Re: [PATCH v2 2/3] mm, page_owner: Add page_owner_stacks file to print out only stacks and their counter

From: Oscar Salvador <osalvador@suse.de>
To: Marco Elver <elver@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Michal Hocko <mhocko@suse.com>, Vlastimil Babka <vbabka@suse.cz>,
	Eric Dumazet <edumazet@google.com>,
	Waiman Long <longman@redhat.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Andrey Konovalov <andreyknvl@gmail.com>,
	Alexander Potapenko <glider@google.com>
Subject: Re: [PATCH v2 2/3] mm, page_owner: Add page_owner_stacks file to print out only stacks and their counter
Date: Thu, 8 Sep 2022 05:32:28 +0200	[thread overview]
Message-ID: <YxliTBGU/gEzLr+S@localhost.localdomain> (raw)
In-Reply-To: <CANpmjNNn0cSpszRJyeF9_ogER9m859MYh745bqOr1DLEp7kibQ@mail.gmail.com>

On Wed, Sep 07, 2022 at 09:14:35AM +0200, Marco Elver wrote:
> Why are you casting a stack_record** to a stack_record*? stack_table
> is already appropriately typed, and there should be no need to cast
> things around.
> 
> 'stacks' is supposed to be the bucket? In which case you need to
> dereference it to get the first entry in the bucket: bucket =
> stack_table[table_i];
> 
> stack_i cannot be used to index into the bucket, because the elements
> in it are a linked list and not necessarily adjacent in memory. You
> have to traverse the linked list stack_i elements to get to the start:

Yea, I figured that much after thinking about more, but I was overly
eager.

>   for (int i = 0; stack && i < stack_i; stack = stack->next, ++i);

But this seems suboptimal.
With this code, we have to walk the list till we find the right index
every time we enter the function, while the actual code of v2
or even the patch from v1 [1], we do not really need to do that
because we already have the pointer to the stack.

So I much rather prefer that, than having to traverse the stacks
till the find the right one.

-- 
Oscar Salvador
SUSE Labs