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

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Message-ID: <20200309155945.GA4124965@arrakis.emea.arm.com>
Date:   Mon, 9 Mar 2020 15:59:45 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     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>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        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 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