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

[Pekka Enberg]

Hi Catalin,

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).

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

- Pekka