linux-kernel - Re: [PATCH v8 2/5] mm,page_owner: Implement the tracking of the stacks count

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Date: Tue, 13 Feb 2024 14:42:25 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Oscar Salvador <osalvador@...e.de>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Michal Hocko <mhocko@...e.com>, Marco Elver <elver@...gle.com>,
 Andrey Konovalov <andreyknvl@...il.com>,
 Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v8 2/5] mm,page_owner: Implement the tracking of the
 stacks count

On 2/12/24 23:30, Oscar Salvador wrote:
> page_owner needs to increment a stack_record refcount when a new allocation
> occurs, and decrement it on a free operation.
> In order to do that, we need to have a way to get a stack_record from a
> handle.
> Implement __stack_depot_get_stack_record() which just does that, and make
> it public so page_owner can use it.
> 
> Also implement {inc,dec}_stack_record_count() which increments
> or decrements on respective allocation and free operations, via
> __reset_page_owner() (free operation) and __set_page_owner() (alloc
> operation).
> 
> Traversing all stackdepot buckets comes with its own complexity,
> plus we would have to implement a way to mark only those stack_records
> that were originated from page_owner, as those are the ones we are
> interested in.
> For that reason, page_owner maintains its own list of stack_records,
> because traversing that list is faster than traversing all buckets
> while keeping at the same time a low complexity.
> inc_stack_record_count() is responsible of adding new stack_records
> into the list stack_list.
> 
> Modifications on the list are protected via a spinlock with irqs
> disabled, since this code can also be reached from IRQ context.
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>  include/linux/stackdepot.h |  9 +++++
>  lib/stackdepot.c           |  8 +++++
>  mm/page_owner.c            | 73 ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 90 insertions(+)
>  static inline void __set_page_owner_handle(struct page_ext *page_ext,
> @@ -199,6 +271,7 @@ noinline void __set_page_owner(struct page *page, unsigned short order,
>  		return;
>  	__set_page_owner_handle(page_ext, handle, order, gfp_mask);
>  	page_ext_put(page_ext);
> +	inc_stack_record_count(handle);

What if this is dummy handle, which means we have recursed in page owner,
and we'll by trying to kmalloc() its struct stack and link it to the
stack_list because it was returned for the first time? Also failure_handle.
Could you pre-create static (not kmalloc) struct stack for these handles
with refcount of 0 and insert them to stack_list, all during
init_page_owner()? Bonus: no longer treating stack_list == NULL in a special
way in add_stack_record_to_list() (although you don't need to handle it
extra even now, AFAICS).

>  }
>  
>  void __set_page_owner_migrate_reason(struct page *page, int reason)