Re: [PATCH] mm: page_owner: detect page_owner recursion via task_struct

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  1 Apr 2021 23:30:10 +0100 Sergei Trofimovich <slyfox@gentoo.org> wrote:

> Before the change page_owner recursion was detected via fetching
> backtrace and inspecting it for current instruction pointer.
> It has a few problems:
> - it is slightly slow as it requires extra backtrace and a linear
>   stack scan of the result
> - it is too late to check if backtrace fetching required memory
>   allocation itself (ia64's unwinder requires it).
> 
> To simplify recursion tracking let's use page_owner recursion depth
> as a counter in 'struct task_struct'.

Seems like a better approach.

> The change make page_owner=on work on ia64 bu avoiding infinite
> recursion in:
>   kmalloc()
>   -> __set_page_owner()
>   -> save_stack()
>   -> unwind() [ia64-specific]
>   -> build_script()
>   -> kmalloc()
>   -> __set_page_owner() [we short-circuit here]
>   -> save_stack()
>   -> unwind() [recursion]
> 
> ...
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1371,6 +1371,15 @@ struct task_struct {
>  	struct llist_head               kretprobe_instances;
>  #endif
>  
> +#ifdef CONFIG_PAGE_OWNER
> +	/*
> +	 * Used by page_owner=on to detect recursion in page tracking.
> +	 * Is it fine to have non-atomic ops here if we ever access
> +	 * this variable via current->page_owner_depth?

Yes, it is fine.  This part of the comment can be removed.

> +	 */
> +	unsigned int page_owner_depth;
> +#endif

Adding to the task_struct has a cost.  But I don't expect that
PAGE_OWNER is commonly used in prodction builds (correct?).

> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -213,6 +213,9 @@ struct task_struct init_task
>  #ifdef CONFIG_SECCOMP
>  	.seccomp	= { .filter_count = ATOMIC_INIT(0) },
>  #endif
> +#ifdef CONFIG_PAGE_OWNER
> +	.page_owner_depth	= 0,
> +#endif
>  };
>  EXPORT_SYMBOL(init_task);

It will be initialized to zero by the compiler.  We can omit this hunk
entirely.

> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -20,6 +20,16 @@
>   */
>  #define PAGE_OWNER_STACK_DEPTH (16)
>  
> +/*
> + * How many reenters we allow to page_owner.
> + *
> + * Sometimes metadata allocation tracking requires more memory to be allocated:
> + * - when new stack trace is saved to stack depot
> + * - when backtrace itself is calculated (ia64)
> + * Instead of falling to infinite recursion give it a chance to recover.
> + */
> +#define PAGE_OWNER_MAX_RECURSION_DEPTH (1)

So this is presently a boolean.  Is there any expectation that
PAGE_OWNER_MAX_RECURSION_DEPTH will ever be greater than 1?  If not, we
could use a single bit in the task_struct.  Add it to the
"Unserialized, strictly 'current'" bitfields.  Could make it a 2-bit field if we want
to permit PAGE_OWNER_MAX_RECURSION_DEPTH=larger.