Re: [PATCH v4] kmemleak: survive in a low-memory situation

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Mar 28, 2019 at 01:50:29PM +0200, Pekka Enberg wrote:
> On 27/03/2019 2.59, Qian Cai wrote:
> > > > Unless there is a brave soul to reimplement the kmemleak to embed it's
> > > > metadata into the tracked memory itself in a foreseeable future, this
> > > > provides a good balance between enabling kmemleak in a low-memory
> > > > situation and not introducing too much hackiness into the existing
> > > > code for now.
> 
> On Thu, Mar 28, 2019 at 08:05:31AM +0200, Pekka Enberg wrote:
> > > Unfortunately I am not that brave soul, but I'm wondering what the
> > > complication here is? It shouldn't be too hard to teach calculate_sizes() in
> > > SLUB about a new SLAB_KMEMLEAK flag that reserves spaces for the metadata.
> 
> On 28/03/2019 12.30, Catalin Marinas wrote:> I don't think it's the
> calculate_sizes() that's the hard part. The way
> > kmemleak is designed assumes that the metadata has a longer lifespan
> > than the slab object it is tracking (and refcounted via
> > get_object/put_object()). We'd have to replace some of the
> > rcu_read_(un)lock() regions with a full kmemleak_lock together with a
> > few more tweaks to allow the release of kmemleak_lock during memory
> > scanning (which can take minutes; so it needs to be safe w.r.t. metadata
> > freeing, currently relying on a deferred RCU freeing).
> 
> Right.
> 
> I think SLUB already supports delaying object freeing because of KASAN (see
> the slab_free_freelist_hook() function) so the issue with metadata outliving
> object is solvable (although will consume more memory).

Thanks. That's definitely an area to investigate.

> I can't say I remember enough details from kmemleak to comment on the
> locking complications you point out, though.

They are not too bad, I'd just need some quiet couple of days to go
through them (which I don't have at the moment).

-- 
Catalin