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

[Arnd Bergmann]

On Wed, Mar 11, 2020 at 3:29 PM Catalin Marinas <catalin.marinas@xxxxxxx> 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?

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.

      Arnd