Re: [Bug 9138] New: kernel overwrites MAP_PRIVATE mmap

[Paolo Bonzini <paolo.bonzini@lu.unisi.ch>]

> It is standard behaviour that truncating the inode on which an mmap
> was done will generate SIGBUS on access to pages of the mmap beyond
> the new end of file.  Easier to understand when MAP_SHARED, but even
> when MAP_PRIVATE, and even when private pages have already been
> C-O-Wed from the file.

I would have expected MAP_PRIVATE to establish a snapshot of the file, 
as it appears to do on BSDs.  I find it hard to believe that code in the 
wild wants this behavior for MAP_PRIVATE (on the other hand, it is 
clearly the right thing for MAP_SHARED).

> Might it have been a different version of Smalltalk which was tested
> with the 2.6.8 kernel, a version which didn't cause this to happen?

Two weeks ago it started failing on x86-64 after a kernel update but 
still worked on i686; then, yesterday it also started failing on i686 
(guess what, after another kernel update).  It might well be that the 
bug was latent in 2.6.8 and was uncovered by another mmap-related change 
in the kernel, or something like that.

I can work around it by unlink+open; though it will break hard links, 
that's not a big deal.

Thanks for the explanation.

Paolo

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