linux-kernel - Re: Do we still need SLAB_MEM_SPREAD (and possibly others)?

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Message-ID: <698633db-b066-4f75-b201-7b785819277b@linux.dev>
Date: Thu, 1 Feb 2024 14:27:23 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>, Yosry Ahmed <yosryahmed@...gle.com>,
 Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Kees Cook <keescook@...omium.org>, Christoph Lameter <cl@...ux.com>,
 David Rientjes <rientjes@...gle.com>, Hyeonggon Yoo <42.hyeyoo@...il.com>,
 Xiongwei Song <xiongwei.song@...driver.com>,
 Chengming Zhou <zhouchengming@...edance.com>,
 Zheng Yejian <zhengyejian1@...wei.com>
Subject: Re: Do we still need SLAB_MEM_SPREAD (and possibly others)?

On 2024/2/1 06:40, Vlastimil Babka wrote:
> On 1/31/24 23:25, Yosry Ahmed wrote:
>> On Wed, Jan 31, 2024 at 2:20 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>>>
>>> I was looking into moving eventfs_inode into a slab, and after cutting and
>>> pasting the tracefs allocator:
>>>
>>>         tracefs_inode_cachep = kmem_cache_create("tracefs_inode_cache",
>>>                                                  sizeof(struct tracefs_inode),
>>>                                                  0, (SLAB_RECLAIM_ACCOUNT|
>>>                                                      SLAB_MEM_SPREAD|
>>>                                                      SLAB_ACCOUNT),
>>>                                                  init_once);
>>>
>>> I figured I should know what those slab flags mean. I also looked at what
>>> others in fs use for their slabs. The above is rather common (which I
>>> probably just copied from another file system), but I wanted to know what
>>> they are for.
>>>
>>> When I got to SLAB_MEM_SPREAD, I found that it's a common flag and there's
>>> a lot of caches that just set that and nothing else.
>>>
>>> But I couldn't find how it was used.
>>>
>>> Then I found this commit:
>>>
>>>  16a1d968358a ("mm/slab: remove mm/slab.c and slab_def.h")
>>>
>>> Which I think removed the only use case of SLAB_MEM_SPREAD.
>>>
>>>  $ git grep SLAB_MEM_SPREAD mm
>>> mm/slab.h:                            SLAB_MEM_SPREAD | \
>>>
>>> That's all I find in the mm directory.
>>>
>>> Is it obsolete now? Can we delete it? Maybe there's other SLAB_* flags that
>>> are no longer used. I don't know, I haven't audited them.
>>
>> Perhaps cpuset_do_slab_mem_spread() as well.
> 
> Yep, good find. Show how obscure mm/slab.c was in the end :)
> 
> CCing a few more new people who did slab changes recently, who'd like some
> low hanging fruit of negative diffcount? :)

Thanks for CCing, I can prepare the patch to do it. IIUC, what I need to do is:

1. delete SLAB_MEM_SPREAD and all its uses.

2. cpuset_do_slab_mem_spread() is not used anymore, should we keep the interface?
   Since it's the interface exported by cgroup-v1 "cpuset.memory_spread_slab".