linux-kernel - Re: [PATCH v5 1/1] mm: report per-page metadata information

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Message-ID: <2023110216-labrador-neurosis-1e6e@gregkh>
Date:   Thu, 2 Nov 2023 15:28:58 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     Sourav Panda <souravpanda@...gle.com>, corbet@....net,
        rafael@...nel.org, akpm@...ux-foundation.org,
        mike.kravetz@...cle.com, muchun.song@...ux.dev, rppt@...nel.org,
        david@...hat.com, rdunlap@...radead.org, chenlinxuan@...ontech.com,
        yang.yang29@....com.cn, tomas.mudrunka@...il.com,
        bhelgaas@...gle.com, ivan@...udflare.com, yosryahmed@...gle.com,
        hannes@...xchg.org, shakeelb@...gle.com,
        kirill.shutemov@...ux.intel.com, wangkefeng.wang@...wei.com,
        adobriyan@...il.com, vbabka@...e.cz, Liam.Howlett@...cle.com,
        surenb@...gle.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, willy@...radead.org, weixugc@...gle.com
Subject: Re: [PATCH v5 1/1] mm: report per-page metadata information

On Thu, Nov 02, 2023 at 10:24:04AM -0400, Pasha Tatashin wrote:
> On Thu, Nov 2, 2023 at 1:42 AM Greg KH <gregkh@...uxfoundation.org> wrote:
> >
> > On Wed, Nov 01, 2023 at 04:08:16PM -0700, Sourav Panda wrote:
> > > Adds a new per-node PageMetadata field to
> > > /sys/devices/system/node/nodeN/meminfo
> >
> > No, this file is already an abuse of sysfs and we need to get rid of it
> > (it has multiple values in one file.)  Please do not add to the
> > nightmare by adding new values.
> 
> Hi Greg,
> 
> Today, nodeN/meminfo is a counterpart of /proc/meminfo, they contain
> almost identical fields, but show node-wide and system-wide views.

And that is wrong, and again, an abuse of sysfs, please do not continue
to add to it, that will only cause problems.

> Since per-page metadata is added into /proc/meminfo, it is logical to
> add into nodeN/meminfo, some nodes can have more or less struct page
> data based on size of the node, and also the way memory is configured,
> such as use of vmemamp optimization etc, therefore this information is
> useful to users.
> 
> I am not aware of any example of where a system-wide field from
> /proc/meminfo is represented as a separate sysfs file under node0/. If
> nodeN/meminfo is ever broken down into separate files it will affect
> all the fields in it the same way with or without per-page metadata

All of the fields should be individual files, please start adding them
if you want to add new items, I do not want to see additional abuse here
as that will cause problems (as you are seeing with the proc file.)

> > Also, even if you did want to do this, you didn't document it properly
> > in Documentation/ABI/ :(
> 
>  The documentation for the fields in nodeN/meminfo is only specified
> in  Documentation/filesystems/proc.rst, there is no separate sysfs
> Documentation for the fields in this file, we could certainly add
> that.

All sysfs files need to be documented in Documentation/ABI/ otherwise
you should get a warning when running our testing scripts.

thanks,

greg k-h