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

[afedorov]

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.

[markj]

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.

[afedorov]

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.

[afedorov]

@vmaffione , do you have any objections?

[editor_callfortesting.org]

This applies cleanly to 13.0R and corrects the issue. Tested with dd(1) and fio using /dev/ada0 backed by a malloc md device.

Great work!