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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 11 Mar 2020 23:21:01 +0100
Message-ID: <CAK8P3a0QTKsqoxE7HS7aNrASSHOfFJHfp3+KZNTVoQ12wHi3VQ@mail.gmail.com>
Subject: Re: [PATCH] vfs: keep inodes with page cache off the inode shrinker LRU
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Russell King - ARM Linux admin <linux@armlinux.org.uk>,
        Nishanth Menon <nm@ti.com>,
        Santosh Shilimkar <santosh.shilimkar@oracle.com>,
        Tero Kristo <t-kristo@ti.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Michal Hocko <mhocko@suse.com>,
        Rik van Riel <riel@surriel.com>,
        Santosh Shilimkar <ssantosh@kernel.org>,
        Dave Chinner <david@fromorbit.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Yafang Shao <laoar.shao@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Johannes Weiner <hannes@cmpxchg.org>,
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        kernel-team@fb.com, Kishon Vijay Abraham I <kishon@ti.com>,
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On Wed, Mar 11, 2020 at 6:26 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
> On Wed, Mar 11, 2020 at 05:59:53PM +0100, Arnd Bergmann wrote:
> > On Wed, Mar 11, 2020 at 3:29 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > Do you have an estimate of how long writing to TTBR0_64 takes on Cortex-A7
> > and A15, respectively?
>
> I don't have numbers but it's usually not cheap since you need an ISB to
> synchronise the context after TTBR0 update (basically flushing the
> pipeline).

Ok.

> > Another way might be to use a use a temporary buffer that is already
> > mapped, and add a memcpy() through L1-cache to reduce the number
> > of ttbr0 changes. The buffer would probably have to be on the stack,
> > which limits the size, but for large copies get_user_pages()+memcpy()
> > may end up being faster anyway.
>
> IIRC, the x86 attempt from Ingo some years ago was using
> get_user_pages() for uaccess. Depending on the size of the buffer, this
> may be faster than copying twice.

I guess the tradeoffs for that were rather different, as x86 back
then had no ASIDs, so changing the page tables required a full
TLB flush.

         Arnd