Date:   Mon, 19 Oct 2020 12:36:59 +0300 (MSK)
From:   Roman Anufriev <dotdot@...dex-team.ru>
To:     Jan Kara <jack@...e.cz>
cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        dmtrmonakhov@...dex-team.ru
Subject: Re: [PATCH 2/2] ext4: export quota journalling mode via sysfs attr
 quota_mode

On Mon, 19 Oct 2020, Jan Kara wrote:

> On Sat 17-10-20 21:26:37, Roman Anufriev wrote:
>> Hi, sorry for the delay.
>>
>> On Thu, 15 Oct 2020, Jan Kara wrote:
>>
>>> On Thu 15-10-20 14:32:52, Roman Anufriev wrote:
>>>> Right now, it is hard to understand what quota journalling type is enabled:
>>>> you need to be quite familiar with kernel code and trace it or really
>>>> understand what different combinations of fs flags/mount options lead to.
>>>>
>>>> This patch exports via sysfs attr /sys/fs/ext4/<disk>/quota_mode current
>>>> quota jounalling mode, making it easier to check at a glance/in autotests.
>>>> The semantics is similar to ext4 data journalling modes:
>>>>
>>>> * journalled - quota accounting and journaling are enabled
>>>> * writeback  - quota accounting is enabled, but journalling is disabled
>>>> * none       - quota accounting is disabled
>>>> * disabled   - kernel compiled without CONFIG_QUOTA feature
>>>>
>>>> Signed-off-by: Roman Anufriev <dotdot@...dex-team.ru>
>>>> Reviewed-by: Dmitry Monakhov <dmtrmonakhov@...dex-team.ru>
>>>
>>> Hum, I'm not sure about this. The state of quota can be found out with
>>> "quotaon -p <mntpoint>" (or corresponding quotactl if you need this from
>>> C). The only thing you won't learn is journalled / writeback mode and
>>> generally you should not care about this although I agree that for fs
>>> crash testing purposes you may care. But is that big enough usecase for a
>>> new sysfs file when all the information is already available for userspace
>>> just not in a convenient form?
>>
>> Rationale behind this patch was mainly the addition of an easy way to check
>> whether quota journalled or not as this is quite wanted feature in out
>> production environment. TBH, I was not sure about sysfs file too, but it
>> seemed to me like the most natural place to put it. Maybe if sysfs is an
>> overkill - just add printing to dmesg on mount? At least, you'll be able to
>> check what quota type you can enable right after mounting.
>>
>>> BTW, I've now realized ext4_any_quota_enabled() has actually misleading
>>> name in the sysfs file reports wrong information. It is rather
>>> ext4_any_quota_may_be_enabled() since presence of QUOTA mount option only
>>> says that quotaon(8) will enable quotas if it is run, not that quota
>>> accounting is enabled. sb_any_quota_loaded() tells you if accounting is
>>> actually enabled or not (however this can change anytime so that's why we
>>> use more relaxed checks for the purpose of journal credit estimates).
>>
>> My bad! Totally forgot about the case when 'quota' mount option is present
>> but quota accounting is not enabled, as our helper-tool around 'quotactl()'
>> do remounting+enabling in one go.
>>
>> I'll rename the function to smth like 'ext4_quota_capable()' in v2. And if
>> printing to dmesg is OK, I'll probably still use this function to print on
>> mount what the quota type will be after accounting is enabled.
>
> Yeah, if a message in dmesg is fine for your purposes, I'd rather go with
> that than with a sysfs file.

I think it'll be enough. I've implemented this in v3 (skip the v2, please 
- made a syntax mistake there): 
https://lore.kernel.org/linux-ext4/1603099162-25028-1-git-send-email-dotdot@yandex-team.ru/

 								Roman

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