Re: [Qemu-devel] [Qemu-block] Limiting coroutine stack usage

[Peter Lieven]

Am 06.03.2018 um 12:51 schrieb Stefan Hajnoczi:
> On Tue, Feb 20, 2018 at 06:04:02PM +0100, Peter Lieven wrote:
>> I remember we discussed a long time ago to limit the stack usage of all 
>> functions that are executed in a coroutine
>> context to a very low value to be able to safely limit the coroutine stack 
>> size as well.
>>
>> I checked through all functions in block/, migration/ and nbd/ and there are 
>> only very few larger or unbound stack
>> allocations that can easily be fixed.
>>
>> Now my question: Is there an easy way to add a cflag like -Wstack-usage=2048 
>> to all objects in a given directory only?
>> I tried to add a llimit to the whole project, but fixing this will be a 
>> larger task.
> 2KB is fine for QEMU code but actual coroutine stack sizes will have to
> be at least 8KB, I guess, in order for third-party libraries to work
> (e.g. curl, rbd).  PATH_MAX is 4KB on Linux.
>
> Nested event loops in QEMU code can also result in deep call stacks.
> This happens when aio_poll() invokes an fd handler or BH that also
> invokes aio_poll().

The plan was to limit the stack usage only as a compiler option. I would leave 
the coroutine stack size at 1MB
for now until we have a way to identify the worst case usage.

Peter