linux-kernel - Re: [PATCH] mm/cma: Add sysfs file 'release_pages

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Date: Wed, 7 Feb 2024 10:04:31 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, alexandru.elisei@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/cma: Add sysfs file 'release_pages_success'

On 2/6/24 23:08, Andrew Morton wrote:
> On Tue,  6 Feb 2024 10:27:31 +0530 Anshuman Khandual <anshuman.khandual@....com> wrote:
> 
>> This adds the following new sysfs file tracking the number of successfully
>> released pages from a given CMA heap area. Also like before - this will be
>> available via CONFIG_CMA_SYSFS, and help in determining active CMA pages
>> available on the system.
> 
> "like before" is mysterious.  Is this referring to some other patch? 
> To existing code?  Please be explicit and complete.

This was just referring to existing sysfs files i.e 'alloc_pages_success'
and 'alloc_pages_fail'.

> 
>> /sys/kernel/mm/cma/<cma-heap-area>/release_pages_success
>>
>> It adds an element 'nr_pages_released' (with CONFIG_CMA_SYSFS config) into
>> 'struct cma' which gets updated during cma_release().
> 
> The changelog doesn't explain why Linux needs this feature.  The value
> to our users.  Perhaps that info is buried in the link which is buried
> below the ^---$, but as this is the most important part of a changelog,
> it really should be spelled out here, completely and carefully please.

Does this look better ?

mm/cma: Add sysfs file 'release_pages_success'

This adds the following new sysfs file tracking the number of successfully
released pages from a given CMA heap area. This file will be available via
CONFIG_CMA_SYSFS and help in determining active CMA pages available on the
CMA heap area. This adds a new 'nr_pages_released' (CONFIG_CMA_SYSFS) into
'struct cma' which gets updated during cma_release().

/sys/kernel/mm/cma/<cma-heap-area>/release_pages_success

After this change, an user will be able to find active CMA pages available
in a given CMA heap area via the following method.

Active pages = alloc_pages_success - release_pages_success

That's valuable information for both software designers, and system admins
as it allows them to tune the number of CMA pages available in the system.
This increases user visibility for allocated CMA area and its utilization.

> 
>> ---
>> This patch applies on v6.8-rc3
>>
>> Some earlier relevant discussions regarding arm64 MTE dynamic tag storage
>> in this regard can be found here.
>>
>> https://lore.kernel.org/all/ZbpKyNVHfhf1-AAv@raptor/