Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Reclamation interactions with RCU

NeilBrown
2024-03-01T04:09:19Z

On Fri, 01 Mar 2024, Kent Overstreet wrote:
> On Thu, Feb 29, 2024 at 10:52:06PM -0500, Kent Overstreet wrote:
> > On Fri, Mar 01, 2024 at 10:33:59AM +0700, James Bottomley wrote:
> > > On Thu, 2024-02-29 at 22:09 -0500, Kent Overstreet wrote:
> > > > Or maybe you just want the syscall to return an error instead of
> > > > blocking for an unbounded amount of time if userspace asks for
> > > > something silly.
> > >
> > > Warn on allocation above a certain size without MAY_FAIL would seem to
> > > cover all those cases. If there is a case for requiring instant
> > > allocation, you always have GFP_ATOMIC, and, I suppose, we could even
> > > do a bounded reclaim allocation where it tries for a certain time then
> > > fails.
> >
> > Then you're baking in this weird constant into all your algorithms that
> > doesn't scale as machine memory sizes and working set sizes increase.
> >
> > > > Honestly, relying on the OOM killer and saying that because that now
> > > > we don't have to write and test your error paths is a lazy cop out.
> > >
> > > OOM Killer is the most extreme outcome. Usually reclaim (hugely
> > > simplified) dumps clean cache first and tries the shrinkers then tries
> > > to write out dirty cache. Only after that hasn't found anything after
> > > a few iterations will the oom killer get activated
> >
> > All your caches dumped and the machine grinds to a halt and then a
> > random process gets killed instead of simply _failing the allocation_.
> >
> > > > The same kind of thinking got us overcommit, where yes we got an
> > > > increase in efficiency, but the cost was that everyone started
> > > > assuming and relying on overcommit, so now it's impossible to run
> > > > without overcommit enabled except in highly controlled environments.
> > >
> > > That might be true for your use case, but it certainly isn't true for a
> > > cheap hosting cloud using containers: overcommit is where you make your
> > > money, so it's absolutely standard operating procedure. I wouldn't
> > > call cheap hosting a "highly controlled environment" they're just
> > > making a bet they won't get caught out too often.
> >
> > Reading comprehension fail. Reread what I wrote.
> >
> > > > And that means allocation failure as an effective signal is just
> > > > completely busted in userspace. If you want to write code in
> > > > userspace that uses as much memory as is available and no more, you
> > > > _can't_, because system behaviour goes to shit if you have overcommit
> > > > enabled or a bunch of memory gets wasted if overcommit is disabled
> > > > because everyone assumes that's just what you do.
> > >
> > > OK, this seems to be specific to your use case again, because if you
> > > look at what the major user space processes like web browsers do, they
> > > allocate way over the physical memory available to them for cache and
> > > assume the kernel will take care of it. Making failure a signal for
> > > being over the working set would cause all these applications to
> > > segfault almost immediately.
> >
> > Again, reread what I wrote. You're restating what I wrote and completely
> > missing the point.
> >
> > > > Let's _not_ go that route in the kernel. I have pointy sticks to
> > > > brandish at people who don't want to deal with properly handling
> > > > errors.
> > >
> > > Error legs are the least exercised and most bug, and therefore exploit,
> > > prone pieces of code in C. If we can get rid of them, we should.
> >
> > Fuck no.
> >
> > Having working error paths is _basic_, and learning how to test your
> > code is also basic. If you can't be bothered to do that you shouldn't be
> > writing kernel code.
> >
> > We are giving far too much by going down the route of "oh, just kill
> > stuff if we screwed the pooch and overcommitted".
> >
> > I don't fucking care if it's what the big cloud providers want because
> > it's convenient for them, some of us actually do care about reliability.
> >
> > By just saying "oh, the OO killer will save us" what you're doing is
> > making it nearly impossible to fully utilize a machine without having
> > stuff randomly killed.
> >
>
> And besides all that, as a practical matter you can't just "not have
> erro paths" because, like you said, you'd still have to have a max size
> where you WARN() - and _fail the allocation_ - and you've still got to
> unwind.

No. You warn and DON'T fail the allocation. Just like lockdep warns of
possible deadlocks but lets you continue.
These will be found in development (mostly) and changed to use
__GFP_RETRY_MAYFAIL and have appropriate error-handling paths.

>
> The OOM killer can't kill processes while they're stuck blocking on an
> allocation that will rever return in the kernel.

But it can depopulate the user address space (I think).

NeilBrown

>
> I think we can safely nip this idea in the bud.
>
> Test your damn error paths...
>