OOMA kill with vm.pfault_oom_attempts="-1" on RPi3 at r357147 (a vm_pfault_oom_attempts < 0 handling bug as of head -r357026)

[Mark Millard]

On 2020-Jan-28, at 11:02, bob prohaska <fbsd at www.zefox.net> wrote:

> On Tue, Jan 28, 2020 at 09:42:17AM -0800, Mark Millard wrote:
>> 
>> 
>> 
> The (partly)modified kernel compiled and booted without
> obvious trouble. It's trying to finish buildworld now.
> 
>> If you are testing with vm.pfault_oom_attempts="-1" then
>> the vm_fault printf message should never happen anyway.
>> 
> Would it not be interesting if the message appeared in that
> case? 

Thanks for the question: looking at the new code found a bug
causing oom where it used to be avoided in head -r357025 and
before.

After vm_waitpfault(dset, vm_pfault_oom_wait * hz)
the -r357026 code does a vm_pageout_oom(VM_OOM_MEM_PF) no
matter what, even when vm_pfault_oom_attempts < 0 ||
fs->oom < vm_pfault_oom_attempts :

New code in head -r357026
( nothing to avoid the vm_pageout_oom(VM_OOM_MEM_PF)
for vm_pfault_oom_attempts < 0 ||
fs->oom < vm_pfault_oom_attempts ):

	if (fs->m == NULL) {
		unlock_and_deallocate(fs);
		if (vm_pfault_oom_attempts < 0 ||
		    fs->oom < vm_pfault_oom_attempts) {
			fs->oom++;
			vm_waitpfault(dset, vm_pfault_oom_wait * hz);
		}
		if (bootverbose)
			printf(
"proc %d (%s) failed to alloc page on fault, starting OOM\n",
			    curproc->p_pid, curproc->p_comm);
		vm_pageout_oom(VM_OOM_MEM_PF);
		return (KERN_RESOURCE_SHORTAGE);
	}

Old code in head -r357025
( has the goto RetryFault_oom after vm_waitpfault(. . .),
thereby avoiding the vm_pageout_oom(VM_OOM_MEM_PF) for
vm_pfault_oom_attempts < 0 || fs->oom < vm_pfault_oom_attempts ) :

			if (fs.m == NULL) {
				unlock_and_deallocate(&fs);
				if (vm_pfault_oom_attempts < 0 ||
				    oom < vm_pfault_oom_attempts) {
					oom++;
					vm_waitpfault(dset,
					    vm_pfault_oom_wait * hz);
					goto RetryFault_oom;
				}
				if (bootverbose)
					printf(
	"proc %d (%s) failed to alloc page on fault, starting OOM\n",
					    curproc->p_pid, curproc->p_comm);
				vm_pageout_oom(VM_OOM_MEM_PF);
				goto RetryFault;
			}

I expect this is the source of the behavioral
difference folks have been seeing for OOM kills.


As for "gather evidence" messages . . .

>> You may be able to just look and manually delete or
>> comment out the bootverbose line in the more modern
>> source that currently looks like:
>> 
>> 		if (bootverbose)
>> 			printf(
>> "proc %d (%s) failed to alloc page on fault, starting OOM\n",
>> 			    curproc->p_pid, curproc->p_comm);
>> 		vm_pageout_oom(VM_OOM_MEM_PF);
>> 		return (KERN_RESOURCE_SHORTAGE);
>> 
> 
> I can find those lines in /usr/src/sys/vm/vm_fault.c, but
> unclear on the motivation to comment the lines out. Perhaps 
> to eliminate the return(...) ?  Anyway, is it sufficient 
> to insert /* before and */ after? 

The only line to delete or comment out in that
code block is:

		if (bootverbose)

Disabling that line makes the following printf
always happen, even when a verbose boot was not
done.

Based on the above reported code change, having
a message before vm_pageout_oom(VM_OOM_MEM_PF) is
important to getting a report of the kill being
via that code.

>> and is now in vm_fault_allocate(. . .). (That file has
>> hd a reorganization since where I'm synchronized.)
>> 
>> Having the message indicate vm_fault_allocate is
>> optional but would look like:
>> 
>> "vm_fault_allocate: proc %d (%s) failed to alloc page on fault, starting OOM\n",
>> 
>> Doing the delete/comment-out would avoid waiting for me.
>> 
>> 
> I'll do it after the next stoppage.



===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)