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Date: Sun, 31 Jul 2005 23:18:58 +0300
From: Giorgos Keramidas <keramida@freebsd.org>
To: Joseph Koshy <joseph.koshy@gmail.com>
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Cc: freebsd-current@freebsd.org
Subject: Re: mmap bug?
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On 2005-07-31 23:45, Joseph Koshy <joseph.koshy@gmail.com> wrote:
> gk> That's something I didn't test.  No, the 'extra' data
> gk> disappears.
>
> So the 'extra' data isn't reaching the FS and is probably
> being served up from a cached VM data the second time your
> test program ran.
>
> This is still a bug though: the mmap(2) manual page
> says:
>     ...
>     If len is not a multiple of
>     the page-size, the mapped region may extend past the
>     specified range.  Any such extension beyond the end of the
>     mapped object will be zero-filled.
>     ...
>
> We are clearly not doing the zero-filling.

The mapping is allocated as MAP_SHARED, so when I unmap() it from a
process that has attached to the specific object/file/whatever that is,
it shouldn't be zeroed.

The bug seems to be elsewhere, namely to the fact that the filesystem
code never realizes the file has changed size after I use mmap() to map
a region beyond its current size and write past its current end.