OK, then why all of the talk earlier saying that journal recovery will
corrupt a swapfile? That was the reason journaling was brought into the
discussion in the first place:
"And now you have kernel which expects data still in journal (that was
state before suspend), but reality on disk is quite different (journal
was replayed). Data corruption." -- Pavel
If the filesystem was not unmounted and remounted, then no replay will happen.
End of story. If the suspend code is doing something like:
1) save memory contents to disk
2) suspend/power off
3) reboot kernel, mount filesystem(s), etc
4) check for presence of suspend image
5) replace currently-running kernel with suspended kernel
Then you are in for a world of hurt regardless of whether the data is in a
swap file or a swap partition. The data in the swapfile isn't touched by
journal replay at all (so that is safe regardless), but the rest of the
filesystem is, which could cause strange disk corruption since the in-memory
data doesn't match the on-disk data.
If that is the case, then the only way to avoid this would be to call
sync_super_lockfs() on each filesystem before the suspend, which will
force the journal to be empty when it returns. That API is supported
by all of the journaling filesystems, and is probably a good thing to
do anyways, as it will potentially free a lot of dirty data from RAM,
and also ensure that the on-disk data is consistent in case the resume
isn't handled gracefully.
> > What is also important to note is that during normal filesystem operation,
> > the ext3 journaling code never reads back any data from the journal, with
> > the exception of a couple of fields in the journal superblock. I would
> > hazard a guess that if you did a suspend, wiped the journal, and then
> > resumed the journaling code wouldn't know the difference.
> That's possible, but I do not want suspend to know about filesystem
This was just given as a counterexample saying that the in-kernel ext3
journal code does not actually care what is in the journal on disk, since
it is never read unless there is a crash. So, the suspend code does not
need to know anything about the filesystem specifics at all in this regard.
-- Andreas Dilger http://sourceforge.net/projects/ext2resize/ http://www-mddsp.enel.ucalgary.ca/People/adilger/
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