page table fault, which should map kernel virtual address space

[Alan Cox]

On Thu, Sep 30, 2010 at 6:28 AM, Svatopluk Kraus <onwahe at gmail.com> wrote:

> On Tue, Sep 21, 2010 at 7:38 PM, Alan Cox <alan.l.cox at gmail.com> wrote:
> > On Mon, Sep 20, 2010 at 9:32 AM, Svatopluk Kraus <onwahe at gmail.com>
> wrote:
> >> Beyond 'kernel_map', some submaps of 'kernel_map' (buffer_map,
> >> pager_map,...) exist as result of 'kmem_suballoc' function call.
> >> When this submaps are used (for example 'kmem_alloc_nofault'
> >> function) and its virtual address subspace is at the end of
> >> used kernel virtual address space at the moment (and above 'NKPT'
> >> preallocation), then missing page tables are not allocated
> >> and double fault can happen.
> >>
> >
> > No, the page tables are allocated.  If you create a submap X of the
> kernel
> > map using kmem_suballoc(), then a vm_map_findspace() is performed by
> > vm_map_find() on the kernel map to find space for the submap X.  As you
> note
> > above, the call to vm_map_findspace() on the kernel map will call
> > pmap_growkernel() if needed to extend the kernel page table.
> >
> > If you create another submap X' of X, then that submap X' can only map
> > addresses that fall within the range for X.  So, any necessary page table
> > pages were allocated when X was created.
>
> You are right. Mea culpa. I was focused on a solution and made
> too quick conclusion. The page table fault hitted in 'pager_map',
> which is submap of 'clean_map' and when I debugged the problem
> I didn't see a submap stuff as a whole.
>
> > That said, there may actually be a problem with the implementation of the
> > superpage_align parameter to kmem_suballoc().  If a submap is created
> with
> > superpage_align equal to TRUE, but the submap's size is not a multiple of
> > the superpage size, then vm_map_find() may not allocate a page table page
> > for the last megabyte or so of the submap.
> >
> > There are only a few places where kmem_suballoc() is called with
> > superpage_align set to TRUE.  If you changed them to FALSE, that is an
> easy
> > way to test this hypothesis.
>
> Yes, it helps.
>
> My story is that the problem started up when I updated a project
> ('coldfire' port)
> based on FreeBSD 8.0. to FreeBSD current version. In the current version
> the 'clean_map' submap is created with superpage_align set to TRUE.
>
> I have looked at vm_map_find() and debugged the page table fault once
> again.
> IMO, it looks that a do-while loop does not work in the function as
> intended.
> A vm_map_findspace() finds a space and calls pmap_growkernel() if needed.
> A pmap_align_superpage() arrange the space but never call
> pmap_growkernel().
> A vm_map_insert() inserts the aligned space into a map without error
> and never call pmap_growkernel() and does not invoke loop iteration.
>
> I don't know too much about an idea how a virtual memory model is
> implemented
> and used in other modules. But it seems that it could be more reasonable to
> align address space in vm_map_findspace() internally and not to loop
> externally.
>
> I have tried to add a check in vm_map_insert() that checks the 'end'
> parameter
> against 'kernel_vm_end' variable and returns KERN_NO_SPACE error if needed.
> In this case the loop in vm_map_find() works and I have no problem with
> the page table fault. But 'kernel_vm_end' variable must be initializated
> properly before first use of vm_map_insert(). The 'kernel_vm_end' variable
> can be self-initializated in pmap_growkernel() in FreeBSD 8.0 (it is too
> late),
> but it was changed in current version ('i386' port).
>
> Thanks for your answer, but I'm still looking for permanent
> and approved solution.
>

I have a patch that implements one possible fix for this problem.  I'll
probably commit that patch in the next day or two.

Regards,
Alan