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

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Date:   Thu, 1 Aug 2019 08:41:53 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        Qian Cai <cai@....pw>
Subject: Re: [PATCH v2] mm: kmemleak: Use mempool allocations for kmemleak
 objects

On Wed 31-07-19 16:44:50, Catalin Marinas wrote:
> On Tue, Jul 30, 2019 at 01:02:15PM -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).
> > 
> > btw, the checkpatch warnings are valid:
> > 
> > WARNING: usage of NR_CPUS is often wrong - consider using cpu_possible(), num_possible_cpus(), for_each_possible_cpu(), etc
> > #70: FILE: mm/kmemleak.c:197:
> > +static int min_object_pool = NR_CPUS * 4;
> > 
> > WARNING: usage of NR_CPUS is often wrong - consider using cpu_possible(), num_possible_cpus(), for_each_possible_cpu(), etc
> > #71: FILE: mm/kmemleak.c:198:
> > +static int min_scan_area_pool = NR_CPUS * 1;
> > 
> > There can be situations where NR_CPUS is much larger than
> > num_possible_cpus().  Can we initialize these tunables within
> > kmemleak_init()?
> 
> We could and, at least on arm64, cpu_possible_mask is already
> initialised at that point. However, that's a totally made up number. I
> think we would better go for a Kconfig option (defaulting to, say, 1024)
> similar to the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE and we grow it if
> people report better values in the future.

If you really want/need to make this configurable then the command line
parameter makes more sense - think of distribution kernel users for
example. But I am still not sure why this is really needed. The initial
size is a "made up" number of course. There is no good estimation to
make (without a crystal ball). The value might be increased based on
real life usage.
-- 
Michal Hocko
SUSE Labs