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From:   Alexander Potapenko <glider@google.com>
Date:   Thu, 17 Dec 2020 11:54:36 +0100
Message-ID: <CAG_fn=UXQUGiDqmChqD-xX-yF5Jp+7K+oHwKPrO9DZL-zW_4KQ@mail.gmail.com>
Subject: Re: [PATCH v3] lib: stackdepot: Add support to configure STACK_HASH_SIZE
To:     Vijayanand Jitta <vjitta@codeaurora.org>
Cc:     Minchan Kim <minchan@kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        dan.j.williams@intel.com, broonie@kernel.org,
        Masami Hiramatsu <mhiramat@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Konovalov <andreyknvl@google.com>, qcai@redhat.com,
        ylal@codeaurora.org, vinmenon@codeaurora.org,
        kasan-dev <kasan-dev@googlegroups.com>
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> > Can you provide an example of a use case in which the user wants to
> > use the stack depot of a smaller size without disabling it completely,
> > and that size cannot be configured statically?
> > As far as I understand, for the page owner example you gave it's
> > sufficient to provide a switch that can disable the stack depot if
> > page_owner=off.
> >
> There are two use cases here,
>
> 1. We don't want to consume memory when page_owner=off ,boolean flag
> would work here.
>
> 2. We would want to enable page_owner on low ram devices but we don't
> want stack depot to consume 8 MB of memory, so for this case we would
> need a configurable stack_hash_size so that we can still use page_owner
> with lower memory consumption.
>
> So, a configurable stack_hash_size would work for both these use cases,
> we can set it to '0' for first case and set the required size for the
> second case.

Will a combined solution with a boolean boot-time flag and a static
CONFIG_STACKDEPOT_HASH_SIZE work for these cases?
I suppose low-memory devices have a separate kernel config anyway?

My concern is that exposing yet another knob to users won't really
solve their problems, because the hash size alone doesn't give enough
control over stackdepot memory footprint (we also have stack_slabs,
which may get way bigger than 8Mb).