Re: [PATCH 0/2] kdump/mmap: Fix mmap of /proc/vmcore for s390

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Jun 03, 2013 at 03:27:18PM +0200, Michael Holzheu wrote:

[..]
> > If not, how would remap_pfn_range() work with HSA region when
> > /proc/vmcore is mmaped()?
> 
> I am no memory management expert, so I discussed that with Martin
> Schwidefsky (s390 architecture maintainer). Perhaps something like
> the following could work:
> 
> After vmcore_mmap() is called the HSA pages are not initially mapped in
> the page tables. So when user space accesses those parts
> of /proc/vmcore, a fault will be generated. We implement a mechanism
> that in this case the HSA is copied to a new page in the page cache and
> a mapping is created for it. Since the page is allocated in the page
> cache, it can be released afterwards by the kernel when we get memory
> pressure.
> 
> Our current idea for such an implementation:
> 
> * Create new address space (struct address_space) for /proc/vmcore.
> * Implement new vm_operations_struct "vmcore_mmap_ops" with
>   new vmcore_fault() ".fault" callback for /proc/vmcore.
> * Set vma->vm_ops to vmcore_mmap_ops in mmap_vmcore().
> * The vmcore_fault() function will get a new page cache page,
>   copy HSA page to page cache page add it to vmcore address space.
>   To see how this could work, we looked into the functions
>   filemap_fault() in "mm/filemap.c" and relay_buf_fault() in
>   "kernel/relay.c".
> 
> What do you think?

I am not mm expert either but above proposal sounds reasonable to me.

So remap_pfn_range() call will go in arch dependent code so that arch
can decide which range can be mapped right away and which ranges will
be filed in when fault happens? I am assuming that s390 will map
everything except for pfn between 0 and HSA_SIZE.

And regular s390 kdump will map everyting right away and will not
have to rely on fault mechanism?

Thanks
Vivek