Re: [RFC 0/12] introduce down_write_killable for rw_semaphore

[Ingo Molnar]

* Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> On Wed 09-03-16 14:17:10, Ingo Molnar wrote:
> > 
> > * Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > 
> > > >  [...] this is a follow up work for oom_reaper [1]. As the async OOM killing 
> > > >  depends on oom_sem for read we would really appreciate if a holder for write 
> > > >  stood in the way. This patchset is changing many of down_write calls to be 
> > > >  killable to help those cases when the writer is blocked and waiting for 
> > > >  readers to release the lock and so help __oom_reap_task to process the oom 
> > > >  victim.
> > > > 
> > > > there seems to be a misunderstanding: if a writer is blocked waiting for 
> > > > readers then no new readers are allowed - the writer will get its turn the 
> > > > moment all existing readers drop the lock.
> > > 
> > > Readers might be blocked e.g. on the memory allocation which cannot proceed due 
> > > to OOM. Such a reader might be operating on a remote mm.
> > 
> > Doing complex allocations with the mm locked looks fragile no matter what: we 
> > should add debugging code that warns if allocations are done with a remote mm 
> > locked. (it should be trivial)
> 
> No matter how fragile is that it is not something non-existent. Just
> have a look at use_mm for example. We definitely do not want to warn
> about those, right?

Sure we care about eliminating fragility, and usage does not seem to be widespread 
at all:

 triton:~/tip> git grep -w use_mm

 drivers/staging/rdma/hfi1/user_sdma.c:          use_mm(req->pq->user_mm);
 drivers/usb/gadget/function/f_fs.c:             use_mm(io_data->mm);
 drivers/usb/gadget/legacy/inode.c:      use_mm(mm);
 drivers/vhost/vhost.c:  use_mm(dev->mm);

I think we also want to keep our general flexibility wrt. eventually turning the 
mmap_sem into a spinlock ...

> > > I am not against interruptible variant as well but I suspect that some paths 
> > > are not expected to return EINTR. I haven't checked them for this but 
> > > killable is sufficient for the problem I am trying to solve. That problem is 
> > > real while latencies do not seem to be that eminent.
> > 
> > If they don't expect EINTR then they sure don't expect SIGKILL either!
> 
> Why? Each syscall already is killable as the task might be killed by the OOM 
> killer.

Not all syscalls are interruptible - for example sys_sync() isn't:

SYSCALL_DEFINE0(sync)
{
        int nowait = 0, wait = 1;

        wakeup_flusher_threads(0, WB_REASON_SYNC);
        iterate_supers(sync_inodes_one_sb, NULL);
        iterate_supers(sync_fs_one_sb, &nowait);
        iterate_supers(sync_fs_one_sb, &wait);
        iterate_bdevs(fdatawrite_one_bdev, NULL);
        iterate_bdevs(fdatawait_one_bdev, NULL);
        if (unlikely(laptop_mode))
                laptop_sync_completion();
        return 0;
}

> > There's a (very) low number of system calls that are not interruptible, but 
> > the vast majority is.
> 
> That might be true. I just fail to see how this is related to the
> particular problem I am trying to solve. As I've said those callsites
> which cause problems with latencies can be later converted to
> interruptible waiting trivially.

So my problem as I see it is the following: you are adding a rare API to an 
already complex locking interface, further complicating already complicated MM 
code paths in various ways. Only to help a case that is a third type of rare: 
OOM-kill.

That's a surefire whack-a-mole nest of bugs, if I've ever seen one.

What I am suggesting instead is a slight modification of the concept: to re-phrase 
the problem set and think in broader terms of interruptability: make certain MM 
operations, especially ones which tend to hinder OOM-kill latencies, more 
interruptible - which implicitly also makes them more OOM-killable.

That's a win-win as I see it: as both your usecase and a lot of other usecases 
will be improved - and it will also be tested a lot more than any OOM-kill path 
will be tested.

I might be wrong in the end, but your counterarguments were not convincing so far 
(to me).

Thanks,

	Ingo
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