Re: wrong madvise(MADV DONTNEED) semantic

[Jamie Lokier]

Michael Kerrisk wrote:
> For a MAP_SHARED region, the kernel may discard modified 
> pages, depending on the architecture.  (I seem to recall 
> getting this information about architecture variation from 
> an email conversation with Jamie Lokier; I haven't 
> verified it.  On x86 at least, I haven't seen an pages
> discarded for MADV_DONTNEED on a MAP_SHARED region.)  

The discard in MAP_SHARED case isn't explicit.  It's due to CPU cache
incoherency, so the data (or just some of the data) is lost simply by
not syncing properly between userspace and the page cache.

It wouldn't be apparent on fully-coherent architectures like x86.

That was when I last looked at the code (during the 2.5 time frame).
I didn't test it, only examined the code, but quite thoroughly.

-- Jamie
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