Change Details

We talked about it in the bug, Andriy / Warner proposed a concrete implementation, and Kirk was supportive. So let's actually do the thing! This Differential is prompted by real-world pain with a sometimes-RO device that does not report WP (nor SWP) correctly. (It is sometimes RW; we cannot blacklist it with a quirk.) Basic `ggate` example repro from the PR description seems to work fine (will copy in results later). Further testing needs to be done. Hoping to try `gnop` with `-w 100` and custom error code(s) to make it more interesting. Future work would be extending something like this to other filesystems that set a dirty bit on mount.

We talked about it in the bug, Andriy / Warner proposed a concrete implementation, and Kirk was supportive. So let's actually do the thing! This Differential is prompted by real-world pain with a sometimes-RO device that does not report WP (nor SWP) correctly. (It is sometimes RW; we cannot blacklist it with a quirk.) Basic `ggate` example repro from the PR comment #5 seems to work fine. ``` $ dd if=/dev/zero of=./image bs=1M count=100 $ newfs_msdos ./image $ ggatel create -o ro ./image ggate0 $ mount -t msdosfs /dev/ggate0 /mnt/tst ; echo $? g_vfs_done():ggate0[WRITE(offset=512, length=4096)]error = 1 mount_msdosfs: /dev/ggate0: Operation not permitted 1 $ sysctl vfs.numdirtybuffers vfs.numdirtybuffers: 0 ``` Further testing with `gnop` to ensure similar handling across a range of possible errors. ========================================================================== ENODEV: ``` $ mdconfig -a -t swap -s 100M md0 $ newfs_msdos /dev/md0 // ENODEV = 19 $ gnop create -e 19 -r 0 -w 100 -v md0 GEOM_NOP: Device md0.nop created. $ mount -t msdosfs /dev/md0.nop /mnt/tst ; echo $? g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 19 vfs_donmount: R/W mount failed, possibly R/O media, trying R/O mount 0 // Oops, autoro. $ mount ... /dev/md0.nop on /mnt/tst (msdosfs, local, read-only) $ umount /mnt/tst // Try again with explicit -o rw. $ mount -t msdosfs -o rw /dev/md0.nop /mnt/tst ; echo $? g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 19 mount_msdosfs: /dev/md0.nop: Operation not supported by device 1 $ sysctl vfs.numdirtybuffers vfs.numdirtybuffers: 0 ``` EIO: ``` $ gnop destroy md0.nop $ gnop create -e 5 -r 0 -w 100 -v md0 $ mount -t msdosfs -o rw /dev/md0.nop /mnt/tst ; echo $? g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 5 mount_msdosfs: /dev/md0.nop: Input/output error 1 ``` EACCES: ``` $ gnop destroy md0.nop $ gnop create -e 13 -r 0 -w 100 -v md0 $ mount -t msdosfs -o rw /dev/md0.nop /mnt/tst ; echo $? ; sysctl vfs.numdirtybuffers g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 13 mount_msdosfs: /dev/md0.nop: Permission denied 1 vfs.numdirtybuffers: 0 ``` ENXIO: ``` $ gnop destroy md0.nop $ gnop create -e 6 -r 0 -w 100 -v md0 $ mount -t msdosfs -o rw /dev/md0.nop /mnt/tst ; echo $? ; sysctl vfs.numdirtybuffers g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 6 mount_msdosfs: /dev/md0.nop: Device not configured 1 vfs.numdirtybuffers: 0 ``` Finally, test the ro -> rw upgrade path. ========================================================================== ``` $ gnop destroy md0.nop $ gnop create -e 19 -r 0 -w 100 -v md0 $ mount -t msdosfs -o ro /dev/md0.nop /mnt/tst ; echo $? 0 $ mount /dev/md0.nop on /mnt/tst (msdosfs, local, read-only) $ mount -u -o current,rw /mnt/tst ; echo $? g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 19 mount_msdosfs: /dev/md0.nop: Operation not supported by device 1 $ mount /dev/md0.nop on /mnt/tst (msdosfs, local, read-only) $ sysctl vfs.numdirtybuffers vfs.numdirtybuffers: 0 ``` Looks good. Future work would be extending something like this to other filesystems that set a dirty bit on mount.

