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

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Message-ID: <13316568.uLZWGnKmhe@suse>
Date:   Wed, 21 Jun 2023 15:24:29 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Sumitra Sharma <sumitraartsy@...il.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>
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, Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH v2] drm/i915: Replace kmap() with kmap_local_page()

On mercoledì 21 giugno 2023 11:06:51 CEST 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(),

Migration is disabled.

> so while I haven't seen any guidelines around the usage
> of this api for long-held mappings,

It would be advisable to not use it for long term mappings, if possible. These 
"local" mappings should better be help for not too long duration. 

> I figure it's wise to keep the
> mapping duration short, or at least avoid sleeping with a
> kmap_local_page() map active.

Nothing prevents a call of preempt_disable() around the section of code 
between kmap_local_page() / kunmap_local(). If it is needed, local mappings 
could also be acquired under spinlocks and/or with disabled interrupts.

I don't know the code, however, everything cited above could be the subject of 
a subsequent patch.

Regards,

Fabio

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