Q: behaviour of mlockall(MCL_FUTURE) and VM_GROWSDOWN segments

So in this case, the behaviour I would prefer is MCL_FUTURE for
all vma's *except* the stack. Stack pages should be locked
only when they are faulted in. Hard call.

Quoted 3 lines · Andrew Morton

So in this case, the behaviour I would prefer is MCL_FUTURE for
all vma's *except* the stack. Stack pages should be locked
only when they are faulted in. Hard call.

There is just one problem: linuxthread stacks are just ordinary mappings and they are in no way special to the kernel; they aren't VM_GROWSDOWN. You would need to add a way to the kernel first to tag the linux thread stacks in a way that is recognizable to mlockall and then do that from linuxthreads.

I think for the normal stack - real VM_GROWSDOWN segments - mlockall already does the right thing.

Quoted 3 lines · Andrew Morton

So in this case, the behaviour I would prefer is MCL_FUTURE for
all vma's *except* the stack. Stack pages should be locked
only when they are faulted in. Hard call.

There is just one problem: linuxthread stacks are just ordinary mappings and they are in no way special to the kernel; they aren't VM_GROWSDOWN. You would need to add a way to the kernel first to tag the linux thread stacks in a way that is recognizable to mlockall and then do that from linuxthreads.

I think for the normal stack - real VM_GROWSDOWN segments - mlockall already does the right thing.

it doesn't (of course depends "what's the right thing"), and that's why

What the current kernel is doing with page faults, is to fault in only the touched pages, not the pages in between as well, this isn't a security concern because the faulted in pages won't be swapped out, but it may matter for some RT app, OTOH the RT apps would better memset the whole stack they need before assuming they won't get page faults, first of all because of all other kernels out there (this is what I mean with a matter of API).

For the stack they can get minor faults anyways when they allocate new stack space below ESP. There is no good way to fix that from the kernel; the application has to preallocate its memory on stack. I think it's reasonable if it does the same for holes on the stack.

Quoted 1 line · Andrea Arcangeli

it doesn't (of course depends "what's the right thing"), and that's why

I think it does. Allocating all possible in future allocated pages is just not possible for VM_GROWSDOWN, because the stack has really no suitable limit (other than rlimits, which are far too big to mlock them)

BTW expand_stack seems to have a small bug: it adds to mm->locked_vm the complete offset from last vm_start; if it covers more than one page the locked_vm value will be too large.

Quoted 7 lines

What the current kernel is doing with page faults, is to fault in only the touched pages, not the pages in between as well, this isn't a security concern because the faulted in pages won't be swapped out, but it may matter for some RT app, OTOH the RT apps would better memset the whole stack they need before assuming they won't get page faults, first of all because of all other kernels out there (this is what I mean with a matter of API).

For the stack they can get minor faults anyways when they allocate new stack space below ESP. There is no good way to fix that from the kernel; the

application has to preallocate its memory on stack. I think it's reasonable if it does the same for holes on the stack.

(quoted diff) +0 -2

-Andi
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Quoted 6 lines · Andi Kleen

For the stack they can get minor faults anyways when they allocate new stack space below ESP. There is no good way to fix that from the kernel; the application has to preallocate its memory on stack. I think it's reasonable if it does the same for holes on the stack.

Ok, everyone agrees that mlockall() should not grow VM_GROWSDOWN segments to their maximum size.

Should the page fault handler fill the hole created by

void * grow_stack(void)

{
	char data[100000];
	data[0] = '0';
	return data;
}

The principle of least surprise would mean filling holes, but OTHO sane apps would use memset(data,0,sizeof(data)).