I just found a case that could certainly result in endless page faults,
and an endless stream of __get_free_page() calls. It's been there forever,
and I bascially thought it could never happen, but thinking about it some
more it can happen a lot more easily than I thought.

The problem is that the page fault handling code will give up if it cannot
allocate a page table entry. We have code in place to handle the final
page allocation failure, but the "mid-way" failures just failed, and
caused the page fault to be done over and over again.

More importantly, this could happen from kernel mode when a system call
was trying to fill in a user page, in which case it wouldn't even be
interruptible.

It's really unlikely to happen (because the page tables tend to be set up
already), but I suspect it can be triggered by execve'ing a new process
which is not going to have any existing page tables. Even then we're
likely to have old pages available (the ones we free'd from the previous
process), but at least it doesn't sound impossible that this could be a
problem.

I've not seen this behaviour myself, but it could have caused Andrea's
problems, especially the harder to find ones. Andrea, can you check this
patch (against clean 2.1.126) out and see if it makes any difference to
your testing?

(Right now it does the wrong error code: it will cause a SIGSEGV instead
of a SIGBUS when we run out of memory, but that's a small detail).
Essentially, instead of trying to call "oom()" and sending a signal (which
doesn't work for kernel level accesses anyway), the code returns the
proper return value from handle_mm_fault(), which allows the caller to do
the right thing (which can include following the exception tables). That
way we can handle the case of running out of memory from a kernel mode
access too..

(This is also why the fault gets the wrong signal - I didn't bother to fix
up the x86 fault handler all that much ;)
Btw, the reason I'm sending out these patches in emails instead of just
putting them on ftp.kernel.org is that the machine has had disk problems
for the last week, and finally gave up completely last Friday or so. So
ftp.kernel.org is down until we have a new raid array or the old one
magically recovers.  Sorry about the spamming.

                Linus