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Date:   Mon, 9 Mar 2020 15:59:45 +0000
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Nishanth Menon <nm@ti.com>,
        Santosh Shilimkar <santosh.shilimkar@oracle.com>,
        Tero Kristo <t-kristo@ti.com>,
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        Santosh Shilimkar <ssantosh@kernel.org>,
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        Russell King - ARM Linux admin <linux@armlinux.org.uk>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>
Subject: Re: [PATCH] vfs: keep inodes with page cache off the inode shrinker
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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.

-- 
Catalin