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SubjectRe: [PATCHv2 0/3] Find mirrored memory, use for boot time allocations
On Fri, 8 May 2015 09:44:21 -0700 Tony Luck <tony.luck@intel.com> wrote:

> Some high end Intel Xeon systems report uncorrectable memory errors
> as a recoverable machine check. Linux has included code for some time
> to process these and just signal the affected processes (or even
> recover completely if the error was in a read only page that can be
> replaced by reading from disk).
>
> But we have no recovery path for errors encountered during kernel
> code execution. Except for some very specific cases were are unlikely
> to ever be able to recover.
>
> Enter memory mirroring. Actually 3rd generation of memory mirroing.
>
> Gen1: All memory is mirrored
> Pro: No s/w enabling - h/w just gets good data from other side of the mirror
> Con: Halves effective memory capacity available to OS/applications
> Gen2: Partial memory mirror - just mirror memory begind some memory controllers
> Pro: Keep more of the capacity
> Con: Nightmare to enable. Have to choose between allocating from
> mirrored memory for safety vs. NUMA local memory for performance
> Gen3: Address range partial memory mirror - some mirror on each memory controller
> Pro: Can tune the amount of mirror and keep NUMA performance
> Con: I have to write memory management code to implement
>
> The current plan is just to use mirrored memory for kernel allocations. This
> has been broken into two phases:
> 1) This patch series - find the mirrored memory, use it for boot time allocations
> 2) Wade into mm/page_alloc.c and define a ZONE_MIRROR to pick up the unused
> mirrored memory from mm/memblock.c and only give it out to select kernel
> allocations (this is still being scoped because page_alloc.c is scary).

Looks good to me. What happens to these patches while ZONE_MIRROR is
being worked on?


I'm wondering about phase II. What does "select kernel allocations"
mean? I assume we can't say "all kernel allocations" because that can
sometimes be "almost all memory". How are you planning on implementing
this? A new __GFP_foo flag, then sprinkle that into selected sites?

Will surplus ZONE_MIRROR memory be available for regular old movable
allocations?

I suggest you run the design ideas by Mel before getting into
implementation.


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