Re: [PATCH] mm/page_alloc.c: Allow error injection

[Vlastimil Babka <vbabka@suse.cz>]

On 12/14/18 8:43 AM, Benjamin Poirier wrote:
> Model call chain after should_failslab(). Likewise, we can now use a kprobe
> to override the return value of should_fail_alloc_page() and inject
> allocation failures into alloc_page*().

I'd be more explicit about both the advantages and disadvantages, as we
discussed internally. E.g. something along:

This will allow injecting allocation failures using the BCC tools even
without building kernel with CONFIG_FAIL_PAGE_ALLOC and booting it with
a fail_page_alloc= parameter, which incurs some overhead even when
failures are not being injected. On the other hand, this patch adds an
unconditional call to should_fail_alloc_page() from page allocation
hotpath. That overhead should be rather negligible with
CONFIG_FAIL_PAGE_ALLOC=n when there's no kprobe attached, though.

> Signed-off-by: Benjamin Poirier <bpoirier@suse.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  include/asm-generic/error-injection.h |  1 +
>  mm/page_alloc.c                       | 10 ++++++++--
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/include/asm-generic/error-injection.h b/include/asm-generic/error-injection.h
> index 296c65442f00..95a159a4137f 100644
> --- a/include/asm-generic/error-injection.h
> +++ b/include/asm-generic/error-injection.h
> @@ -8,6 +8,7 @@ enum {
>  	EI_ETYPE_NULL,		/* Return NULL if failure */
>  	EI_ETYPE_ERRNO,		/* Return -ERRNO if failure */
>  	EI_ETYPE_ERRNO_NULL,	/* Return -ERRNO or NULL if failure */
> +	EI_ETYPE_TRUE,		/* Return true if failure */
>  };
>  
>  struct error_injection_entry {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2ec9cc407216..64861d79dc2d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3053,7 +3053,7 @@ static int __init setup_fail_page_alloc(char *str)
>  }
>  __setup("fail_page_alloc=", setup_fail_page_alloc);
>  
> -static bool should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
> +static bool __should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
>  {
>  	if (order < fail_page_alloc.min_order)
>  		return false;
> @@ -3103,13 +3103,19 @@ late_initcall(fail_page_alloc_debugfs);
>  
>  #else /* CONFIG_FAIL_PAGE_ALLOC */
>  
> -static inline bool should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
> +static inline bool __should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
>  {
>  	return false;
>  }
>  
>  #endif /* CONFIG_FAIL_PAGE_ALLOC */
>  
> +static noinline bool should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
> +{
> +	return __should_fail_alloc_page(gfp_mask, order);
> +}
> +ALLOW_ERROR_INJECTION(should_fail_alloc_page, TRUE);
> +
>  /*
>   * Return true if free base pages are above 'mark'. For high-order checks it
>   * will return true of the order-0 watermark is reached and there is at least
>