On Thu, Sep 30, 2010 at 6:28 AM, Svatopluk Kraus <[email protected]> wrote:
> On Tue, Sep 21, 2010 at 7:38 PM, Alan Cox <[email protected]> wrote: > > On Mon, Sep 20, 2010 at 9:32 AM, Svatopluk Kraus <[email protected]> > 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 _______________________________________________ [email protected] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[email protected]"
