Re: corruption of active mmapped files in btrfs snapshots

[Chris Mason <chris.mason@fusionio.com>]

Quoting Chris Mason (2013-03-22 14:07:05)
> [ mmap corruptions with leveldb and btrfs compression ]
> 
> I ran this a number of times with compression off and wasn't able to
> trigger problems.  With compress=lzo, I see errors on every run.
> 
> Compile: gcc -Wall -o mmap-trunc mmap-trunc.c
> Run: ./mmap-trunc file_name
> 
> The basic idea is to create a 256MB file in steps.  Each step ftruncates
> the file larger, and then mmaps a region for writing.  It dirties some
> unaligned bytes (a little more than 8K), and then munmaps.
> 
> Then a verify stage goes back through the file to make sure the data we
> wrote is really there.  I'm using a simple rotating pattern of chars
> that compress very well.

Going through the code here, when I change the test to truncate once in
the very beginning, I still get errors.  So, it isn't an interaction
between mmap and truncate.  It must be a problem between lzo and mmap.

-chris