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

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Date:   Wed, 21 Jun 2023 09:35:43 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>, Sumitra Sharma <sumitraartsy@...il.com>,
        Ira Weiny <ira.weiny@...el.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()

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?

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

Thanks!
Ira