linux-kernel - Re: [PATCH] vfs: keep inodes with page cache off the inode shrinker LRU

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Message-ID: <20200311172631.GN3216816@arrakis.emea.arm.com>
Date:   Wed, 11 Mar 2020 17:26:31 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Nishanth Menon <nm@...com>,
        Santosh Shilimkar <santosh.shilimkar@...cle.com>,
        Tero Kristo <t-kristo@...com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Santosh Shilimkar <ssantosh@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Yafang Shao <laoar.shao@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kernel-team@...com, Kishon Vijay Abraham I <kishon@...com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>
Subject: Re: [PATCH] vfs: keep inodes with page cache off the inode shrinker
 LRU

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@....com> wrote:
> 
> > > - Flip TTBR0 on kernel entry/exit, and again during user access.
> > >
> > > This is probably more work to implement than your idea, but
> > > I would hope this has a lower overhead on most microarchitectures
> > > as it doesn't require pinning the pages. Depending on the
> > > microarchitecture, I'd hope the overhead would be comparable
> > > to that of ARM64_SW_TTBR0_PAN.
> >
> > This still doesn't solve the copy_{from,to}_user() case where both
> > address spaces need to be available during copy. So you either pin the
> > user pages in memory and access them via the kernel mapping or you
> > temporarily map (kmap?) the destination/source kernel address. The
> > overhead I'd expect to be significantly greater than ARM64_SW_TTBR0_PAN
> > for the uaccess routines. For user entry/exit, your suggestion is
> > probably comparable with SW PAN.
> 
> Good point, that is indeed a larger overhead. The simplest implementation
> I had in mind would use the code from arch/arm/lib/copy_from_user.S and
> flip ttbr0 between each ldm and stm (up to 32 bytes), but I have no idea
> of the cost of storing to ttbr0, so this might be even more expensive. 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).

> 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.

-- 
Catalin