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Re: USB umass hard drive "failed to create xfers" when attaching

With a 9.99.46 kernel built from sources dated 2020-02-07 16:26:35 UTC
I get the following errors when plugging in a USB hard drive:

umass0 at uhub1 port 2 configuration 1 interface 0
umass0: Western Digital (0x1058) Ext HDD 1021 (0x1021), rev 2.00/20.02, addr 32
umass0: using SCSI over Bulk-Only
umass0: autoconfiguration error: failed to create xfers

This worked correctly with a 9.99.42 kernel built from sources dated
2020-01-25 19:35:05 UTC

When it was working, this is what dmesg reported:

umass0 at uhub1 port 1 configuration 1 interface 0
umass0: Western Digital (0x1058) Ext HDD 1021 (0x1021), rev 2.00/20.02, addr 6
umass0: using SCSI over Bulk-Only
scsibus0 at umass0: 2 targets, 1 lun per target
sd0 at scsibus0 target 0 lun 0: <WD, Ext HDD 1021, 2002> disk fixed
sd0: fabricating a geometry
sd0: 1863 GB, 1907727 cyl, 64 head, 32 sec, 512 bytes/sect x 3907024896 sectors
sd0: fabricating a geometry

Anyone got any clues on how this got broke?  Or how to fix?

More info...

First, this is on a amd64 system, witwh 8core/16thread and 128GB of RAM.

On IRC it was suggested (thanks, maya!) that the error message might be
related to memory fragmentation.  I didn't believe it (given how much
RAM I have), but a quick check with top(1) showed that I had more than
100GB of 'file cache' active.  So, I unmounted all my development trees
(to force the cache to get flushed).  Sure enough, I am now able to
successfully mount the USB drive!

So, sounds like "something somewhere isn't quite right (tm)".  I would
have expected a memory allocation failure to automatically trigger some
mechanism to reclaim some of the file cache...

And, based on more discussion, this would seem to be a kernel virtual
address fragmentation issue, and not related to physical memory being
available.  The concensus on IRC is that this is a bug in the xhci(4)

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