Re: [Xen-devel] [PATCH] stack overflow during pv-guest restore

[Don Dutile <ddutile@xxxxxxxxxx>]

Jan (& Keir)

That's what I did the first time (used a simple static) I generated
a fix & tested it, because a lock is taken out (long) before __cpu_up() is 
invoked,
to ensure only one cpu is in that code path anyhow.

*But* as I explained below, a modified version of cpu_initialize_context()
was already upstream and it went with a kzalloc-kfree implementation, so
maybe you want to ask Jeremy why he didn't go with a simple static as well.

So, the bottom line: the fix tries to follow upstream... typically that's
what we do (or change upstream, so all of us can follow it).

- Don

Jan Beulich wrote:
> I think this can be fixed with a much smaller change: Since the only caller
> of cpu_initialize_context() is __cpu_up(), and since __cpu_up() invocation
> is serialized, the variable in question could simply be static.
>
> Jan
>
> > > > Don Dutile <ddutile@xxxxxxxxxx> 31.01.08 21:05 >>>
> When secondary cpus are initialized during an i386 pv-guest restore
> (due to save/restore or live migration), and the guest has
> a load that generates a fair number of interrupts (e.g., parallel kernel make),
> a stack overflow can occur because cpu_initialize_context() has
> a 2800 byte structure it declares on its stack.  linux-i386 has 4K stacks, by 
> default.
> Using 2800 bytes out of 4K by a single function in a call list isn't nice;
> add the beginning of interrupt handling at just the right point, before the
> switch to the interrupt stack is made... and... the scale tips...
>
> Simple fix: malloc & free structure as needed.
>
> Would fail save/restore testing of an i386-guest running a parallel, kernel 
> make after
> 50->100 save/restores;  with the fix, haven't seen it fail after 650 
> save/restores.
>
> Note: this is a basic port of this function in Jeremy Fitzharinge's Xen 
> implementation of pv-ops
>        in upstream Linux, part of 15/32 patch, Xen SMP guest support.
>
> Original patch done on rhel5; did a simple diff & merge to xen-unstable's 
> version
> of smpboot.c to generate the attached patch, so it cleanly applies; but
> haven't built/run/tested the xen-unstable version.
>
> Signed-off-by: Donald Dutile <ddutile@xxxxxxxxxx>
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