On 10/25/22 16:05, Hannes Reinecke wrote: > 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? For the NCQ read command, FUA text says: If the Forced Unit Access (FUA) bit is set to one, the device shall retrieve the data from the non-volatile media regardless of whether the device holds the requested information in the volatile cache. If the device holds a modified copy of the requested data as a result of having volatile cached writes, the modified data shall be written to the non-volatile media before being retrieved from the non-volatile media as part of this operation. If the FUA bit is cleared to zero, the data shall be retrieved either from the device's non-volatile media or cache. So all well for NCQ, everything as expected, no surprises at all here. But no equivalent behavior defined for non-NCQ read commands, and with libata.fua enabled, ata_rwcmd_protocol() will cause any REQ_FUA read to fail, which is the safe thing to do, but that is not what one gets when libata.fua is disabled. So enabling libata fua by default could potentially break some use cases out there of people using REQ_FUA reads without realizing that it is doing nothing in non-NCQ case. I have not checked the block layer though. Does the preflush/sync machinery also triggers for reads ? I do not think so. I think REQ_FUA is ignored for reads. > >> 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? Absolutely, yes. Running NCQ commands at QD = 1 is perfectly fine. I think that this "do not use NCQ when QD == 1" is there mostly to deal with drives that have a buggy NCQ implementation. Not sure. It has been like this forever. Would need to do some git archeology about that one. >> 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.) Yes, that is my thinking too. Will try this and see how it looks. > > Cheers, > > Hannes -- Damien Le Moal Western Digital Research
