Re: [PATCH v7 05/13] fat: make fat_update_time get its own timestamp

[OGAWA Hirofumi]

Jeff Layton <jlayton@xxxxxxxxxx> writes:

> On Thu, 2023-08-10 at 00:17 +0900, OGAWA Hirofumi wrote:
>> Jan Kara <jack@xxxxxxx> writes:

[...]

> My mistake re: lazytime vs. relatime, but Jan is correct that this
> shouldn't break anything there.

Actually breaks ("break" means not corrupt fs, means it breaks lazytime
optimization). It is just not always, but it should be always for some
userspaces.

> The logic in the revised generic_update_time is different because FAT is
> is a bit strange. fat_update_time does extra truncation on the timestamp
> that it is handed beyond what timestamp_truncate() does.
> fat_truncate_time is called in many different places too, so I don't
> feel comfortable making big changes to how that works.
>
> In the case of generic_update_time, it calls inode_update_timestamps
> which returns a mask that shows which timestamps got updated. It then
> marks the dirty_flags appropriately for what was actually changed.
>
> generic_update_time is used across many filesystems so we need to ensure
> that it's OK to use even when multigrain timestamps are enabled. Those
> haven't been enabled in FAT though, so I didn't bother, and left it to
> dirtying the inode in the same way it was before, even though it now
> fetches its own timestamps from the clock. Given the way that the mtime
> and ctime are smooshed together in FAT, that seemed reasonable.
>
> Is there a particular case or flag combination you're concerned about
> here?

Yes. Because FAT has strange timestamps that different granularity on
disk . This is why generic time truncation doesn't work for FAT.

Well anyway, my concern is the only following part. In
generic_update_time(), S_[CM]TIME are not the cause of I_DIRTY_SYNC if
lazytime mode.

-	if ((flags & S_VERSION) && inode_maybe_inc_iversion(inode, false))
+	if ((flags & (S_VERSION|S_CTIME|S_MTIME)) && inode_maybe_inc_iversion(inode, false))
		dirty_flags |= I_DIRTY_SYNC;

If reverted this part to check only S_VERSION, I'm fine.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>