linux-kernel - Re: [PATCH v3] lib: stackdepot: Add support to configure STACK_HASH

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Date:   Thu, 17 Dec 2020 11:19:24 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Vijayanand Jitta <vjitta@...eaurora.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Mark Brown <broonie@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>, qcai@...hat.com,
        ylal@...eaurora.org, vinmenon@...eaurora.org
Subject: Re: [PATCH v3] lib: stackdepot: Add support to configure STACK_HASH_SIZE

On Thu, Dec 17, 2020 at 6:42 AM Vijayanand Jitta <vjitta@...eaurora.org> wrote:
>
> On 12/16/2020 7:04 PM, Alexander Potapenko wrote:
> >>> To reiterate, I think you don't need a tunable stack_hash_order
> >>> parameter if the only use case is to disable the stack depot.
> >>> Maybe it is enough to just add a boolean flag?
> >>
> >> There are multiple users of stackdepot they might still want to use
> >> stack depot but with a lower memory footprint instead of MAX_SIZE
> >> so, a configurable size might help here ?
> >
> > 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.
>
> >>> Or even go further and disable the stack depot in the same place that
> >>> disables page owner, as the user probably doesn't want to set two
> >>> flags instead of one?
> >>>
> >>
> >> Since, page owner is not the only user of stack depot we can't take that
> >> decision of disabling stack depot if page owner is disabled.
> >
> > Agreed, but if multiple subsystems want to use stackdepot together, it
> > is even harder to estimate the total memory consumption.
> > How likely is it that none of them will need MAX_SIZE?
> >
> >>>> Minchan,
> >>>> This should be fine right ? Do you see any issue with disabling
> >>>> stack depot completely ?

+kasan-dev