linux-kernel - Re: Default value of CMA

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Message-ID: <9ec621c0-ab26-442e-a07d-06905699fbc6@redhat.com>
Date: Wed, 6 Aug 2025 10:51:37 +0200
From: David Hildenbrand <david@...hat.com>
To: Jean Delvare <jdelvare@...e.de>, Minchan Kim <minchan@...nel.org>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Frank van der Linden <fvdl@...gle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Minchan Kim <minchan@...nel.org>
Subject: Re: Default value of CMA_SYSFS

On 06.08.25 10:07, Jean Delvare wrote:
> Hi all,

Hi!

> 
> CMA_SYSFS looks like a generally useful and low footprint option. Is
> there any reason why it isn't enabled by default when its dependencies
> are met?

Looking at the introducing commit

commit 43ca106fa8ec7d684776fbe561214d3b2b7cb9cb
Author: Minchan Kim <minchan@...nel.org>
Date:   Tue May 4 18:37:28 2021 -0700

     mm: cma: support sysfs
     
     Since CMA is getting used more widely, it's more important to keep
     monitoring CMA statistics for system health since it's directly related to
     user experience.
     
     This patch introduces sysfs statistics for CMA, in order to provide some
     basic monitoring of the CMA allocator.
     
      * the number of CMA page successful allocations
      * the number of CMA page allocation failures
     
     These two values allow the user to calcuate the allocation
     failure rate for each CMA area.

I cannot really tell why we made this configurable.

> 
> I was wondering if it would make sense to add "default y" to this
> option. What do you think?

I'm wondering if we should remove the option completely -- or turn it into an
internal option only such that it's no longer user-selectable.

CCing Minchan Kim

-- 
Cheers,

David / dhildenb