Re: Can get_user_pages( ,write=1, force=1, ) result in a read-only pte and _count=2?

[Hugh Dickins <hugh@veritas.com>]

On Thu, 19 Jun 2008, Robin Holt wrote:
> On Thu, Jun 19, 2008 at 12:09:15PM +0100, Hugh Dickins wrote:
> > 
> > (I assume Robin is not forking, we do know that causes this kind
> > of problem, but he didn't mention any forking so I assume not.)
> 
> There has been a fork long before this mapping was created.  There was a
> hole at this location and the mapping gets established and pages populated
> following all ranks of the MPI job getting initialized.

There's usually been a fork somewhen in the past!  That's no problem.

The fork problem comes when someone has done a get_user_pages to break
all the COWs, then another thread does a fork which writeprotects and
raises page_mapcount, so the next write from userspace breaks COW again
and writes to a different page from that which the kernel is holding.

That one kept on coming up, but I've not heard of it again since we
added madvise MADV_DONTFORK so apps could exclude such parts of the
address space from copy_page_range.

Hugh

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