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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Daniel Vetter <daniel@ffwll.ch>
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Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends
In-Reply-To: <CAKMK7uENFDANQKebS_H0bhHeQRijrp1aVHQqyZPute3KBZ+fVQ@mail.gmail.com>
References: <20200919091751.011116649@linutronix.de> <CAKMK7uHTVJL2jGtCg61zG=myiF1BSk+yDdRYikcm-Mq_1TQWMQ@mail.gmail.com> <CAKMK7uENFDANQKebS_H0bhHeQRijrp1aVHQqyZPute3KBZ+fVQ@mail.gmail.com>
Date:   Sun, 20 Sep 2020 08:23:26 +0200
Message-ID: <87pn6hc6g1.fsf@nanos.tec.linutronix.de>
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On Sat, Sep 19 2020 at 12:37, Daniel Vetter wrote:
> On Sat, Sep 19, 2020 at 12:35 PM Daniel Vetter <daniel@ffwll.ch> wrote:
>> I think it should be the case, but I want to double check: Will
>> copy_*_user be allowed within a kmap_temporary section? This would
>> allow us to ditch an absolute pile of slowpaths.
>
> (coffee just kicked in) copy_*_user is ofc allowed, but if you hit a
> page fault you get a short read/write. This looks like it would remove
> the need to handle these in a slowpath, since page faults can now be
> served in this new kmap_temporary sections. But this sounds too good
> to be true, so I'm wondering what I'm missing.

In principle we could allow pagefaults, but not with the currently
proposed interface which can be called from any context. Obviously if
called from atomic context it can't handle user page faults.

In theory we could make a variant which does not disable pagefaults, but
that's what kmap() already provides.

Thanks,

        tglx