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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 8 Apr 2021 14:33:43 -0700
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Subject: Re: [RFC bpf-next 0/1] bpf: Add page cache iterator
To:     Daniel Xu <dxu@dxuuu.xyz>
Cc:     bpf <bpf@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>, jolsa@kernel.org,
        Johannes Weiner <hannes@cmpxchg.org>,
        Yonghong Song <yhs@fb.com>
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On Wed, Apr 7, 2021 at 2:47 PM Daniel Xu <dxu@dxuuu.xyz> 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 ...
>   * etc.
>

So, you need the list of inodes for a filesystem that are in memory.
With that list, you can open, map and do mincore on them?

I don't know of a way to get that inode list.

> 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.
>

Oh you want to share page cache between different containers/jobs?
That would complicate charging.