On Mon, 2013-02-04 at 21:12 +0100, Rafael J. Wysocki wrote: > On Monday, February 04, 2013 12:46:24 PM Toshi Kani wrote: > > On Mon, 2013-02-04 at 20:48 +0100, Rafael J. Wysocki wrote: > > > On Monday, February 04, 2013 09:02:46 AM Toshi Kani wrote: > > > > On Mon, 2013-02-04 at 14:41 +0100, Rafael J. Wysocki wrote: > > > > > On Sunday, February 03, 2013 07:23:49 PM Greg KH wrote: > > > > > > On Sat, Feb 02, 2013 at 09:15:37PM +0100, Rafael J. Wysocki wrote: > > > > > > > On Saturday, February 02, 2013 03:58:01 PM Greg KH wrote: > > > > : > > > > > > > Yes, but those are just remove events and we can only see how destructive they > > > > > > > were after the removal. The point is to be able to figure out whether or not > > > > > > > we *want* to do the removal in the first place. > > > > > > > > > > > > Yes, but, you will always race if you try to test to see if you can shut > > > > > > down a device and then trying to do it. So walking the bus ahead of > > > > > > time isn't a good idea. > > > > > > > > > > > > And, we really don't have a viable way to recover if disconnect() fails, > > > > > > do we. What do we do in that situation, restore the other devices we > > > > > > disconnected successfully? How do we remember/know what they were? > > > > > > > > > > > > PCI hotplug almost had this same problem until the designers finally > > > > > > realized that they just had to accept the fact that removing a PCI > > > > > > device could either happen by: > > > > > > - a user yanking out the device, at which time the OS better > > > > > > clean up properly no matter what happens > > > > > > - the user asked nicely to remove a device, and the OS can take > > > > > > as long as it wants to complete that action, including > > > > > > stalling for noticable amounts of time before eventually, > > > > > > always letting the action succeed. > > > > > > > > > > > > I think the second thing is what you have to do here. If a user tells > > > > > > the OS it wants to remove these devices, you better do it. If you > > > > > > can't, because memory is being used by someone else, either move them > > > > > > off, or just hope that nothing bad happens, before the user gets > > > > > > frustrated and yanks out the CPU/memory module themselves physically :) > > > > > > > > > > Well, that we can't help, but sometimes users really *want* the OS to tell them > > > > > if it is safe to unplug something at this particualr time (think about the > > > > > Windows' "safe remove" feature for USB sticks, for example; that came out of > > > > > users' demand AFAIR). > > > > > > > > > > So in my opinion it would be good to give them an option to do "safe eject" or > > > > > "forcible eject", whichever they prefer. > > > > > > > > For system device hot-plug, it always needs to be "safe eject". This > > > > feature will be implemented on mission critical servers, which are > > > > managed by professional IT folks. Crashing a server causes serious > > > > money to the business. > > > > > > Well, "always" is a bit too strong a word as far as human behavior is concerned > > > in my opinion. > > > > > > That said I would be perfectly fine with not supporting the "forcible eject" to > > > start with and waiting for the first request to add support for it. I also > > > would be fine with taking bets on how much time it's going to take for such a > > > request to appear. :-) > > > > Sounds good. In my experience, though, it actually takes a LONG time to > > convince customers that "safe eject" is actually safe. Enterprise > > customers are so afraid of doing anything risky that might cause the > > system to crash or hang due to some defect. I would be very surprised > > to see a customer asking for a force operation when we do not guarantee > > its outcome. I have not seen such enterprise customers yet. > > But we're talking about a kernel that is supposed to run on mobile phones too, > among other things. I think using this feature for RAS i.e. replacing a faulty device on-line, will continue to be limited for high-end systems. For low-end systems, it does not make sense for customers to pay much $$ for this feature. They can just shut the system down for replacement, or they can simply buy a new system instead of repairing. That said, using this feature on VM for workload balancing does not require any special hardware. So, I can see someone willing to try out to see how it goes with a force option on VM for personal use. Thanks, -Toshi -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>