> Update: Having reread the report, it appears that this may only be a problem in the new 4.0 kernel?
No, reporter uses 4.0 kernel which is why bug report lists that version. I've looked at commit message for the fix but it doesn't talk about any regression, so this bug may be existing for a long time.
An entire extent could be lost(or zeroed?) which means serious corruption to your file(s) at time of writing if the cards fall in the right order.
I see no indication that this is a 4.0-specific issue. This code has existed for a long time. They fixed it in "stable kernels 4.0.3 as well as much older stable kernels."
edit: major corruption is possible, not just slight, because an extent can be 128Mib on a 4K block size filesystem.
I assume/hope that if this was a common occurrence it would have been found much earlier...
Update: Having reread the report, it appears that this may only be a problem in the new 4.0 kernel?
Update2: Nope, not just 4.0 kernels (see below).