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SubjectRe: [PATCH 3/4] unionfs: restructure unionfs_setattr
On Tue, 18 Dec 2007, Erez Zadok wrote:
> In message <>, Hugh Dickins writes:
> > In order to fix unionfs truncation, we need to move the lower notify_change
> > out of the loop in unionfs_setattr. But when I came to do that, I couldn't
> [...]
> Hugh, I want to understand how patches 3/4 and 4/4 are related. In patch 3
> you say "in order to fix truncation" but you mention a truncation problem
> only in patch 4; is there a patch ordering problem, or they're both related
> to the same issue (with 3/4 being a code cleanup, and 4/4 actually fixing
> the problem)?

I needed to move that notify_change out of the loop, to fix the truncation
problem, but had great difficulty understanding the loop. So, just as you
say, made the code cleanup first in 3/4, then fixed the problem in 4/4.

But that cleanup does need your review and testing.

> What tests did you conduct to tickle this truncation problem: I assume
> fsx-linux through unionfs, mounted on tmpfs?

Exactly. The familiar "fsx foo -q -c 100 -l 10000000" on its own is
enough to trigger it, though my habit is to run that while forcing
swapout and swapoff too. The problem isn't peculiar to tmpfs,
but I expect other filesystems are more likely just to fail the
fsx test rather than oops.

> Did that include both series
> of patches (your 9 tmpfs patches, plus the two memcgrpoup?).

Any kernel without the unionfs 4/4 should show the problem, but
differently with different sets of patches. A lot of my testing
was with an earlier set of tmpfs patches for unionfs (I didn't
realize the tmpfs 5/9 issue until later on), and with those it
manifested as fsx failure.

But if you run without any of these patches,
I believe it manifests as shmem_writepage's
BUG_ON(!(info->flags & SHMEM_TRUNCATE));
or its
Whereas in tmpfs 4/9 I removed that latter BUG_ON(!entry) -
we don't usually bother to BUG on NULL pointers, just let the
dereference oops.


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