[PATCH v8] mm,kfence: decouple kfence from page granularity mapping judgement

[Marco Elver]

On Wed, Mar 15, 2023 at 02:51PM +0800, Zhenhua Huang wrote:
[...]
> > Is it possible to free this early allocated memory later in
> > mm_init()->kfence_alloc_pool()? if that is not possible, can we think of
> > adding early param for kfence?
> 
> If we freed that buffer, there may be no chance to get that page granularity
> mapped buffer again.. as all these allocation/free are through normal buddy
> allocator.
> 
> At this stage, seems only additional early param can work.. Marco previously
> wanted to reuse sample_interval but seems not doable now.
> 
> Hi Marco,
> 
> Sorry, Can we thought of the solution again? like
> ARM64:
> 1. intercepts early boot arg and gives early alloc memory to KFENCE
> 2. KFENCE to disable dynamic switch
> 3. disable page gran and save memory overhead
> The purpose is in the case of w/o boot arg, it's just same as now.. arch
> specific kfence buffer will not allocate. And w/ boot arg, we can get
> expected saving.

You can get kfence.sample_interval with early_param(). mm/kfence/core.c
should be left as is with a module param, so it can be set at runtime in
/sys/modules/kfence/parameters/.

However you can add this to the #ifdef CONFIG_KFENCE in arm64 code
you're adding:

  static bool kfence_early_init __initdata = !!CONFIG_KFENCE_SAMPLE_INTERVAL;
  static int __init parse_kfence_early_init(char *p) {
  	int val;

  	if (get_option(&p, &val))
  		kfence_early_init = !!val;
  	return 0;
  }
  early_param("kfence.sample_interval", parse_kfence_early_init);

Nothing is preventing us from parsing kfence.sample_interval twice
during boot. At this stage you don't need the actual sample_interval,
only if kfence.sample_interval was provided on the cmdline and is not 0.

That will avoid adding another new param.

Thanks,
-- Marco