linux-kernel - Re: [PATCH v2] mm: kmemleak: Use mempool allocations for kmemleak objects

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Date:   Tue, 30 Jul 2019 13:39:39 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Qian Cai <cai@....pw>
Cc:     Catalin Marinas <catalin.marinas@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2] mm: kmemleak: Use mempool allocations for kmemleak
 objects

On Tue, 30 Jul 2019 16:22:37 -0400 Qian Cai <cai@....pw> wrote:

> On Tue, 2019-07-30 at 12:57 -0700, Andrew Morton wrote:
> > On Sat, 27 Jul 2019 14:23:33 +0100 Catalin Marinas <catalin.marinas@....com>
> > wrote:
> > 
> > > Add mempool allocations for struct kmemleak_object and
> > > kmemleak_scan_area as slightly more resilient than kmem_cache_alloc()
> > > under memory pressure. Additionally, mask out all the gfp flags passed
> > > to kmemleak other than GFP_KERNEL|GFP_ATOMIC.
> > > 
> > > A boot-time tuning parameter (kmemleak.mempool) is added to allow a
> > > different minimum pool size (defaulting to NR_CPUS * 4).
> > 
> > Why would anyone ever want to alter this?  Is there some particular
> > misbehaviour which this will improve?  If so, what is it?
> 
> So it can tolerant different systems and workloads. For example, there are some
> machines with slow disk and fast CPUs. When they are under memory pressure, it
> could take a long time to swap before the OOM kicks in to free up some memory.
> As the results, it needs a large mempool for kmemleak or suffering from higher
> chance of a kmemleak metadata allocation failure.

This sort of thing should be in the changelog and in the user-facing
documentation please.  Also, we should document the user-visible
effects of this failure so that users can determine whether this tunable
will help them.