Re: [PATCHv2 0/3] Find mirrored memory, use for boot time allocations

From: Tony Luck
Date: Fri May 08 2015 - 16:38:58 EST

On Fri, May 8, 2015 at 1:03 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> Looks good to me. What happens to these patches while ZONE_MIRROR is
> being worked on?

I think these patches can go into the kernel now while I figure
out the next phase - there is some value in just this part. We'll
have all memory <4GB mirrored to cover the kernel code/data.
Adding the boot time allocations mostly means the page structures
(in terms of total amount of memory).

> 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?

Some of that is TBD - there are some clear places where we have bounded
amounts of memory that we'd like to pull into the mirror area. E.g. loadable
modules - on a specific machine an administrator can easily see which modules
are loaded, tally up the sizes, and then adjust the amount of mirrored memory.
I don't think we necessarily need to get to 100% ... if we can avoid 9/10
errors crashing the machine - that moves the reliability needle enough to
make a difference. Phase 2 may turn into phase 2a, 2b, 2c etc. as we
pick on certain areas.

Oh - they'll be some sysfs or debugfs stats too - so people can
check that they have the right amount of mirror memory under application
load. Too little and they'll be at risk because kernel allocations will
fall back to non-mirrored. Too much, and they are wasting memory.

> Will surplus ZONE_MIRROR memory be available for regular old movable
> allocations?
ZONE_MIRROR and ZONE_MOVABLE are pretty much opposites. We
only want kernel allocations in mirror memory, and we can't allow any
kernel allocations in movable (cause they'll pin it).

> I suggest you run the design ideas by Mel before getting into
> implementation.
Good idea - when I have something fit to be seen, I'll share
with Mel.

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