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@xxxxxxxxxx>
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
--
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