md(4): Add dummy support of the BIO_FLUSH command for malloc and swap backend.
ClosedPublic
Actions

Authored by afedorov on Feb 13 2022, 1:15 PM.

Details

Reviewers

vmaffione
markj
jhb

Commits

rGf40063d32977: md(4): Add dummy support of the BIO_FLUSH command for malloc and swap
rGcb28dfb27d12: md(4): Add dummy support of the BIO_FLUSH command for malloc and swap

Summary

Some userspace applications may call ioctl(fd, DIOCGFLUSH) for an md(4) device with a memory or swap backend.

An example of such an application is bhyve. achi-hd controller emulation calls blockif_flush(), which fails if md(4) (malloc or swap) is used as the disk image.

The guest system in this case displays an error message:

(ada0:ahcich0:0:0:0): FLUSHCACHE48. ACB: ea 00 00 00 00 40 00 00 00 00 00 00
(ada0:ahcich0:0:0:0): CAM status: ATA Status Error
(ada0:ahcich0:0:0:0): ATA status: 41 (DRDY ERR), error: 04 (ABRT )
(ada0:ahcich0:0:0:0): RES: 41 04 00 00 00 40 00 00 00 00 00
(ada0:ahcich0:0:0:0): Retrying command, 0 more tries remain
(ada0:ahcich0:0:0:0): FLUSHCACHE48. ACB: ea 00 00 00 00 40 00 00 00 00 00 00
(ada0:ahcich0:0:0:0): CAM status: ATA Status Error
(ada0:ahcich0:0:0:0): ATA status: 41 (DRDY ERR), error: 04 (ABRT )
(ada0:ahcich0:0:0:0): RES: 41 04 00 00 00 40 00 00 00 00 00
(ada0:ahcich0:0:0:0): Error 5, Retries exhausted
(ada0:ahcich0:0:0:0): Synchronize cache failed

See detailed description in PR: 260200

Since BIO_FLUSH is useless in the case of memory or swap, just add dummy support for it.

Test Plan

After the patch, the error in the guest system goes away.

Diff Detail

Lint

Lint Skipped

Unit

Tests Skipped

Event Timeline

afedorov requested review of this revision.Feb 13 2022, 1:15 PM

afedorov created this revision.

BTW, mdstart_vnode() does not set bio_resid after handling a BIO_FLUSH. I suspect it does not matter in practice, since by convention BIO_FLUSH commands have bio_length == 0.

This revision is now accepted and ready to land.Feb 13 2022, 3:15 PM

afedorov updated this revision to Diff 102740.Feb 14 2022, 12:32 PM

This revision now requires review to proceed.Feb 14 2022, 12:32 PM

In D34260#775362, @markj wrote:

BTW, mdstart_vnode() does not set bio_resid after handling a BIO_FLUSH. I suspect it does not matter in practice, since by convention BIO_FLUSH commands have bio_length == 0.

Indeed. Thanks for pointing this.

markj accepted this revision.Feb 14 2022, 2:01 PM

This revision is now accepted and ready to land.Feb 14 2022, 2:01 PM

@vmaffione , do you have any objections?

vmaffione accepted this revision.Feb 15 2022, 9:24 PM