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Date:   Fri, 14 Oct 2022 18:44:25 +0200
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To:     Damien Le Moal <damien.lemoal@opensource.wdc.com>
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From:   "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Subject: Re: [PATCH 2/2] ata: allow enabling FUA support in Kconfig
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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@oracle.com>
>>>>>>>
>>>>>>> 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?

Thanks,
Maciej