We talked about it in the bug, Andriy / Warner proposed a concrete implementation, and Kirk was supportive. So let's actually do the thing! This Differential is prompted by real-world pain with a sometimes-RO device that does not report WP (nor SWP) correctly. (It is sometimes RW; we cannot blacklist it with a quirk.) Basic `ggate` example repro from the PR descriptioncomment #5 seems to work fine (will. ``` copy in results later). Further testing needs to be done. Hoping to try$ dd if=/dev/zero of=./image bs=1M count=100 `gnop` with `-w 100` and custom error code(s) to make it more interesting$ newfs_msdos ./image $ ggatel create -o ro ./image ggate0 $ mount -t msdosfs /dev/ggate0 /mnt/tst ; echo $? g_vfs_done():ggate0[WRITE(offset=512, length=4096)]error = 1 mount_msdosfs: /dev/ggate0: Operation not permitted 1 $ sysctl vfs.numdirtybuffers vfs.numdirtybuffers: 0 ``` Further testing with `gnop` to ensure similar handling across a range of possible errors. ========================================================================== ENODEV: ``` $ mdconfig -a -t swap -s 100M md0 $ newfs_msdos /dev/md0 // ENODEV = 19 $ gnop create -e 19 -r 0 -w 100 -v md0 GEOM_NOP: Device md0.nop created. $ mount -t msdosfs /dev/md0.nop /mnt/tst ; echo $? g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 19 vfs_donmount: R/W mount failed, possibly R/O media, trying R/O mount 0 // Oops, autoro. $ mount ... /dev/md0.nop on /mnt/tst (msdosfs, local, read-only) $ umount /mnt/tst // Try again with explicit -o rw. $ mount -t msdosfs -o rw /dev/md0.nop /mnt/tst ; echo $? g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 19 mount_msdosfs: /dev/md0.nop: Operation not supported by device 1 $ sysctl vfs.numdirtybuffers vfs.numdirtybuffers: 0 ``` EIO: ``` $ gnop destroy md0.nop $ gnop create -e 5 -r 0 -w 100 -v md0 $ mount -t msdosfs -o rw /dev/md0.nop /mnt/tst ; echo $? g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 5 mount_msdosfs: /dev/md0.nop: Input/output error 1 ``` EACCES: ``` $ gnop destroy md0.nop $ gnop create -e 13 -r 0 -w 100 -v md0 $ mount -t msdosfs -o rw /dev/md0.nop /mnt/tst ; echo $? ; sysctl vfs.numdirtybuffers g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 13 mount_msdosfs: /dev/md0.nop: Permission denied 1 vfs.numdirtybuffers: 0 ``` ENXIO: ``` $ gnop destroy md0.nop $ gnop create -e 6 -r 0 -w 100 -v md0 $ mount -t msdosfs -o rw /dev/md0.nop /mnt/tst ; echo $? ; sysctl vfs.numdirtybuffers g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 6 mount_msdosfs: /dev/md0.nop: Device not configured 1 vfs.numdirtybuffers: 0 ``` Finally, test the ro -> rw upgrade path. ========================================================================== ``` $ gnop destroy md0.nop $ gnop create -e 19 -r 0 -w 100 -v md0 $ mount -t msdosfs -o ro /dev/md0.nop /mnt/tst ; echo $? 0 $ mount /dev/md0.nop on /mnt/tst (msdosfs, local, read-only) $ mount -u -o current,rw /mnt/tst ; echo $? g_vfs_done():md0.nop[WRITE(offset=512, length=4096)]error = 19 mount_msdosfs: /dev/md0.nop: Operation not supported by device 1 $ mount /dev/md0.nop on /mnt/tst (msdosfs, local, read-only) $ sysctl vfs.numdirtybuffers vfs.numdirtybuffers: 0 ``` Looks good. Future work would be extending something like this to other filesystems that set a dirty bit on mount.