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

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Date:   Thu, 28 Mar 2019 10:30:21 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Pekka Enberg <penberg@....fi>
Cc:     Qian Cai <cai@....pw>, akpm@...ux-foundation.org, cl@...ux.com,
        mhocko@...nel.org, willy@...radead.org, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] kmemleak: survive in a low-memory situation

On Thu, Mar 28, 2019 at 08:05:31AM +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.
> 
> 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.

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).

Anyway, I think it is possible, just not straight forward.

-- 
Catalin