Re: OOM detection regressions since 4.7

[Jeff Layton <jlayton@poochiereds.net>]

On Mon, 2016-08-29 at 10:28 -0700, Linus Torvalds wrote:
> > On Mon, Aug 29, 2016 at 7:52 AM, Olaf Hering <olaf@aepfle.de> wrote:
> > 
> > 
> > Today I noticed the nfsserver was disabled, probably since months already.
> > Starting it gives a OOM, not sure if this is new with 4.7+.
> 
> That's not an oom, that's just an allocation failure.
> 
> And with order-4, that's actually pretty normal. Nobody should use
> order-4 (that's 16 contiguous pages, fragmentation can easily make
> that hard - *much* harder than the small order-2 or order-2 cases that
> we should largely be able to rely on).
> 
> In fact, people who do multi-order allocations should always have a
> fallback, and use __GFP_NOWARN.
> 
> > 
> > [93348.306406] Call Trace:
> > [93348.306490]  [<ffffffff81198cef>] __alloc_pages_slowpath+0x1af/0xa10
> > [93348.306501]  [<ffffffff811997a0>] __alloc_pages_nodemask+0x250/0x290
> > [93348.306511]  [<ffffffff811f1c3d>] cache_grow_begin+0x8d/0x540
> > [93348.306520]  [<ffffffff811f23d1>] fallback_alloc+0x161/0x200
> > [93348.306530]  [<ffffffff811f43f2>] __kmalloc+0x1d2/0x570
> > [93348.306589]  [<ffffffffa08f025a>] nfsd_reply_cache_init+0xaa/0x110 [nfsd]
> 
> Hmm. That's kmalloc itself falling back after already failing to grow
> the slab cache earlier (the earlier allocations *were* done with
> NOWARN afaik).
> 
> It does look like nfsdstarts out by allocating the hash table with one
> single fairly big allocation, and has no fallback position.
> 
> I suspect the code expects to be started at boot time, when this just
> isn't an issue. The fact that you loaded the nfsd kernel module with
> memory already fragmented after heavy use is likely why nobody else
> has seen this.
> 
> Adding the nfsd people to the cc, because just from a robustness
> standpoint I suspect it would be better if the code did something like
> 
>  (a) shrink the hash table if the allocation fails (we've got some
> examples of that elsewhere)
> 
> or
> 
>  (b) fall back on a vmalloc allocation (that's certainly the simpler model)
> 
> We do have a "kvfree()" helper function for the "free either a kmalloc
> or vmalloc allocation" but we don't actually have a good helper
> pattern for the allocation side. People just do it by hand, at least
> partly because we have so many different ways to allocate things -
> zeroing, non-zeroing, node-specific or not, atomic or not (atomic
> cannot fall back to vmalloc, obviously) etc etc.
> 
> Bruce, Jeff, comments?
> 
>              Linus

Yeah, that makes total sense.

Hmm...we _do_ already auto-size the hash at init time already, so
shrinking it downward and retrying if the allocation fails wouldn't be
hard to do. Maybe I can just cut it in half and throw a pr_warn to tell
the admin in that case.

In any case...I'll take a look at how we can improve it.

Thanks for the heads-up!
-- 
Jeff Layton <jlayton@poochiereds.net>