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

[Russell King - ARM Linux admin]

On Mon, Mar 09, 2020 at 03:59:45PM +0000, Catalin Marinas wrote:
> On Sun, Mar 08, 2020 at 11:58:52AM +0100, Arnd Bergmann wrote:
> > - revisit CONFIG_VMSPLIT_4G_4G for arm32 (and maybe mips32)
> >   to see if it can be done, and what the overhead is. This is probably
> >   more work than the others combined, but also the most promising
> >   as it allows the most user address space and physical ram to be used.
> 
> A rough outline of such support (and likely to miss some corner cases):
> 
> 1. Kernel runs with its own ASID and non-global page tables.
> 
> 2. Trampoline code on exception entry/exit to handle the TTBR0 switching
>    between user and kernel.
> 
> 3. uaccess routines need to be reworked to pin the user pages in memory
>    (get_user_pages()) and access them via the kernel address space.
> 
> Point 3 is probably the ugliest and it would introduce a noticeable
> slowdown in certain syscalls.

We also need to consider that it has implications for the single-kernel
support; a kernel doing this kind of switching would likely be horrid
for a kernel supporting v6+ with VIPT aliasing caches.  Would we be
adding a new red line between kernels supporting VIPT-aliasing caches
(present in earlier v6 implementations) and kernels using this system?

-- 
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