linux-kernel - Re: [PATCH V2 2/2] Documentation: filesystems: introduce proc/slabinfo to users

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Date: Mon, 19 Feb 2024 09:09:13 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: zhang fangzheng <fangzheng.zhang1003@...il.com>,
 Matthew Wilcox <willy@...radead.org>
Cc: Fangzheng Zhang <fangzheng.zhang@...soc.com>,
 Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
 David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, Greg KH <gregkh@...uxfoundation.org>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, tkjos@...gle.com,
 Yuming Han <yuming.han@...soc.com>, Chunyan Zhang <zhang.lyra@...il.com>
Subject: Re: [PATCH V2 2/2] Documentation: filesystems: introduce
 proc/slabinfo to users

On 2/19/24 07:23, zhang fangzheng wrote:
> On Mon, Feb 19, 2024 at 12:24 PM Matthew Wilcox <willy@...radead.org> wrote:
>>
>> On Mon, Feb 19, 2024 at 11:19:11AM +0800, Fangzheng Zhang wrote:
>> > +Note, <slabreclaim> comes from the collected results in the file
>> > +/sys/kernel/slab/$cache/reclaim_account. Next, we will mark /proc/slabinfo
>> > +as deprecated and recommend the use of either sysfs directly or
>> > +use of the "slabinfo" tool that we have been providing in linux/tools/mm.
>>
>> Wait, so you're going to all of the trouble of changing the format of
>> slabinfo (with the associated costs of updating every tool that currently
>> parses it), only to recommend that we stop using it and start using
>> tools/mm/slabinfo instead?
>>

Hi,

> The initial purpose was to obtain the type of each slab through
> a simple command 'cat proc/slabinfo'. So here, my intention is not to
> update all slabinfo-related tools for the time being, but to modify
> the version number of proc/slabinfo and further display the results
> of using the command.

I'm not sure you understand the concern. There are existing consumers of
/proc/slabinfo, that might become broken by patch 1/2. We don't even know
them all, they might not be all opensource etc. So we can't even make sure
all of them are updated. What can happen after patch 1/2:
- they keep working and ignore the new column (good)
- they include a version check and notice a new unsupported version and
refuse to work
- confused by the new column they start throwing error, or report wrong
stats (that's worse)

>> How about we simply do nothing?

Agreed wrt modifying /proc/slabinfo

> The note here means what changes will occur after
> we modify the version number of proc/slabinfo to 2.2.
> As for the replacement of tools/mm/slabinfo (that inspired
> by Christoph’s suggestions), it will be implemented in the next version
> or even the later version.

So what is your motivation for all this in the first place? You have some
monitoring tool that relies on /proc/slabinfo and want to distinguish
reclaimable caches? So you can change it to parse the /sys directories. Is
it more work? Yes, but you only have to do that once per boot, because
unlike the object/memory stats in /proc/slabinfo, the reclaimable flag will
not change for a cache.

Would tools/mm/slabinfo almost work for you, but you're missing something?
Then send patches for that in the first place. Changing /proc/slabinfo (and
breaking other consumers) for a quick and easy fix with a different solution
planned for the future is simply not feasible.

HTH,
Vlastimil

> Thanks!