usecases where you have teeny tiny layers, each of which might contain a single package (eg a webserver or a database). Because you're going to run tens of thousands of instances, you don't want each machine to keep a copy of the program text in pagecache; you want to have it in-memory once and then DAX-map it in each guest. While it's OK to waste a certain amount of each guest's physical memory, when you have hundreds or thousands of these tiny layers, it adds up.
