linux-kernel - Re: [PATCH v2] drm/i915: Replace kmap() with kmap_local

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Message-ID: <d58c3dfd-e1d5-a8c5-7f0f-295dbf99e91a@shipmail.org>
Date:   Wed, 21 Jun 2023 20:51:34 +0200
From:   Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>
To:     Ira Weiny <ira.weiny@...el.com>,
        Sumitra Sharma <sumitraartsy@...il.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, Fabio <fmdefrancesco@...il.com>,
        Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH v2] drm/i915: Replace kmap() with kmap_local_page()


On 6/21/23 18:35, Ira Weiny wrote:
> Thomas Hellström (Intel) wrote:
>> I think one thing worth mentioning in the context of this patch is that
>> IIRC kmap_local_page() will block offlining of the mapping CPU until
>> kunmap_local(), so while I haven't seen any guidelines around the usage
>> of this api for long-held mappings, I figure it's wise to keep the
>> mapping duration short, or at least avoid sleeping with a
>> kmap_local_page() map active.
>>
>> I figured, while page compression is probably to be considered "slow"
>> it's probably not slow enough to motivate kmap() instead of
>> kmap_local_page(), but if anyone feels differently, perhaps it should be
>> considered.
> What you say is all true.  But remember the mappings are only actually
> created on a HIGHMEM system.  HIGHMEM systems are increasingly rare.  Also
> they must suffer such performance issues because there is just no other
> way around supporting them.
>
> Also Sumitra, and our kmap conversion project in general, is focusing on
> not using kmap* if at all possible.  Thus the reason V1 tried to use
> page_address().
>
> Could we guarantee the i915 driver is excluded from all HIGHMEM systems?

The i915 maintainers might want to chime in here, but I would say no, we 
can't, although we don't care much about optimizing for them. Same for 
the new xe driver.

Thanks,

/Thomas


>
>> With that said, my Reviewed-by: still stands.
> Thanks!
> Ira