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

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Message-ID: <61af19ca-5f9a-40da-a04d-b04ed27b8754@suse.cz>
Date: Wed, 31 Jan 2024 23:40:20 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: 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 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,
Vlastimil