linux-kernel - Re: [RFC bpf-next 0/1] bpf: Add page cache iterator

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Date:   Thu, 8 Apr 2021 16:13:32 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Daniel Xu <dxu@...uu.xyz>
Cc:     bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, jolsa@...nel.org, hannes@...xchg.org,
        yhs@...com
Subject: Re: [RFC bpf-next 0/1] bpf: Add page cache iterator

On Wed, Apr 07, 2021 at 02:46:10PM -0700, Daniel Xu wrote:
> There currently does not exist a way to answer the question: "What is in
> the page cache?". There are various heuristics and counters but nothing
> that can tell you anything like:
> 
>   * 3M from /home/dxu/foo.txt
>   * 5K from ...

5K?  That's an extraordinary Weird Machine(tm).

>   * etc.
> 
> The answer to the question is particularly useful in the stacked
> container world. Stacked containers implies multiple containers are run
> on the same physical host. Memory is precious resource on some (if not
> most) of these systems. On these systems, it's useful to know how much
> duplicated data is in the page cache. Once you know the answer, you can
> do something about it. One possible technique would be bind mount common
> items from the root host into each container.

Um, are you describing a system that uses BPF to deduplicating the page
cache by using bind mounts?  Can the containers scribble on these files
and thereby mess up the other containers?  What happens if the container
wants to update itself and clobbers the root host's copy instead?  How
do you deal with a software update process failing because the root host
fights back against the container trying to change its files?

Also, I thought we weren't supposed to share resources across security
boundaries anymore?

--D

> 
> NOTES: 
> 
>   * This patch compiles and (maybe) works -- totally not fully tested
>     or in a final state
> 
>   * I'm sending this early RFC to get comments on the general approach.
>     I chatted w/ Johannes a little bit and it seems like the best way to
>     do this is through superblock -> inode -> address_space iteration
>     rather than going from numa node -> LRU iteration
> 
>   * I'll most likely add a page_hash() helper (or something) that hashes
>     a page so that userspace can more easily tell which pages are
>     duplicate
> 
> Daniel Xu (1):
>   bpf: Introduce iter_pagecache
> 
>  kernel/bpf/Makefile         |   2 +-
>  kernel/bpf/pagecache_iter.c | 293 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 294 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/bpf/pagecache_iter.c
> 
> -- 
> 2.26.3
>