On Tue, Jan 28, 2020 at 11:28:14AM -0800, Mark Millard wrote:
>
>
> 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.
> >
Stopped already, with
Jan 28 11:41:59 www kernel: pid 29909 (cc), jid 0, uid 0, was killed: fault's
page allocation failed
> >> 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.
Glad to be of service, even if inadvertently 8-)
> 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.
Oops, it's commented out now and the kernel is rebuilding.
>
> 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.
>
Thank you!
bob prohaska
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