fat: changed filesystem dirty bit behavior

Anssi Hannula
2016-04-21T13:16:34Z

Hi all!

Starting with commit b88a105802e9aeb [1] ("fat: mark fs as dirty on
mount and clean on umount") FAT(32) filesystems are now marked as
"dirty" on mount and clean on unmount.

The commit message says that this is similar to Win 7 behavior - "Win 7,
set dirty flag on first write and remove it on umount".
However, I have been unable to coerce my Windows 7 system to set this
flag on a FAT32 filesystem, when tested both with portable (USB) and
fixed disks. Have they maybe changed this with an update, or does
someone else still see this bit set on Win7 or later?

This change is a bit problematic on a legacy embedded application I'm
working on, as the user interface doesn't have any separate
eject/unmount button for a USB stick - it relies on the user not
unplugging the stick while a data transfer is in progress.
So, when the system is upgraded to a modern vanilla kernel version, this
bit is set when the USB stick is unplugged, causing Windows to always
prompt whether to scan and fix the drive, which is annoying/confusing
for users.

Would a patch to add a filesystem option to restore the previous (and
seemingly Win7) behavior be accepted?

Or is this a case where the application is just considered to be broken?
(as setting the dirty bit seems technically correct, even if Windows
doesn't do it)

Or anything else I'm missing?

For reference, on FAT32 this is bit 0 of byte 0x41 (61) of the first sector.

[1]
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=b88a105802e9aeb6e234e8106659f5d1271081bb

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Anssi Hannula / Bitwise Oy