Re: [PATCH v2 0/3] Improve libata support for FUA

[Hannes Reinecke <hare@suse.de>]

On 10/25/22 01:26, Damien Le Moal wrote:
> On 10/25/22 07:09, Damien Le Moal wrote:
>> On 10/25/22 03:48, Maciej S. Szmigiero wrote:
>>> On 24.10.2022 09:26, Damien Le Moal wrote:
>>>> These patches cleanup and improve libata support for the FUA device
>>>> feature. Patch 3 enables FUA support by default for any drive that
>>>> reports supporting the feature.
>>>>
>>>> Changes from v1:
>>>>    - Removed Maciej's patch 2. Instead, blacklist drives which are known
>>>>      to have a buggy FUA support.
>>>>
>>>> Damien Le Moal (3):
>>>>     ata: libata: cleanup fua handling
>>>>     ata: libata: blacklist FUA support for known buggy drives
>>>>     ata: libata: Enable fua support by default
>>>>
>>>
>>> Thanks for the updated series.
>>>
>>> In general (besides the small commit message thing that Sergey had
>>> already mentioned) it looks good to me, so:
>>> Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
>>
>> Thanks. I need to do some more testing using some very old drives I found.
>> So far, no issues: detection works, some drives have FUA, other not. For
>> the ones that have FUA, I am running fstests (ext4 and xfs) to check for
>> weird behavior with REQ_FUA writes. Once I complete all tests I will queue
>> this.
> 
> Actually, I need to take this back. Checking again the code, I found an
> issue with this entire FUA support: for a drive that does not support NCQ,
> or one that has NCQ but has its queue depth set to one, then for a REQ_FUA
> write request, ATA_CMD_WRITE_MULTI_FUA_EXT or ATA_CMD_WRITE_FUA_EXT will
> be used. All good, BUT ! sd.c may also send read requests with the FUA bit
> set if the read request has REQ_FUA set. For read commands, the regular,
> non FUA commands ATA_CMD_READ_MULTI, ATA_CMD_READ_MULTI_EXT, ATA_CMD_READ
> or ATA_CMD_READ_EXT will be used since ATA does not define a FUA version
> of these. This means that the REQ_FUA flag will be ignored: this entire
> code is broken as it is assuming that the read command processing on the
> drive is consistent with executions of ATA_CMD_WRITE_MULTI_FUA_EXT or
> ATA_CMD_WRITE_FUA_EXT. I do not want to bet on that, especially with old
> drives.
> 
Now you got me confused.
What exactly would be the semantics of a READ request with the FUA bit 
set? Ignore the cache and read from disk?
That would only make sense if the cache went out of sync with the drive, 
which really shouldn't happen, no?

> I would be tempted to restrict FUA support to drives that support NCQ,
> given that with NCQ, both READ FPDMA QUEUED and READ FPDMA WRITE have the
> FUA bit. But then, the problem is that if the user changes the queue depth
> of the drive to 1 through sysfs, ncq is turned off and we are back to
> using the EXT read & write commands, that is, only write has FUA.
> 
Hmm. Is this a requirement? We _could_ use the NCQ variants even with a 
queue depth of 1, no?

> So if we want a solid ata FUA support, we would need to always use NCQ
> regardless of the drive max queue depth setting...
> 
Sure, that would be the way I would be going.
If the drive supports NCQ we should always be using the FPDMA variants, 
irrespective of the queue depth.
Additionally we _might_ make FUA dependent on NCQ, and disallow FUA for 
non-NCQ drives.
(Where it's questionable anyway; if you only have a single command 
outstanding the pressure on any internal cache is far less as with NCQ.)

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
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