Re: Repost: could ia32 mmap() allocations grow downward?

From: Hugh Dickins (
Date: Thu Dec 13 2001 - 12:10:16 EST

On Thu, 13 Dec 2001, Wayne Whitney wrote:
> So it seems like for MAGMA I should be able to work around the fact that
> mmap()'s start at 0x40000000. But as difficulties with other programs
> come up here fairly regularly, I still think it makes sense to fully
> understand the downside of modifying the kernel to allocate mmap() VMAs
> going downward. If the downside is small, I think it is a good tradeoff.

My fear is that you may encounter an indefinite number of buggy apps,
which expect an mmap() to follow the mmap() before: easy bug to commit,
and to go unnoticed, until you reverse the layout.

As to where to place your stack: I don't know what assumptions are made
elsewhere, but a seemingly good place is just below the program's text
at 0x08048000. People sometimes ask why i386 ELF text is usually placed
there: I think it's a convention of some other UNIX implementations,
which used to put stack below text and data above it, all sharing
the one page table (if it's a smallish process).


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