linux-kernel - Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap

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Date:   Sun, 20 Sep 2020 10:58:50 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Paul McKenney <paulmck@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
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        Will Deacon <will@...nel.org>,
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        linux-sparc <sparclinux@...r.kernel.org>
Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of
 kmap_atomic & friends

On Sun, Sep 20, 2020 at 10:42 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Yeah, that looks much easier to explain. Ack.

Btw, one thing that might be a good idea at least initially is to add
a check for p->kmap_ctrl.idx being zero at fork, exit and maybe
syscall return time (but that last one may be too cumbersome to really
worry about).

The kmap_atomic() interface basically has a lot of coverage for leaked
as part of all the "might_sleep()" checks sprinkled around,  The new
kmap_temporary/local/whatever wouldn't have that kind of incidental
debug checking, and any leaked kmap indexes would be rather hard to
debug (much) later when they cause index overflows or whatever.

                Linus