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Subjectunionfs: several more problems
I think it may be time to move away from that by now misleading
"msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland" thread.
And I've cut most out of the Cc list, but by all means add any back.

On Saturday I said:
> I'm glad to report that this unionfs, not the one in 2.6.24-rc2-mm1
> but the one including those 9 patches you posted, now gets through
> my testing with tmpfs without a problem. I do still get occasional
> "unionfs: new lower inode mtime (bindex=0, name=<directory>)"
> messages, but nothing worse seen yet: a big improvement.

I have seen worse now: details below.

> I deceived myself for a while that the danger of shmem_writepage
> hitting its BUG_ON(entry->val) was dealt with too; but that's wrong,
> I must go back to working out an escape from that one (despite never
> seeing it).

Once I tried a more appropriate test (fsx while swapping) I hit that
easily. After some thought and testing, I'm happy with the mm/shmem.c
+mm/swap_state.c fixes I've arrived at for that; but since it's not
easy to reproduce in normal usage, and hasn't been holding you up,
I'd prefer for the moment to hold on to that patch. I need to make
changes around the same pagecache<->swapcache area to solve some mem
cgroup issues: there might turn out to be some interaction, so I'd
rather finalize both patches in the same series if I can.

For a while I thought the further unionfs problems I'm seeing were
tmpfs ones, but now I seem to be seeing all(?) of them with ext2:
so I'd better spend no more time on them, hand them over to you.

Please try running LTP (e.g. ltp-full-20071031.tgz) after something like
mkfs -t ext2 /dev/sdb1
mount -t ext2 /dev/sdb1 /mnt
mount -t unionfs -o dirs=/mnt unionfs /tmp

That throws up quite a number of errors which don't occur
with a straightforward ext2 mounted on /tmp. Notably, when it
mkdir seems to collapse into lots of
mkdir: cannot create directory `dirN': No space left on device
mkdir dirN FAILED
(the same happened when I was using a tmpfs instead of an ext2).

But perhaps before fixing up the several LTP tests, you'll want
to concentrate on a more directed test. Please try this sequence:

# Running with mem=512M, probably irrelevant
swapoff -a # Merely to rule out one potential confusion
mkfs -t ext2 /dev/sdb1
mount -t ext2 /dev/sdb1 /mnt
df /mnt # I have 2280 Used out of 1517920 KB
cp -a 2.6.24-rc3 /mnt # Copy a kernel source tree into ext2
rm -rf /mnt/2.6.24-rc3 # Delete the copy
df /mnt # Again 2280 Used, just as you'd expect
mount -t unionfs -o dirs=/mnt unionfs /tmp
cp -a 2.6.24-rc3 /tmp # Copy a kernel source tree into unionfs
rm -rf /tmp/2.6.24-rc3 # Generates 176 unionfs: filldir error messages
df /mnt # Now 68380 Used (df /tmp shows the same)
ls -a /mnt # Shows . .. .wh.2.6.24-rc3 lost+found
echo 1 >/proc/sys/vm/drop_caches # to free pagecache
df /mnt # Still 68380 Used (df /tmp shows the same)
echo 2 >/proc/sys/vm/drop_caches # to free dentries and inodes
df /mnt # Now 2280 Used as it should be (df /tmp same)
ls -a /mnt # But still shows that .wh.2.6.24-rc3
umount /tmp # Restore
umount /mnt # Restore
swapon -a # Restore

Three different problems there:

1. Whiteouts seem to get left behind (at this top level anyway):
I'm getting an increasing number of files there.
I'm not familiar with the correct behaviour for whiteouts (and it's
not clear to me why whiteouts are needed at all in this degenerate
case of a single directory in the union, but never mind).

2. Why does copying then deleting a tree leave blocks allocated,
which remain allocated indefinitely, until memory pressure or
drop_caches removes them? Hmm, I should have done "df -i" instead,
that would be more revealing. This may well be the same as the LTP
mkdir problem - inodes remaining half-allocated after they're unlinked.

3. The unionfs filldir error messages:
unionfs: filldir: possible I/O error: a file is duplicated in the same
branch 0: page_offset.h
<... 174 more include/asm-* header files named until ...>
unionfs: filldir: possible I/O error: a file is duplicated in the same
branch 0: sfafsr.h
It's tempting to suppose these are related to the unfreed inodes, but
retrying again gives me 176 messages, whereas inodes fall from 2672
to 30. And I didn't see those messages with tmpfs, just with ext2.

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