mtdblock mount issue

Sergei Sharonov sergei.sharonov at
Thu Feb 7 11:28:00 EST 2008

> I am trying to mount a cramfs partition from flash. 

A word of warning. CRAMFS is broken in kernels < 2.6.18. As far as I can tell,
under heavy load (e.g. reading multiple files in parallel that are not cached)
it may fail with decompression errors. I think the issue is not exactly with
cramfs but with some interaction with page cache. Backporting just cramfs files
to 2.6.15 did not help. If you have drop_caches interface (kernel > 2.6.16 or
just backport it) it is easy to reproduce. Otherwise it happens _very_
infrequently because most of the time files are already cached. However the
consequences are severe - corrupted executables, etc.
Here is a script to reproduce the problem:

# Copy large file from cramfs to tmpfs
cp /bin/smbd /tmp/

# First concurrent access to files/directories 
while [ 1 ]; do cat /bin/* > /dev/null; done &

# Second concurrent access to test file and cache flush
for((i=0;i<99999;i++)) do echo -n "cycle=$i "; 
  cmp /bin/smbd /tmp/smbd ; 
  echo 3 > /proc/sys/vm/drop_caches ; 

Note that /bin is in cramfs. smbd can be any large file.
I've also seen statements that drop_caches interface is not 100% reliable.
Nevertheless I have experienced decompression errors even without forcing 
cache purge.
Above script has caused failures on PowerPC/2.6.16 and ARM9/2.6.15.
2.6.18 and 2.6.23 seem ok.



P.S. Yes, cramfs has little to do with MTD but since ppl here use it with flash
I felt it was a good idea to post this warning.

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