linux-kernel - Re: xtensa question, was Re: [PATCH V1 00/10] Remove duplicated kmap code

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Message-ID: <CAMo8BfKYii4=_rQK22x4F4n-59KYY9JHKQ6E3H5iJ-N3J63rwg@mail.gmail.com>
Date:   Fri, 1 May 2020 02:50:27 -0700
From:   Max Filippov <jcmvbkbc@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     ira.weiny@...el.com, LKML <linux-kernel@...r.kernel.org>,
        "open list:TENSILICA XTENSA PORT (xtensa)" 
        <linux-xtensa@...ux-xtensa.org>
Subject: Re: xtensa question, was Re: [PATCH V1 00/10] Remove duplicated kmap code

On Fri, May 1, 2020 at 2:19 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Fri, May 01, 2020 at 02:02:19AM -0700, Max Filippov wrote:
> > Hi Christoph,
> >
> > On Fri, May 1, 2020 at 1:46 AM Christoph Hellwig <hch@...radead.org> wrote:
> > > any idea why xtensa uses PAGE_KERNEL_EXEC instead of PAGE_KERNEL
> > > for kmap_prot?  Mapping all mapped highmem as executable seems rather
> > > dangerous.
> >
> > I sure do: to allow instruction cache flushing when writing to high user
> > pages temporarily mapped with kmap. Instruction cache management
> > opcodes that operate on virtual addresses would raise an exception if
> > the address is not executable.
>
> Seems like this should use kmap_atomic_prot with PAGE_KERNEL_EXEC just
> for that case.  Which of course didn't exist on xtensa so far, but with
> this series will.

Yeah, except it's the __access_remote_vm that does the kmap and then
calls copy_to_user_page...

-- 
Thanks.
-- Max