linux-kernel - Re: [PATCH v2 5/6] alloc_tag: make page allocation tag reference size configurable

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Message-Id: <20240901220931.53d3ad335ae9ac3fe7ef3928@linux-foundation.org>
Date: Sun, 1 Sep 2024 22:09:31 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: kent.overstreet@...ux.dev, corbet@....net, arnd@...db.de,
 mcgrof@...nel.org, rppt@...nel.org, paulmck@...nel.org, thuth@...hat.com,
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Subject: Re: [PATCH v2 5/6] alloc_tag: make page allocation tag reference
 size configurable

On Sun,  1 Sep 2024 21:41:27 -0700 Suren Baghdasaryan <surenb@...gle.com> wrote:

> Introduce CONFIG_PGALLOC_TAG_REF_BITS to control the size of the
> page allocation tag references. When the size is configured to be
> less than a direct pointer, the tags are searched using an index
> stored as the tag reference.
> 
> ...
>
> +config PGALLOC_TAG_REF_BITS
> +	int "Number of bits for page allocation tag reference (10-64)"
> +	range 10 64
> +	default "64"
> +	depends on MEM_ALLOC_PROFILING
> +	help
> +	  Number of bits used to encode a page allocation tag reference.
> +
> +	  Smaller number results in less memory overhead but limits the number of
> +	  allocations which can be tagged (including allocations from modules).
> +

In other words, "we have no idea what's best for you, you're on your
own".

I pity our poor users.

Can we at least tell them what they should look at to determine whether
whatever random number they chose was helpful or harmful?