vmalloc problem with DEBUG_PAGEALLOC config option

[Zdenek Kabelac]

Hello

Recently I've spent some time with hunting weird problem which
appeared to be caused by the CONFIG_DEBUG_PAGEALLOC option. After the
crash of 'nash' sometimes my application started to behave strange -
after some debugging and testing when I've basically tried everything
else :) because it wouldn't come to my mind what could be wrong I' ve
noticed, that inside the kernel driver 'vmalloc-ated' memory chunk
simply doesn't get the right memory copy from the funtion call
copy_from_user().
When I want to get 16kb copied from user space and I prefil it with
some byte pattern, I get zeroed area. When I use 'get_user()'  I see
correct user-space data - so the problem is with the copying and
DEBUG_PAGEALLOC enabled.

When I'm not using this debug option - I'm not able to replicate my
problem with copy_from_user().

I'm using 64bit x86 kernel on 2GB T61

Zdenek
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