Kimball Murray wrote:
Attached is a git patch that implements a feature that is used by Stratus
fault-tolerant servers running on Intel x86_64 platforms. It provides the
kernel mechanism that allows a loadable module to be able to keep track of
recently dirtied pages for the purpose of copying live, currently active
memory, to a spare memory module.
In Stratus servers, this spare memory module (and CPUs) will be brought into
lockstep with the original memory (and CPUs) in order to achieve fault
tolerance.
In order to make this feature work, it is necessary to track pages which have
been recently dirtied. A simplified view of the algorithm used in the kernel
module is this:
1. Turn on the tracking functionality.
2. Copy all memory from active to spare memory module.
3. Until done:
a) Identify all pages that were dirtied in the active memory since
the last copy operation.
b) Copy all pages identified in 3a to the spare memory module.
c) If number of pages still dirty is less than some threshhold,
i. "black out" the system (enter SMI)
ii. copy remaining pages in blackout context
iii. goto step 4
Else
goto 3a.
4. synchronize cpus
5. leave SMI, return to OS
6. System is now "Duplexed", and fault tolerant.
Silly question, why can't you do all this from stop_machine_run context (or
your SMI) that doesn't have to worry about other CPUs dirtying memory?
Please consider this feature for inclusion in the kernel tree, as it is very
important to Stratus.
Given that it doesn't touch core mm/ code, I don't really care about it[*]
except that it doesn't make sense to have the tracking hooks in generic code
because it is pretty specific to your module.
Also, as far as a "notifier" goes, it is of course completely broken unless
your module is the only one that is ever going to use it. Which I suspect
may be the case ;) But you do really need to at least WARN_ON or fail if
someone is going to break it like this.
[*] Though if it gets included, it would not stop me lamenting the
proliferation of complexities to support *tiny* obscure userbases. Can
we wait until your hardware is smart enough to snoop the cc? :)
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