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Re: [dm-devel] NetBSD libdevmapper port



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On Jun,Friday 13 2008, at 6:51 PM, Mikulas Patocka wrote:

Hi

If you are rewriting it --- have you somehow thought about avoiding suspend?

device suspend/resume are unsupported for now. I was not sure what these commands should really do.
Because GPL/BSD licensing issues I haven't look at linux code.



A big source of problems in Linux is, that when you suspend a device, you can do only a limited set of calls --- basically, you must avoid anything that could possibly wait for I/O or allocate memory --- or you might end up waiting for the suspended device and deadlock. And I know still about 3 places in kernel that have this error possibility.

ok but why I should want to do something like suspending of device ? to avoid IO operations when I'm changing device table or what problem are device suspend/resume want to solve. If this is problem I can use one mutex shared between dm_dev_suspend/resume_ioctl call and dmstrategy(this routine does IO) so I can avoid IO's when
device is suspended.



A Linux LVM does something like: suspend old table, write to disk with direct i/o, resume new table. I'd suggest that you invent some method how to batch these operations into single syscall --- or you run into a several years of deadlock problems on NetBSD --- basically, on Linux, we have to preallocate a stack and heap (so that running LVM process won't cause a page fault --- the question --- how to do it portably on all NetBSD architectures?), mlock the process, make sure that we don't open files or write anything to terminal while suspended, make sure that the ioctl syscall doesn't allocate anything (currently false, it won't deadlock but it could randomly fail), make sure that O_DIRECT write syscall doesn't allocate anything or wait for other I/O (currently it false, there is a deadlock possibility) ... etc.

--- if you port lvm2 as it is, you'll have to audit (and maybe rewrite) many parts of NetBSD kernel for not waiting for I/O. If you do it badly, you'll get deadlocks.

I want definitely avoid such massive changes to our kernel.



This suspend thing was a big misdesign and if you are writing it from scratch, try to avoid it.

Regards

Adam.

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