Re: [PATCH 2/2] ata: allow enabling FUA support in Kconfig

[Damien Le Moal]

On 10/15/22 01:44, Maciej S. Szmigiero wrote:
> On 8.10.2022 00:41, Damien Le Moal wrote:
>> On 10/7/22 23:14, Maciej S. Szmigiero wrote:
>>> On 7.10.2022 00:53, Damien Le Moal wrote:
>>>> On 10/7/22 07:20, Damien Le Moal wrote:
>>>>> On 10/6/22 22:06, Maciej S. Szmigiero wrote:
>>>>>> On 6.10.2022 01:38, Damien Le Moal wrote:
>>>>>>> On 9/27/22 04:51, Maciej S. Szmigiero wrote:
>>>>>>>> From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>
>>>>>>>>
>>>>>>>> Currently, if one wants to make use of FUA support in libata it is
>>>>>>>> necessary to provide an explicit kernel command line parameter in order to
>>>>>>>> enable it (for drives that report such support).
>>>>>>>>
>>>>>>>> In terms of Git archaeology: FUA support was enabled by default in early
>>>>>>>> libata versions but was disabled soon after.
>>>>>>>> Since then there were a few attempts to enable this support by default:
>>>>>>>> [1] (for NCQ drives only), [2] (for all drives).
>>>>>>>> However, the second change had to be reverted after a report came of
>>>>>>>> an incompatibility with the HDD in 2011 Mac Mini.
>>>>>>>>
>>>>>>>> Enabling FUA avoids having to emulate it by issuing an extra drive cache
>>>>>>>> flush for every request that have this flag set.
>>>>>>>> Since FUA support is required by the ATA/ATAPI spec for any drive that
>>>>>>>> supports LBA48 and so these days should be pretty widespread let's provide
>>>>>>>> an ability to enable it by default in Kconfig.
>>>>>>>
>>>>>>> This can be done by adding "libata.fua=1" to the CONFIG_CMDLINE option. So
>>>>>>> I do not see the need to add yet another config option.
>>>>>>
>>>>>> A specific Kconfig option is more structured than a free-form
>>>>>> CONFIG_CMDLINE (which is also technically a per-arch option, but seems
>>>>>> to be widely supported across arches).
>>>>>>
>>>>>> That's why there is a lot (100+) of similar Kconfig default-changing
>>>>>> options, a quick sample of these (in no particular order):
>>>>>> SOUND_OSS_CORE_PRECLAIM, SND_INTEL_BYT_PREFER_SOF, LSM,
>>>>>> SECURITY_SELINUX_CHECKREQPROT_VALUE, SECURITY_LOADPIN_ENFORCE,
>>>>>> SECURITY_APPARMOR_DEBUG_MESSAGES, IP_VS_TAB_BITS, IP_SET_MAX,
>>>>>> MAC80211_HAS_RC, SLUB_DEBUG_ON, KFENCE_SAMPLE_INTERVAL, PRINTK_TIME,
>>>>>> DEBUG_OBJECTS_ENABLE_DEFAULT, RCU_NOCB_CPU_DEFAULT_ALL, ...
>>>>>>
>>>>>> libata currently has only one similar option: SATA_MOBILE_LPM_POLICY,
>>>>>> so it's not like a person performing kernel configuration is
>>>>>> overloaded with questions here.
>>>>>>
>>>>>> But at the same time, I respect your decision as a maintainer of
>>>>>> this code.
>>>>>
>>>>> I am not dead set on pushing back on this, but as usual, whenever we can
>>>>> avoid adding config options, we should. Given that libata has had fua
>>>>> disabled forever, I am not convinced yet that there is a strong need for
>>>>> that new option. But if distros prefer the config option approach, we can
>>>>> make that happen.
>>>>>
>>>>> If anything, I would be tempted to switch fua support to on by default
>>>>> after some time if we do not get many reports of broken drives. You did
>>>>> mention that old mac minis drives did not like it, but these are not even
>>>>> blacklisted in libata-scsi. They should. Only one model of maxtor drives
>>>>> is. We should add an ATA_HORKAGE_NO_FUA flag and start a proper blacklist
>>>>> of drives not liking fua. Without that in place, switching to on by
>>>>> default as your config option allows could break many (old) systems.
>>>>
>>>> To be extra clear, I think that this fua module parameter is silly. If a
>>>> drive says it supports fua, we should use it and not have a global
>>>> parameter to disable it for all drives. So no config option needed for it.
>>>>
>>>> That is also why I am not keen on taking that config option. It is not
>>>> really improving anything at all and would prefer nuking the fua module
>>>> argument and have a proper blacklisting of buggy drives.
>>>>
>>>> But such a change is painful as we'll be able to update the blacklist with
>>>> users getting corrupted FSes on buggy drives. The time may have come to do
>>>> this change though as the number of buggy drives out there is hopefully
>>>> small enough now.
>>>
>>> So your proposal is basically to switch the existing fua option default
>>> to "on" and deal with the fallout (hopefully minimal) by blacklisting
>>> misbehaving drives as they get reported, right?
>>
>> Yes. The risk though is that if the fallout are not minimal and we get too
>> many bug reports, we'll likely have to revert. So this needs to be
>> attempted early at the beginning of a cycle to get plenty of testing.
>>
>>> In this case, my vote would be to still keep the "libata.fua" parameter
>>> available (at least for the time being) so people have some way of
>>> working broken drives around without having to recompile their kernel
>>> (maybe also print a kernel log message if libata.fua=0 is provided asking
>>> people to report these drive models to linux-ide@).
>>
>> If we add a proper "nofua" horkage flag to blacklist buggy drives, we need
>> to move the fua parameter to be an argument of the force parameter so that
>> disabling fua can be done per drive (port) or for all drives, similarly to
>> other tweak (noncq, nodmalog, etc)
> 
> So would you like an updated patch set or do you prefer to do the changes
> yourself?

Almost done already :) Need a lot of testing though.
Will post the patches when done.

Note that for now, I kept the fua module parameter, for compatibility with
existing setups. But I added the parameter force=[no]fua which allows
doing the same global enable/disable but also per drive enable/disable.
And known bad drives can be marked with the horkage flag.

> 
> Thanks,
> Maciej
> 

-- 
Damien Le Moal
Western Digital Research